===== Welcome to the barometer wiki! =====

Although this wiki is mostly about barometers for the [[https://www.ecowitt.com/shop/goodsPage/1/43|Ecowitt]] line of personal weather stations, barometers operate pretty much the same way for all weather stations.

Owning your own personal weather station is an enjoyable hobby. Most amateur weather stations now have internet capabilities and you can upload your weather data to create your own weather website or upload your data to various national and international weather service organizations.

Barometers and their pressure readings and related calculations can be difficult to understand, so much so that until now, only a very small percentage of station owners have successfully managed to properly set-up or even use their barometers. However, this all changed with the introduction of Ecowitt's new automatic barometer firmware update in 2025.

Ecowitt's new sea level pressure (SLP) firmware update is a radical departure from the previous algorithm, where sea level pressures were somewhat crudely approximated using a simple fixed offset. SLP (mean sea level pressure) is now automatically and continuously calculated based on changes in station pressure, outside temperatures and humidity. This new update greatly increases the accuracy of sea level pressure calculations. It will also make the initial set-up of the barometer much easier, requiring the user to only input their barometer altitude above sea level. No more manual calculations required!


<alert>
**//Although I will be referring to mainly Ecowitt weather stations, these tutorials can also apply to other weather station brands. The same barometric principles apply. Depending on the brand and model of your amateur-grade weather station, these stations should have the ability to measure the current atmospheric pressure (station pressure) and be able to calculate Altimeter setting/QNH and/or SLP (sea level pressure).//**</alert>



Barometers are critically important sensors for weather forecasting purposes.The purpose of this wiki is to provide help and guidance along with plenty of examples and tutorials to get your barometer set up, calibrated and operational!
----
==== Introduction ====

<alert>**//"...the atmospheric pressure measured with your own barometer remains the most important indication of weather changes at your location. To evaluate present weather or to forecast coming weather, we need accurate barometric pressure… ////- Dr. David Burch, author of "The Barometer Handbook"//**</alert>

If we need __accurate__ pressure, how do we best obtain it? The following series of tutorials and guides are aimed at either a brand-new owner of an weather station or an existing owner that hasn’t properly calibrated their barometer because it is too complicated, the manual is unclear, barometers involve quantum mechanics to figure out, don’t have much time etc, etc. If so, these guides are for you.

Barometric sensors measure the ever-changing weight/pressure of the atmosphere around us. Measuring these changes is one of the most important aspects of meteorology, and for aviation it is safety-critical as it keeps aircraft away from the ground and from each other.

If you were like me, I skipped over the barometer section of the manual when I first purchased my weather station. Although I consider myself a reasonably technical person, the instructions in the manual were baffling. After several frustrating attempts to follow the scant barometer instructions, I gave up and went on to set up the rest of the station. I left the weather station uncalibrated for some months until I stumbled across [[https://www.wxforum.net/|wxforum.net]].

However, a helpful wxforum.net forum member got me going and soon after, my barometer was finally set up and operational.

Time to pay it forward!

Things have changed a great deal since I purchased my first weather station in 2019. The new barometer set up instructions and calibration procedures presented here, will greatly increase accuracy and simplify the initial set-up of your barometer. Hopefully, you will find this wiki as a useful resource for all things barometric.

====Importance of barometers====
**//updated 10 Aug 2025//**

There is no doubt that of all our weather station sensors, the barometer is one of the most important - if not the most important. The history of meteorology and weather forecasting, in particular, revolves around this instrument.

<alert>
**"Analyzed pressure fields are a fundamental requirement of the science of meteorology. It is imperative that these pressure fields be defined, as they form the basis for all subsequent predictions of the state of the atmosphere. Pressure measurements must be as accurate as technology will allow, within realistic financial constraints, and there must be uniformity in the measurement and calibration procedures across national boundaries."**

//- World Meteorological Organization (WMO), Guide to Meteorological Instruments and Methods of Observation//
</alert>

One of its first important uses was for sailing ships more than three hundred years ago. Sailors noted that changes in atmospheric pressure were related to wind and gale.  Wind causes waves. Big waves sink ships and loses lives. Therefore, barometers became increasingly important. Even today, ships still have analogue (dial type) barometers on board should all the electronics fail.

In modern times, we now have aircraft and they too, have barometers aka "altimeters". And just like for ships, they can also save lives. Altimeters help maintain safe separation between other aircraft and terrain.

Needless to say, in addition to weather forecasting purposes, barometers are safety-critical instruments for air, land and sea. Despite their importance, most people would have no idea how they work or how to use them.

But the barometer is different. Unlike most weather sensors, it has predictive qualities, which is a key element of weather forecasting.

Why are barometers important?

The atmosphere is important. Air is heavy - about 11 tonnes per square meter on the Earth's surface. All that weight means that when the atmosphere is moving around, it can have considerable force as anyone who has experienced a hurricane, cyclone or tornado has found out.

Barometers play a critically important role in measuring and predicting the weather. No serious weather station could do without the essential barometer.

References and sources:

//H.M.S Beagle, Captain Robert Fitzroy and Charles Darwin//
https://en.wikipedia.org/wiki/Robert_FitzRoy

//A Weather Eye//
https://www.nzgeo.com/stories/a-weather-eye/

//Weather Observer's Handbook - Stephen Burt 2nd edition 2024//

//WMO - Guide to Meteorological Instruments and Methods of Observation//
https://www.weather.gov/media/epz/mesonet/CWOP-WMO8.pdf

//Why buy a barometer? - David Burch//
https://www.starpath.com/catalog/accessories/why_buy_a_barometer.htm

==== Understanding the Basics of the Atmosphere ====
**//updated 16 February 2024//**

It is best to start with the sea. We need a starting point – a benchmark or datum to weigh the atmosphere.  The average atmospheric pressure at the average sea level elevation (average because there are tides!) is equal to 1013.25 mb. The world-wide sea also has an average temperature, and it has been set to be 15C.

So those two numbers are our baseline. Intuitively, we know that the higher we go in the atmosphere, atmospheric pressure becomes less, and the temperature becomes colder. After all, on Mt. Everest, the air is thin and cold. The question is: how thin? And how cold? We also know that the density of air changes with temperature – cold air is heavier (denser) and warm air is lighter (less dense) – something that hot air balloons take advantage of.

//Standard Atmosphere (model of the atmosphere)://

  - //Temperature: 15 °C @ 0 meters (mean sea level). The model assumes the temperature is always 15C at sea level elevation. Temperature declines linearly at the rate of 0.0065 °C per meter with altitude.//
  - //Pressure: 1013.25 mb @ 0 meters (mean sea level). The model assumes that the pressure at sea level always stays at 1013.25 mb. Pressure declines in a non-linear fashion (it’s on a curve) with altitude.//

Weather occurs in the lowest part of the atmosphere called the troposphere.

The height of this layer varies greatly with latitude - being much thicker at the equator and quite thin at the poles. The height of the troposphere averages out to be about 10 km to 13 km high.

//Note: There are other parameters in the Standard Atmosphere model, but for our weather stations, pressure and temperature (and their relationships) are the most important ones.//

====Sea Level Pressure - visualizing a concept (Example #1)====
updated 07 Jan 2026

There is no doubt that of all the atmospheric pressures, sea level pressure is the most difficult to understand and at the same time - the most misunderstood.

Sea level pressure could be an actual measurement of pressure (at sea level) or it could also be a mathematical approximation of sea level pressure if you are not at sea level.

Here's an illustration of sea level pressure that might help visualize the concept:

We will use a pair of Ecowitt display consoles for the illustration. Both consoles are identical and each have calibrated barometric sensors.

**Illustration of Sea Level Pressure**

Let's say that you have a display console on the 40th floor of an apartment building and your friend has the other identical display console on the ground floor (at sea level). Note: each floor is 10 feet high.

**Question:**

a) Are both display console's ABS (station pressures) the same or different?

b) Are the REL values (calculated sea level pressure) on the console display on the 40th floor the same or different than the REL display on the ground floor?

**Answer:**

a) **Each console has a different ABS**. ABS (absolute pressure) is just the current atmospheric pressure at your location. The two ABS values would be different because the top floor is way higher than the ground floor located at sea level. The air is thinner on the 40th floor and therefore would show an ABS pressure a fair bit less than the ground floor ABS at sea level. In this illustration, the pressure would be 14.5 hPa lower on the 40th floor than the ground floor at sea level. The ground floor ABS = 1013.2.

b) **Each console has the same REL**. The sea level pressure (REL) is the same on the 40th floor as it is at sea level on the ground floor! This is not so readily apparent at first - but let's take a closer look. //Why is the sea level pressure the same on both floors?//

First, we have to understand what the barometer algorithm in the console's firmware is calculating. The algorithm is calculating REL (relative pressure) which is an estimate of sea level pressure (the atmospheric pressure at mean sea level) 400 feet below.

How about the pressure at ground level floor at sea level? No need to calculate or estimate the pressure at sea level when you are already at sea level. The friend's weather station located at ground level was originally set up with a 0 (zero) altitude so the algorithm calculates zero as the adjustment to sea level. In other words, if you want a sea level pressure at sea level - you just measure it. Therefore, the display console at sea level on the ground floor will display ABS = REL (the same). The display would show ABS = 1013.2 and REL = 1013.2.

"Relative pressure" always refers to a pressure relative to the sea. Therefore, relative pressure (REL on the display), is displaying a sea level pressure.

<alert>
>//NOTE: The official term used in meteorology is pressure "reduced" to sea level however the word "reduced" actually refers to reduced elevation or reduced altitude to sea level. It is the height above sea level that is reduced - not the pressure.//
</alert>

Let go back to the 40 story apartment building scenario:

The weather station on the 40th floor is 400 feet above sea level (10 ft/floor) and is displaying an ABS of 998.7 hPa. You call your friend on the ground floor (at sea level) and ask him what his pressure is. He says it's 1013.2. You glance at your Ecowitt display console and the REL is also displaying 1013.2, the same as your friend's weather station on the ground floor at sea level! Fantastic! The algorithm is working perfectly!

How does my display console "know" what the sea level pressure is at sea level when I am not at sea level??

Actually, you've already told your barometer how high it was above sea level when you first set up your barometer. You had entered 400 feet as your altitude in the console, so the algorithm "knows" to add a 400 foot pressure adjustment (14.5 hPa) to whatever the current ABS pressure is showing on the display. The console's algorithm calculates and adds back an extra 14.5 hPa of pressure to the current ABS pressure to compensate for the thinner air 400 ft above sea level.

Basically, the REL value displayed on the 40th floor display console answers the question: What is the estimated pressure at sea level if I am located 400 feet above sea level?

SUMMARY

Sea level pressure can be measured if you are located at sea level or estimated/calculated if you are not.

As most of us live above sea level, the new Ecowitt SLP algorithm will calculate the sea level pressure based on your barometer's elevation (altitude) and station pressure, Additionally, it will also factor in current outside temperature and humidity to give us a truer representation of the atmosphere.

<alert>
If two or more weather stations are at different altitudes, you can "equalize" or "normalize" each of them to a common elevation/altitude (sea level) by adjusting their station pressures to an estimated sea level pressure.
</alert>

====Sea Level Pressure - visualizing a concept (Example #2)====
**updated 07 March 2026**

If you do a general search for a visual illustration of sea level pressure, you will undoubtedly find quite a few illustrations/drawings of mountains and weather stations at different elevations and calculated (but different) sea level pressures. 
For a new weather station owner, these illustrations may not be all that helpful.

To better illustrate the concept of //sea level pressure,//we will do something a bit different.

Rather than starting with station pressure and calculating sea level pressure, we will do the reverse. We will start with the sea - specifically the standard sea level pressure of 1013.25 hPa and work our way up into the atmosphere and calculate the appropriate temperature and station pressures at our weather stations using the standard model of the atmosphere. 

Let's say we have three Ecowitt weather stations on a small ocean island located at three different altitudes. One of the stations, STN "A" is located exactly at sea level - not far from the beach. The other two stations, STN "B" and STN "C" are located on two small mountains. All three stations are under the same, stable atmospheric pressure system. If we were to drive around this small island in a boat, we can confirm that the pressure at sea level all around the island is indeed, 1013.25.

The current temperature is 15C at sea level and we know that the atmosphere will get cooler as we go up higher into the mountains where STN "B" & STN "C" are located.

STN "A" is located at sea level. It is directly measuring sea level pressure of 1013.25 hPa at sea level. Because all pressures (measured or calculated) are the same at sea level, the REL value will be the same as the ABS value. Therefore, our Ecowitt console located at sea level near the beach (STN "A"), is showing ABS = 1013.25 and REL = 1013.25.

Our second Ecowitt console is STN "B" located at 200 meters above sea level. The station pressure (ABS) @ 200 m is now lower by about 23.75 hPa from what it was at sea level because we are higher into the atmosphere and the air is thinner and cooler. Our Ecowitt console is now displaying ABS = 989.45 and REL = 1013.25. The outside temperature at STN "B" has dropped from 15C to 13.7C.

STN "C" is the highest station located at 500 meters above sea level and because we are much higher, the station pressure has dropped considerably and is now 58.64 hPa less than the pressure at sea level. Therefore, our station pressure @ 500 meters is 954.61. The console display indicates ABS = 954.61 and REL = 1013.25. The outside temperature has dropped a bit more and is down to 11.75C.

As we know, both pressure and temperature drop the higher we go up in the atmosphere and we can see that the ABS values and outside temperatures are dropping as we move farther up into the mountains.

We have three weather stations located at three different elevations. All three have different station pressures and different temperatures: The Ecowitt algorithm is calculating sea level pressure - displayed on our consoles as REL (relative pressure):

1. STN "A"  ABS =1013.25, REL = 1013.25 @ 15C @ 0 m (sea level)

2. STN "B"  ABS = 989.45, REL = 1013.25 @ 13.7C @ 200 m

3. STN "C"  ABS = 954.61, REL = 1013.25 @ 11.75C @ 500 m  

ABS = measurement of absolute pressure/ station pressure
REL = algorithmic calculation of relative pressure/sea level pressure

We can observe the following:

The REL of 1013.25 (sea level pressure) is the same at all three stations even though all three stations are at different elevations, station pressures and temperatures. 

Since the sea level pressures are the same we could plot all three stations on a weather map and draw an isobar (lines of equal pressure) just like the meteorologists do. 

**SUMMARY**

We can now see what they mean by converting (normalizing) a station pressure to an equivalent sea level pressure. All three stations (A, B & C) have different station pressures, different altitudes and have different temperatures. Yet all of them yield the same sea level pressure of 1013.25. Because we have normalized or standardized all station pressures to a common altitude (sea level), we can now make valid comparisons between all three stations.

**What is the point of calculating sea level pressure if your weather station is not at sea level?**

Meteorologists use barometer readings from thousands of weather stations in order to draw isobars. Isobars are the curving lines of equal mean sea level pressure. These lines (isobars) represent the familiar HIGH and LOW pressure systems we see every day on weather maps and forecasts. Drawing isobars is just a matter of connect-the-dots. See link below.

Resource: https://www.noaa.gov/jetstream/analyze-appair-pressure-map


==== Barometers - why do you need to calibrate them? ====
**updated 07 Jan 2026**

Digital barometers are unique sensors. They are micro-mechanical sensors that can detect extremely tiny changes in atmospheric pressure. When pressure changes, it causes something mechanical to flex (like a diaphragm or membrane) or bend ever so slightly. This movement is converted into an electrical voltage, which gives us a pressure reading.

Most first-time owners of personal weather stations assume that all the weather sensors are calibrated and ready-to-go out-of-the-box. Many weather station manufacturers also assume that their suppliers - the sensor manufacturers, have already calibrated the sensors in their production facilities. And that is true - but there is more to the story.

For example, a barometric sensor might have a specification of +/- 5.0 mb with a short-term drift "spec" of say, 2.0 mb (first 12 months). You should also see a long-term drift "spec" say it was 1.0 mb, which tells us that the sensor will continue to drift up to 1.0 mb every year after.

We should always check for asterisks in the sensor's datasheet footnotes. Sure enough, there's an asterisk that says "pre-solder".

Pre-solder? The guaranteed specs from the sensor manufacturer look pretty good on paper until you solder them to the weather station printed circuit board, The specs can change by "x" amount just by the very act of soldering the chip to the board. This type of drift is known as "solder drift" or "board mount drift" Plus, in addition to the initial solder drift error, the sensor's accuracy is expected to continue to drift by "y" amount in the first 12 months (a high wear-in period) and continue to drift by "z" amount every year thereafter.

Some higher-end weather station manufacturers might optionally, certify the accuracy of their sensors and present a new owner with say, a NIST or equivalent certification. The certification may come as a costly option. These certifications are time-limited, meaning that if you have a sensor certified to be accurate today does not mean that it stays that way forever. It would have to re-certified in an accredited calibration lab on a regular basis - usually every year.

Ecowitt gear has no such certification process, therefore they have a barometer calibration requirement in the manual.

What the manufacturer is telling us is that we have to calibrate our own barometers. However, with careful set-up and calibration you can greatly increase the accuracy of the barometer.

Since most electronic barometers drift continuously, they require occasional re-adjustment to restore their full accuracy. Therefore, calibration or checking for accuracy at least once or twice a year is highly recommended. 
==== Reduction of Atmospheric Pressure ====
**//updated 30 Jan 2026//**

>**"In order to make valid comparisons between two (or more) different weather stations that are at different elevations, all pressures must be reduced to sea level."**

You might see various versions of the above statement but the term, "pressure reduction" requires further discussion. Although the World Meteorological Organization (WMO) uses the term "pressure reduction" in its equations to calculate MSLP (mean sea level pressure) values, it is not a good description for what it really is.

Pressure is not reduced at sea level - pressure actually increases as we dive down through the atmosphere and approach sea level. The "reduction", refers to the theoretical reduction in elevation from your location to sea level.  The station pressure (ABS) at your weather station is mathematically adjusted (corrected or reduced) to sea level as if your barometer was located down there.

Alternatively, you could take an actual sea level pressure reading at sea level. Picture lowering your barometer down the side of a cliff with a rope down to sea level and reading the actual pressure at sea level. Or you can travel to the ocean and take a pressure reading there.

And yes, you can do that every time you want to take a sea level pressure reading, but that would be inconvenient. Instead of measuring sea level pressure at sea level with a rope or travelling to the ocean’s edge, you can calculate the pressure at sea level directly from the comfort of your own home even though your weather station may be 1000 meters above sea level.

When many weather stations (even though they might be all at different elevations) convert their pressure readings to the same elevation (sea level) using temperature, humidity and other factors, we can start to draw lines of equal mean sea level pressure (MSLP) called isobars.

Canada’s meteorological service describes isobars as “curving lines joining points of equal mean sea level (MSL) pressure.” On a weather forecast, these are the familiar lines of high and low pressure systems on a weather chart.

SUMMARY

Our REL value (sea level pressure) is derived/calculated from a measured ABS value. In order to calculate sea level pressure, we need to calculate a pressure "correction" or adjustment which is added to our ABS (absolute pressure value). Adding this "correction" to our ABS value will give us the REL (sea level pressure) value. In 2025, Ecowitt updated the barometer algorithm. Mean sea level pressure(SLP) is now calculated automatically.

====Calibration — the best way!====
//**updated 30 Jan 2026**//

Although using a local airport's METAR report comes in handy as a free barometer calibration tool — it is not the best way to calibrate your weather station barometer(s).

Just about every weather station manual or book suggests using a nearby airport as a calibration reference in order to set up your barometer.

Even if you live at the airport, comparing your barometer with the airport's barometer is not so easy. The airport uses different air pressure calculations and algorithms than our equipment. Station pressure (QFE) is a calculated value because airports use runway elevation as the reference point for station elevation. Our personal weather stations uses an actual measured station pressure rather than a calculated value.

Experts tell us that calibrating using one airport alone may not be sufficient, and that a minimum of four or five airports should be used. Ideally, your weather station should be somewhere in the centre of multiple airports. If you live quite close to an airport no farther than 10–15 miles away (max 25 km), you can use it as your sole reference.

If you live farther away, you have to be absolutely sure that your barometer and the airport's barometer are in the same pressure system otherwise comparing readings will be of no value.

All of these requirements make it difficult to set up your barometer properly by comparing readings with an airport.

What is the best way to set up and calibrate a weather station barometer?

The best way is to make sure your ABS value (absolute pressure) is accurate. All the other calculated pressure values depend on having an accurate ABS value. Other than sending your pressure sensor to an expensive calibration lab, the best way to calibrate is to directly compare your barometer's uncalibrated pressure value with a calibrated reference barometer.

Since most of us don't have pressure chambers or expensive calibration lab equipment, our calibrated reference should be another barometer that has higher specifications than yours. Ideally, the reference barometer should have some evidence of an accuracy certification, i.e. NIST traceable or equivalent.

The calibration process is dead simple. All you have to do is place the reference barometer side-by-side at the exact same elevation as your barometric sensor. The reference barometer should be displaying absolute pressure (station pressure) or QFE. Just adjust your barometer's absolute value (ABS) to be the same as the reference barometer's value, and you are done!

No need to use an airport to calibrate (or try to calibrate) your barometer!
==== Accuracy vs precision ====
**//updated 16 February 2024//**

In any discussion regarding weather sensors, the subject of accuracy comes up. Sensors have specifications (specs). Barometric sensors are no exception. Let's take a look at a barometer accuracy “spec” of +/- 0.7 hPa.

To make things more interesting, we shall use an analogue (it has a dial) aneroid barometer as an example:

Let's assume that someone made their first weather related purchase.

You found a remarkable eBay find. Somebody was selling a used aneroid barometer – a Fischer precision model 103 at a great price. When you received it, it was in mint condition and, clearly, it was still working.  You have no idea if it was calibrated or calibrated badly by a previous owner.

//Note: Whether this barometer is calibrated or not (new or used), is not relevant. You will still need to check, verify and calibrate as necessary.//

Like most analogue dial barometers, in order to calibrate, you have to move the needle by turning a set screw on the back. You might have to move the set screw a little or a lot, depending on how much it is “out”. But how do you know if your barometer is accurate or not?

Although the Fischer precision model 103 aneroid barometer is reported to be a very //precise// instrument, it still could be inaccurate if it was set improperly by the former owner.

Accuracy is not the same as precision. The owner knew that the accuracy as specified by the manufacturer is +/- 0.7 hPa. Not only that, the accuracy was also specified as full scale, meaning that any measurement through the whole range of readings on the dial would be within +/- 0.7 hPa.

To calibrate a barometer, you need a reference calibrated barometer to compare readings. As it turns out, a friend of a friend had a very expensive digital portable barometer that had just been re-calibrated at a certified lab. He was willing to come over and lend a hand to calibrate the Fischer barometer.

To the owner's considerable horror, his Fischer barometer was 3 hPa “out” compared to the reference barometer. He was prepared to try to return his purchase back to the eBay seller because he was not getting his promised 0.7 hPa accuracy. His barometer was off by 3 hPa! Obviously, it was broken!

The guy with the reference digital barometer interceded. He said; “Hang on for a second, don't get too excited. All we have to do is match your barometer's reading with mine. That's why they have an adjustment screw in the back.” And that's what he did. He turned the set screw until the two barometers had the same reading, and carefully explained that all he did was set the barometer to the correct pressure reading for his altitude. He assured the new owner that from now on, all your readings will be accurate to +/- 0.7 hPa.

The new owner looked a bit happier. “You mean that was all we had to do?” “We're changing the starting point?”

The friend of the friend replied. “Well, I would have used the term true value, but yeah, we re-calibrated your barometer to the correct starting point. You can't adjust the precision of the instrument, but you can adjust it (as close as possible) to true value.”

With our Ecowitt digital barometers, we are doing the same thing. Instead of turning a set screw, we adjust accuracy using a ABS offset in order to move/adjust our ABS readings to the reference “true value”. True value is established by a reference calibrated barometer.

Let’s get a bit statistical for a moment.

{{ :accuracy_and_precision_.svg |}}
Illustration By SV1XV - Own work, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=25587770

Accuracy is how close the set of sensor measurements are to a “true” value. Precision is about consistency and repeatability. Think bell curves. You are basically comparing the true value to the mean of a set of measurements.

Analyzing the set of measurements tells us how precise the sensor is. It tells us nothing about how accurate the sensor is. For that, we need a reference. We will call this reference the “true” value. True value can be established by another sensor that has already been calibrated to a high standard. The higher the reference standard – the better. Once we have established a true value, we can make valid comparisons and calibrate our barometer.

//Accuracy is the distance from true value to the mean of the distribution curve of the measurements. The distance of the data points from the mean of the distribution curve establishes the level of precision of the sensor,// i.e. low precision vs high precision. In other words, you want a narrow distribution curve with steep slopes versus a flattened, wide one. The process of calibration is to shift the distribution curve and centre it on the reference true value (zeroing).

**EXAMPLE:**

In the illustration above, you will notice that the distribution curve is shifted to the right of the vertical reference line (true value established by a calibrated reference barometer). Suppose the true reference value is 1000 hPa and the average/mean of the "bell" curve is 1003 hPa.

For the example of a dial type barometer above, to calibrate you would just turn the set screw on the barometer to move the needle to 1000 hPa. That's it - calibration is done!

Let's use another example of a digital barometric sensor. If we are using Ecowitt equipment, our display console or gateway would be showing an ABS value (absolute pressure) of 1003 hPa. We need to get this down to 1000 hPa.

In order to calibrate, we have to find a way to shift/move the bell curve to the left (see illustration/graph above, so that it centered on the true value of 1000 hPa. To do this adjustment (calibration), is simply done by entering an ABS offset of -3 hPa. This simple adjustment subtracts 3 hPa from whatever the current ABS value is reading.

You are now calibrated.

==== Altimetry, METAR and Q-codes ====
**//updated 01 Feb 2026//**


The study of meteorology overlaps with aviation. For obvious reasons, weather is vitally important for all aircraft. The science of measuring altitude is called //Altimetry//. The altitude of an aircraft can be measured by a number of different technologies, but most small aircraft rely on the use of a calibrated [[https://en.wikipedia.org/wiki/Barometer#Aneroid_barometers|aneroid barometer]] (pressure altimeter) to measure altitude using atmospheric pressure or differences in pressure. The aviation view of atmospheric pressure, and particularly the study of air density, will give additional insights into how atmospheric pressure is measured with a weather station and how temperature affects pressure.

For weather station owners, we should become familiar with the meteorological pressure definitions of station pressure, Altimeter and SLP. For Ecowitt personal weather stations, ABS = station pressure and REL = SLP (sea level pressure).

Aviators use slightly different nomenclature:

  * QFE = Station pressure
  * QNH = Altimeter(setting)
  * QFF = SLP (Sea Level Pressure)

Other than the larger international US airports, the majority of METARs (meteorological aerodrome reports) located in the U.S. often don’t publish SLP values at all, They usually report only Altimeter. In Canada, METARs always publish both Altimeter and SLP on every report. In Europe, you would undoubtedly see only QNH on a METAR report.

Here are a few examples of METAR reports from the U.S., Canada and Europe. I have highlighted the pressure values in bold. Note the use of “Q -codes” in Europe.

//Note: customary units for Altimeter(setting) is in English/Imperial units. For example, a METAR report would indicate 30.02 inHg (inches of mercury) as **A3002.** Customary units for SLP are in metric, usually in millibars (mb). In a METAR report, 1013.2 mb would be shortened to **SLP132**. The decimals are dropped for both Altimeter and SLP and the preceding 10 for SLP is dropped as well. In Europe and other countries, a reading of 1017 mb would be reported as **Q1017**. In a METAR report using a “Q-code”, the pressure reported is in whole integer units; Q1016, Q1017, Q1018 etc. There are no decimals.//

Here are some actual METAR reports and their reporting of pressure (in **bold**):

Anchorage International Airport, Anchorage, Alaska, US:
PANC 081017Z 16005KT 10SM -RA SCT020 BKN028 OVC060 07/06 **A2905** RMK AO2 P0001 T00670061

Vancouver International Airport, Canada:
CYVR 081000Z 09007KT 15SM FEW080 FEW220 13/11 **A2986** RMK AC1CI1 AC TR CI TR **SLP113**

Heathrow Airport, England:
EGLL 080950Z AUTO 29003KT 220V340 9999 NCD 18/13 **Q1025** NOSIG

//Note: You may notice that station pressure is absent from METAR reports. METAR does not publish station pressure values. To obtain all three pressures; station pressure, altimeter and SLP, you will have to use your national weather service or in some cases you might be able to obtain all pressure values directly from the data stream from a AWOS or ASOS station.//

Here is a excerpt from the data output of a Canadian AWOS station (XML format):

  - //<element name="stn_pres" uom="hPa" value="1001.0">//
  - //<element name="mslp" uom="hPa" value="1025.0">//
  - //<element name="altmetr_setng” uom="inHg” value="30.23”>//

Note that the station pressure that is absent in METAR reports is present in the above data stream from the AWOS. Also note that Canada prefers the use of hPa for station pressure and SLP (mslp) over mb (millibars). Altimeter(setting) is the exception - it is always in inHg (inches of mercury)

This is where meteorology and Altimetry diverges a bit. Pilots, naturally, need to know how high they are flying, so barometers (altimeters) are very important. If you are studying to become a pilot, your flight instructor will certainly cover the subject of indicated altitude versus true altitude. The instructor might say; “Your altimeter is lying to you!” and go over the saying; “High to low — watch out below!”.

The instructor is really referring to the change in air density if you are flying from a high pressure system into a low pressure system or flying into much colder air. Either way, the indicated altitude as shown on the cockpit altimeter could indicate that you are flying higher than you really are. And that is not a good thing if visibility is poor, and you can't eyeball the ground below. Pilots can compensate for these “errors” by applying manual corrections to their altimeters.

Ok. By now, you must be wondering how all of this aviation stuff is of any use to land-based weather stations. After all, we don't fly our barometers!

Actually, this discussion is very relevant to us amateur weather observers, especially for pressure corrections due to temperature changes.

If your data source (METAR, mesonet,AWOS/ASOS data feed) displays both Altimeter and SLP info, you might notice what appears to be odd behaviours in comparing Altimeter with SLP pressure values. At times, Altimeter (setting) can be higher than SLP, yet at other times lower. Occasionally, we might see that both values are the same. The magnitude of these “spreads” may widen or narrow with the seasons. In a warm/hot summer, the spread will narrow and in a cold winter environment, the spread greatly widens.

**The “Q” rules:**

For elevations/altitude above MSL (Mean Sea Level):

  * If the outside temp < ISA temperature then QNH < QFF
  * If the outside temp = ISA temperature, then QNH = QFF
  * If the outside temp > ISA temperature then QNH > QFF

What is the ISA temperature? The ISA (International Standard Atmosphere) temperature for your location depends on your elevation. According to the ISA model of the atmosphere, every elevation (or altitude), has a temperature assigned to it. For example, the ISA temp at sea level is 15 °C, the ISA temp for a 300m elevation is 13.1 °C, the ISA temp for 500m = 11.75 °C, etc.

QFF changes values depending on outside temperatures that are above or below the ISA temp. If the temperature of the outside air is greater than the ISA temp (warm air is less dense than cold air), then QFF pressure must be corrected downwards. Conversely, when the temps fall below the ISA temp (for your specific elevation) QFF must be adjusted upwards (cold air is denser).

Basically, the “Q” rules tells us that there is a relationship between QNH (Altimeter) and QFF (SLP). The spread between QNH and QFF changes dynamically depending on temperature.

Where are we going with this and why are aviation Q-rules relevant for our terrestrial weather stations?

The main takeaway is that the relationship between QFF/SLP and QNH/Altimeter values constantly change due to changes in temperature. Temperature changes air density. Density changes pressure. If you were to graph SLP vs Altimeter, you might see the lines cross and reverse position as temperatures exceed the ISA temperature during the day. The next time you look at a METAR, take a closer look at the “spread” between Altimeter (setting) and SLP.
==== Sea Level Pressure - which one? ====
**//updated 25 Feb 2025//**

In this discussion, we will talk about the REL (relative pressure) value in our Ecowitt consoles. It is important because the REL that you see on your display console or on your app or web browser is an approximation of sea level pressure calculated from your ABS (absolute pressure).

The REL value is a sea level pressure, but it is not the only sea level pressure.

Of all the various pressure terms, sea level pressure is perhaps the most confusing because there is more than one sea level pressure. It is best to think of sea level pressure as a generic term referring to either Altimeter (setting) or SLP. Sea level pressure is a critically important element for weather forecasting purposes. Sea level pressures are indicated by isobars - the curving lines of equal sea level pressure that we see as the familiar high and low pressure systems on our daily weather maps or surface analysis maps.

Sea level pressure is a weather station's pressure that has been reduced/corrected/normalized to a theoretical pressure at sea level elevation. The chief differences between the different types of sea level pressure are the equations and parameters used to calculate them.

One sea level pressure (Altimeter setting) uses only the elevation pressure difference between your location and sea level to convert your station's pressure to sea level pressure and the other sea level pressure (SLP ) uses elevation plus local temperatures (current or average), humidity and other factors to calculate sea level pressure. In 2025 Ecowitt radically changed the way it calculated sea level pressure. Ecowitts new algorithm accurately calculates SLP (mean sea level pressure).

<alert>There are two types of calculated sea level pressures - SLP and Altimeter (setting). Ecowitt's new algorithm now calculates SLP. However some of Ecowitt's models cannot be upgraded (due to hardware limitations), and will have to use the old fixed offset system. The fixed offset system still has the capability of approximating QNH or Altimeter setting. In Europe, for instance, QNH would be the only sea level pressure reported on an aerodrome meteorological report (METAR). MSLP/SLP values would be absent from European METAR reports and would have to be obtained from other sources.</alert>

==== Review: Starpath USB baro (barometer) ====
**updated 08 March 2025**

David Burch is the director of Starpath School of Navigation in Seattle, WA, This is an interesting company that focuses on marine navigation courses, navigational instruments and is the U.S distributor of the highly regarded Fischer line of weather instruments including their notable precision barometers. Starpath also has a calibration lab and provides calibration services for barometers.

David Burch is a prolific writer and blogger. David wrote; //“The Barometer Handbook”// which is a required read for anyone who has an interest in atmospheric pressure and barometers.

I was on the Starpath website recently and discovered that Starpath was offering a brand new digital barometer aptly named **the USB baro** whose name describes what it is. It looks identical to a USB flash drive.

{{:starpath_usb_baro.png?200|}}

What caught my eye immediately was the stated accuracy of the barometer of +/- 1.5 mb or __better__ and the price of only $49 USD,

I decided to buy one. Mr. Burch saw my original query, so I had the opportunity to ask him some technical questions about his new barometer. I asked if these new barometers were checked by Starpath.

I was very pleased to find out that every barometer leaving the door at Starpath has been read and set to the correct pressure. Samples of ten units are taken from each shipment from the factory and thoroughly checked. Also pleased to hear that accuracy could be as good as +/- 0.3 hPa; far better than the stated manufacturer absolute accuracy of 1.5 hPa.

**//Features://**

  * USB barometer: just plug it in to a USB port on a Windows or Mac computer and run the software.
  * live graphing (barograph) with trend indicator and data export capabilities.
  * NMEA compliant - for marine use, the barometer output can be sent to navigation software.
  * barometer outputs QFE pressure (station pressure) or QNH (user inputs their elevation).
  * one or two point calibration option (user can re-calibrate the USB baro if necessary).
  * pressure is set by Starpath in reference to two on-site NIST traceable sources.

As Mr. Burch runs a calibration lab, he pointed out that at this modest price point, setting and checking is, of course, not the same as generating a certified calibration curve for a barometer. He said that a full calibration involves checking the actual pressure over the full range of the sensor at multiple pressure steps. Full calibration of any barometer can be done at Starpath at an additional cost.

**What’s in the box?**

Interestingly, the USB Baro comes packaged in a sturdy metal tin nestled in high density foam. A shipper would have to make a concerted effort to damage it. Although maybe overkill, Starpath also thoughtfully sandwiched the tin with two layers of cardboard for extra protection. As a thank you, Starpath also enclosed a laminated “Polaris” star chart for navigation purposes.

Inside the tin box you will find the USB baro device and a card containing a QR code and a website to obtain further help, instructions, video tutorials and download links for the software.

Note: Do not confuse the Windows/Mac USB baro software with the other Starpath barometer mobile apps. These mobile apps are to be only used with a smartphone's built-in barometer.

**Setting up the USB baro**

  * Install the software (PC or Mac) and plug the USB Baro into a USB port.
  * On the settings page (gear icon) set your preferred units, i.e. time zone, date format, etc. Next, scroll down to the bottom of the page and click on CONNECTION SETTINGS.
  * Turn on the Serial NMEA input.

For Mac computers:

  * Choose the correct COMM port. In the dropdown menu, choose tty.usbmodem_1 Starpath USB.
  * Other settings can be left as default values: 9600 / no parity, 8 bits / 1 bit, No Control.

For PC computers:

  * Choose the correct COM port. You will need to find out the COM port the USB device is on. In the dropdown menu, choose the COM port associated with the USB port you used.
  * Each USB port on the computer has a different com port number. If there are more than one com ports showing in the dropdown, note which ones are there, then remove the USB Baro, restart the app, and look again. The missing com number is the one you want. Plug the USB Baro back into the same port, and the right one will appear again, and choose that one.
  * Note: Other settings can be left as default values: 9600 / no parity, 8 bits / 1 bit, No Control.

In my case, I didn’t have to remove and reinsert the USB device to find the correct COM port. On my computer, there were only two com ports available to choose from. When the first port in the dropdown didn’t work (no pressure reading), I just chose the second COM port, and it started working immediately. If you have connection issues, make sure you try all COMM ports and if necessary, reinsert the USB baro into another USB port and try again. 

Click the graph icon bottom of the page, to display the current pressure and graph. Full installation and usage information is available within the app and is also available on the starpath website. For detailed instructions on the USB Baro, click on the “?” icon on the bottom of the page.

The instructions were well written - clear and concise and in plain language.

**First use**

As mentioned above, there are some initial settings to enter. Most are self-explanatory. Of special interest are the settings for //Precision display, Position format, Pressure Display Level and Elevation//.

//Precision display// settings indicate the “extra precision” (digits) you want to display. For hPa (my preferred units setting), you can choose to see your hPa in one or two decimals, i.e 1013.2 or 1013.25. Pick the precision setting that you prefer.

//Position format// is used in conjunction with navigation software. The USB baro can be tied in with GPS and plot pressure along your course. If you don’t use navigation software, you can leave this setting as is.

//Pressure Display Level// You can choose the type of pressure you want displayed. You can choose QFE or QNH. In weather station terminology, this would be equivalent to Absolute pressure/station pressure or Altimeter (setting) respectively.

Note: Elevation is required in order for the program to calculate QNH (Altimeter). Enter the elevation in feet or meters above sea level.

As a first test, I set the Starpath USB baro to display QFE (station pressure) so I could compare my Ecowitt WS3900 ABS reading with the reference Starpath barometer. I am happy to report that the two barometers had virtually the same reading. It was clear that the lower cost Ecowitt barometer had much more noise than the Starpath so you have to accomodate the zigs and zags of the Ecowitt sensor in order to compare with the Starpath. The Starpath USB baro did not display this behavior.

Note: I have since adjusted the barometer of my Ecowitt WS3900 console by – 0.1 hPa to help “centre” the noise level around the reference Starpath barometer. Over the years, I've noticed that there is always some random jumps present in all the Ambient and Ecowitt barometric sensors that I've tried – usually +/- 0.2 with the odd outlier of 0.3 hPa or more.

After testing the WS3900 barometer, it was dead simple to re-calibrate a Ecowitt GW1000, GW1100 and a Ambient WS-2000 console to adjust and match their ABS readings to the Starpath reference barometer's QFE reading.

<alert>The Starpath USB baro barometer is a high accuracy, low cost instrument that represents outstanding value. Highly recommended. Cost: $49 USD. See https://starpath.com/usbbaro for more information.
</alert>
===== Essential Terms & Definitions =====
**//updated 08 March 2025//**

**Absolute pressure (ABS)** is the pressure reading from your barometric sensor. When calibrated, it is also known as station pressure.

**Altimeter setting** is station pressure that has been reduced to sea level based on elevation.

**Altitude** is the height of an object compared to sea level. Technical definitions aside, for meteorological purposes, either "altitude" or "elevation" may be used.

**Altitude for REL** is where you enter your barometer's altitude above sea level. This entry is found in the Calibration screen of the Ecowitt firmware.

**ABS offset** is used to adjust the absolute pressure reading (ABS) up or down for calibration purposes.

**Barometric pressure** has various meanings. NOAA/NWS defines it as a pressure reported by a barometer, or it could be atmospheric pressure. However, the World Meteorological Organization has no definition for “barometric pressure”. Some weather station manufacturers refer to sea level pressure as “barometric pressure.” Because of multiple and sometimes contradictory definitions, In this wiki, I will define barometric pressure as a pressure from a barometric sensor.

**Plateau effect** Any station at 305 meters (or above) are considered to be plateau stations and requires an additional pressure correction called a “plateau correction”.
The “plateau correction” reduces SLP when the current temperature of the station is greater than the annual mean temperature. Similarly, the “plateau correction” increases SLP when the current temperature is less than the annual mean temperature.

**Relative pressure (REL)** represents what the pressure would be at sea level elevation if our weather station’s barometer was located down there. REL can refer to Altimeter or SLP.

**REL offset** is the fixed difference in pressure between your location's pressure and the pressure at sea level using standard atmosphere conditions. REL offset sets the elevation of the weather station. If you are located anywhere above sea level it is always a positive number. Note. With the new barometer firmware update released in 2025, calculating the REL offset is no longer required and has been replaced by "Altitude for REL".

**Sea level pressure** in a generic sense, can refer to METAR SLP or Altimeter (setting)/QNH. Both are sea level pressures because both are station pressures that have been reduced (corrected) or "normalized" to sea level. Each is calculated differently.

**SLP** is station pressure that has been reduced to sea level based on elevation, temperature, humidity (optionally) and other factors (plateau effect and other empirical adjustments). In aviation terms, SLP is called QFF.

**Station elevation** For personal weather stations, station elevation refers to the elevation of the barometric sensor. In aviation, station elevation is usually the runway elevation.In some weather softwares, the term "station altitude" can also be used.

**Station pressure** For personal weather stations, it is the sensor pressure at station elevation. In aviation, station pressure is called QFE, which is a calculated amount. To calculate QFE, the barometric sensor height above runway elevation is reduced to the reference point(runway) using a special “removal” correction.

===== About =====

The barometer-wiki is written by "gszlag" who developed an interest in barometers at an early age (8 years old) when he decided to recalibrate the local airport's mercury barometer during a [shortened] school field trip. The airport meteorologist was not particularly happy with my recalibration efforts.

Why devote an entire wiki to just one weather sensor?

Barometers, can be one of the most difficult and frustrating weather sensors to set up properly. My informal annual survey of surrounding weather stations reveals that only a few percent of weather station owners have managed to successfully set up and calibrate their barometers. 

Hopefully, these guides, tutorials and how-to's will shed some light on the subject of barometers - what they are measuring, how to set them up and how to keep them calibrated and accurate,

Along with the technical aspects of setting up your barometer and maintaining them, we will also take a look at the science of the atmosphere and help explain and simplify some of these concepts.

===== Barometer troubleshooting =====
updated 14 May 2026

Ecowitt rolled out a new firmware update in 2025 that dynamically calculates SLP (sea level pressure) factoring in elevation, station pressure,temperature and humidity. This new update is a radical departure from the previous algorithm, where sea level pressures were somewhat crudely approximated using a simple fixed offset.

Most barometer issues are usually caused by barometer calibration problems. All barometers drift over time which means they require regular recalibration to restore their full accuracy. New barometers can drift the most during their initial 12 month wear-in period. Therefore, it is important to check and verify barometer accuracy at least once or twice a year.

If you are having difficulties with the setup of your barometer or your barometer readings just don't seem to be quite right, there are some things to check or double-check:

Give a brief description of your barometer issue. i.e, **//"My relative pressure never matches a close-by airport."//**

**Troubleshooting checklist:**

Provide the following information:

  * station altitude = barometer height above mean sea level.
  * ABS = absolute pressure (station pressure)
  * REL = relative pressure (sea level pressure)
  * outside temperature when the pressure readings were taken.
  * name of closest airport - full name or ICAO code.
  * distance from airport.

Procedure:

1. **Verify that your station altitude is correct.** Station altitude is the total height of the barometer above mean sea level. For Ecowitt equipment, the barometer (its sensor) can be located inside the display console, Wi-Fi gateway or in a separate 3-in-1 device that has the inside temperature, humidity, and pressure sensors, e.g. WH32B. Other models or brands may have the barometer on the outside array.

2. Starpath.com (barometer calibration firm) recommends Google Earth in order to **determine your ground elevation above mean sea level.** Your barometer will likely be located inside a house or apartment so you will have to manually **measure and add the additional height above ground level** in order to obtain the total height of your barometer.

3. Many areas of the world are being re-surveyed using the latest digital elevation models so there may be an opportunity to get more accurate elevation measurements. Although GPS technology is improving, avoid using your phone, hiking GPS units, smartwatch, tablets or other consumer devices containing GPS sensors as the results may not be accurate or reliable.

4. **Verify that your current ABS (absolute pressure) values are accurate.** The only truly accurate way to calibrate your barometer is to compare readings with another reference barometer that has been calibrated and is providing true reference values. I can recommend the Starpath USB baro which can be used as a reference calibration tool.

5. Although the results will be less exact, you can alternatively, use a close-by airport's barometer readings as a reference. We will **calculate our station pressure using the airport's altimeter setting or QNH.**

The airport calculates their Altimeter (setting) or QNH from station pressure. Basically,we are reversing the calculation and calculating station pressure using the airport's QNH/altimeter (setting). Given the airport's sea level pressure and our barometer's altitude, we can easily calculate our station pressure. Use the calculator in the Resource section below.

6. Final step. Once we have calculated our station pressure from the on-line calculator, we can compare this value to the current ABS (absolute pressure) on your display, app or software. For Ecowitt weather stations you can **enter an ABS offset  (positive or negative amount) in order to match your weather station's ABS with the calculated QFE (station pressure).**

**Q & A:**

**How do you get a sea level pressure reading if you are not at sea level?**

If you and your close-by airport are in the same pressure system, say it was a high or low pressure system, then both you and the airport have to have the same sea level pressure providing that the stations are close enough to one another.

If you drilled an imaginary well at both stations right down to sea level and measured the air pressure at the bottom of both wells, the air pressures will be the same at both stations. That's why we will be "borrowing" the airport's known sea level pressure and use it as our own sea level pressure. Keep in mind that sea level pressure is an estimate or approximation that is derived from your station pressure (absolute pressure).

**What is a "close-by airport"?**

There is no hard fast definition or rule, but one weather scholar describes "close by" as less than 25 km or so.

**Why choose Altimeter setting or QNH instead of SLP to calculate our station pressure?**

We will choose to use altimeter setting values or QNH instead of using SLP because altimeter setting and QNH ignores local temperatures and humidity making the math far simpler and easier.

Is the above procedure better than using a calibrated reference barometer?. No, it is not. It is a compromise. Consider it as an initial setting.

**SUMMARY:**

In the absence of a calibration lab or a calibrated reference barometer providing true pressure values, the above checklist and procedure can be used to calculate your absolute value/station pressure based on the airport's altimeter value or QNH. Any required adjustments to station pressure can be done by entering an absolute pressure (ABS) offset.

**Resources:** 

QFE calculator: This calculator uses the term QFE instead of station pressure. QFE is the aviation equivalent to station pressure. Click on the QFE button to start and enter in your elevation and sea level pressure/QNH.

See:https://www.sensorsone.com/elevation-station-qfe-sea-level-qnh-pressure-calculator 

Note: For North America and jurisdictions that use altimeter setting instead of QNH, put the altimeter setting value of your airport into the QNH input box of the the calculator.
   
--- 

<alert>
//IMPORTANT: Anytime you are calibrating pressure values using an airport as a reference, __you must be certain__ that both you and the airport are in the same pressure system. You can confirm this by checking METAR reports from close-by airport(s). You can use windy.com to get airport METAR info and check near term isobar forecasts to help confirm you are in the same pressure system. Keep in mind that windy.com only shows pressure __forecasts__ from several atmospheric models. These isobars are not actual pressure values - they are forecasts. Do not enter forecasted pressures in your consoles. Use them as a guide only!//
</alert>


===== FAQ - Frequently Asked Questions ======

**Question**: //I just bought an Ecowitt weather station. How do I set up the barometer?//

**Answer:** If you have acquired an Ecowitt or re-branded clone weather station — new or used, you will have to set up your barometer before first use. All new Ecowitt weather stations are factory set so that ABS = REL. Therefore, the barometer thinks it is located at sea level. If you are not located at sea level, you will have to enter your actual altitude.

Your barometric sensor requires these steps to set up before first use:

  - Set the altitude of your barometer.
  - Check/verify barometer accuracy. 

//IMPORTANT: Do not assume that your barometer is accurately calibrated at the factory. It might be or it might not be. Ecowitt states: *“This kind of correction [ABS correction] is entirely normal as during manufacturing small shifts in the pressure sensor readings can be introduced.”//

**Terms & Definitions:**

**Absolute pressure (ABS)** is the current live pressure reading from your barometric sensor. 

**Relative pressure (REL)** is a calculation derived from an algorithm. It is an estimate or approximation of atmospheric pressure at mean sea level. It represents what the pressure would be at sea level if our weather station’s barometer was located down there. REL is the calculation of mean sea level pressure (abbreviated as SLP or MSLP).The use of the term “relative pressure” means a pressure relative to sea level.

**ABS offset** is used to adjust the ABS (absolute pressure) reading (up or down) for calibration purposes.

**Altitude for REL** is the entry in your Calibration settings screen in the display console, app or web browser user interface where you enter your barometer's total height above sea level. To do this, you will have to look up your ground elevation above sea level and then add your barometer's height above ground level to obtain the total height above mean sea level.

Note: You can use Google Earth or similar tool to look up your ground elevation above sea level.

**Mean Sea Level** is by convention, used as the common reference or datum to compare pressures from different weather stations that are located at different elevations.  

**Question**: //What is the SLP algorithm actually calculating?//

**Answer:** The new algorithm is constantly adding or subtracting pressure in response to cooler or warmer air at your location. Cold air is heavier (more pressure) so the algorithm is compensating by adding pressure. In summer under warm/hot conditions, the algorithm will actually reduce your REL pressure because warm/hot air is lighter (less pressure).

**Question:** //My REL readings don't seem to exactly match the local airport's SLP reading. Why not?//

**Answer:** Even if your barometer is perfectly calibrated, your console REL (relative barometer) display may not exactly match the airport SLP reading in the METAR report. Your console's SLP calculation algorithm differs from your national weather service empirical method (tables) to calculate SLP, so we can't expect that your console or gateway will always match the airport's SLP exactly.

**Question:** //I used to set my barometer to the Altimeter (setting) from a close-by airport but with Ecowitt's new firmware update, I understand that this update can only calculate SLP. What about my old Altimeter (setting)?//

**Answer**: In the old firmware (fixed offset system) you could set your REL to the airport's Altimeter (setting) or, possibly, SLP (for very low elevations). However, there were major accuracy issues with under-reporting SLP pressures in colder temperatures. Much better results were obtained by setting our weather station's REL to the airport's Altimeter (setting). This enabled us to calibrate our barometer with better precision, but there were still issues with Altimeter accuracy "drifting" at high (or low) pressures.

The new firmware barometer algorithm (introduced in 2025) calculates SLP only. If necessary, you can still use the official [[https://www.weather.gov/epz/wxcalc_altimetersetting|NWS Altimeter (setting) online calculator]] to get one-off Altimeter readings.

**Question:** //Why do I need to calibrate the barometer? All my other sensors don't require calibrating. Why should I bother?//

**Answer:** Since most electronic barometers drift continuously, they require occasional re-adjustment to restore their full accuracy. Therefore, calibration or checking for accuracy at least once a year is highly recommended.

IMPORTANT: Do not assume that your new weather station barometer is calibrated at the factory. Ecowitt states: “This kind of correction [ABS correction] is entirely normal as during manufacturing small shifts in the pressure sensor readings can be introduced.”

**Question:** //What is firmware?//

**Answer:** Firmware is the software-on-a chip that operates your display console or gateway. The firmware also includes the algorithms that calculate SLP (mean sea level pressure).

**Question:** //The barometer wiki mentions there are two separate and distinct sea level pressures. One is called the //Altimeter setting// and the other is called //SLP//. I am looking at both of these values on a METAR report, but there is hardly any difference between the two. If they are supposed to be quite different, why are the values close to being the same?//

**Answer:** SLP values can change significantly in response to large changes in outside temperatures. Therefore, if you live in a climate where there are large temperature differences between the seasons or even between day and nighttime temperatures, the difference (spread) between Altimeter and SLP values will expand or contract. The biggest differences between Altimeter and SLP will be in regions where winters are very cold. However, in warm weather, the difference between Altimeter and SLP shrinks. It is not uncommon for them to be around the same value. The other reason where the Altimeter and SLP can be very close is when your elevation is very close to sea level. As one approaches sea level all pressures; station pressure, Altimeter setting and SLP become closer in value. At sea level all three are identical to one another.

**Question:** //If I am comparing my sea level pressure on my weather station with the local airport, do I need to be at the same elevation and temperature as the airport?//

**Answer:** In order to make comparisons, all station pressures regardless of elevation and temperature are converted to (by convention) to sea level. Isobars are lines joining points of equal sea level pressure - not points of equal elevation and equal temperature. To compare sea level pressures, there is no requirement to have the same station elevation, same station pressure or same station temperatures. All parameters are boiled down to an equivalent sea level pressure. If two or more stations have the same sea level pressure (SLP), then you can draw isobar lines joining points of equal SLP.

**Question:** //I noticed that you are talking about multiple sea level pressures. Isn't there just one sea level pressure?//

**Answer:** In any discussion about sea level pressure, It is important to know that there is more than one sea level pressure, so it is easy to get them mixed up. There are three sea level pressures:

  - Pressure directly measured at sea level is a sea level pressure.
  - For meteorological purposes, pressure measured at any elevation higher than sea level is mathematically reduced to sea level elevation. It is the elevation that is reduced - not pressure. "Pressure reduction to sea level" is not a good description for what it really is. It would better described if we said; "station pressure is converted to an equivalent sea level pressure." A mathematical equation calculates the additional pressure required (based on the weather station's elevation above sea level) to convert the station pressure to what the pressure at sea level would be. This estimate is known as a sea level pressure. In aviation, this estimated sea level pressure is referred to as //Altimeter setting or QNH//.
  - As above, the third sea level pressure is also reduced to sea level elevation, although the equation uses additional local climate factors (temperature and humidity) as well as the elevation of the station to estimate the pressure at sea level. This estimate is also known as a sea level pressure. In an airport METAR report, this sea level pressure is abbreviated as SLP. In aviation, SLP is referred to as a "Q-code" called QFF.

----


====== ARCHIVES: ======

**IMPORTANT: Some Ecowitt models and other older legacy models are not hardware capable of updating their firmware to the newer barometer SLP algorithm and will have to continue to use the old fixed offset system. All the tutorials, guides and examples for the old fixed offset system, have been retained in the ARCHIVE section. See archived articles below.**

**For all models: The archived articles would also be relevant for anyone that chose not to upgrade to the new SLP algorithm including non-technical users that don't like to do any firmware upgrades - preferring to keep everything at the original factory default settings.**
----
==== Calibrating — for beginners ====
**//updated 28 March 2024//**

Note: This is an __archived article__ for use with the older fixed offset barometer firmware.

If you have acquired an Ambient Weather/Ecowitt or clone weather station — new or used, you will have to set up your barometer before first use. Unlike a temperature sensor that just “works” as is, your barometric sensor requires additional steps to set up properly. The barometer, arguably, is the most important weather sensor of all.

The following guide covers everything you need to know to get started. Don't worry, many of these steps need to be done once only. If you run into a snag, post your calibration question on wxforum.net and a forum member will be sure to help you out.

Ecowitt defines elevation in terms of pressure. Instead of entering an elevation, you will need to enter a pressure in order to set up the elevation for your barometer. For your barometer to work properly, you have to tell your barometer how high it is above sea level. You do this by looking up the difference in pressure between your elevation and sea level. This pressure difference is called the Relative offset (REL offset).

The next step is to check if your barometer is accurate by adjusting your Absolute value against a calibrated reference – usually an official weather station at a close-by airport. We will be calibrating to the Altimeter reading at the airport.

This tutorial is aimed at brand-new station owners setting up their barometers for the first time or experienced owners that just need to do a quick calibration. Experienced owners can optionally skip some of the steps and refer directly to the [[https://barometer-wiki.ca/barometer/doku.php?id=start#calibration_-_quickstart|Quickstart Method]].

The process of calibrating your weather station barometer accomplishes two things:

  - //It sets the elevation of your barometer.//
  - //It calibrates the barometer for accuracy.//

The following tutorial can be used to set up and calibrate any Ambient/Ecowitt/clone display console or gateway device that contains a barometric sensor.

//IMPORTANT: Do not assume that your barometer is calibrated at the factory. Ecowitt states: “This kind of correction [ABS correction] is entirely normal as during manufacturing small shifts in the pressure sensor readings can be introduced.”//

**Essential Terms & Definitions:**

**Absolute pressure(ABS)** is the live pressure reading from your barometric sensor. Also known as station pressure.

**ABS offset** is used to adjust the absolute pressure reading (ABS) up or down for calibration purposes.

**Relative pressure (REL)** represents what the pressure would be at sea level elevation if our weather station’s barometer was located down there. REL can refer to Altimeter or SLP. The use of the term "relative  pressure" means a pressure relative to sea level.

**REL offset** is the difference in pressure between your location's pressure and the pressure at sea level. You will need to calculate the REL offset and add this number to your ABS value (Absolute value).  The REL offset can be calculated manually, or you can use an online calculator (see Step 10b), to calculate it out for you.

//IMPORTANT: Before you start calibrating and comparing your pressure readings with an airport for reference purposes, it is critically important to do so only when your weather station is in the same pressure system as the airport!//

**Procedure:**

a) To set your elevation, use a pressure difference calculator to give you the REL offset directly.  For instructions on how to use the calculator, see Step 10(b).

b) Calibrate the barometer by adjusting your ABS offset [enter a positive or negative number] until the REL value on your screen or display is equal to the Altimeter reading at the airport.

**Steps:**

1. There should be an official weather station at the airport sufficiently nearby to act as a reliable and accurate reference barometer.  To get the most precise results, change your settings to display pressure in hPa units. After you have calibrated your barometer, you may change the display back to your preferred units.

2. Determine the elevation of your barometric sensor above sea level. Depending on your Ecowitt weather station model, this sensor can be located in your display console, Wi-Fi gateway (GWxxxx) or in the temperature, humidity, and pressure device (WH32B).

Frequently, this question comes up. Can I use the GPS feature on my phone to determine my elevation? The accuracy of GPS can be highly variable depending on the quality of the GPS sensor chip used. GPS is accurate for lat/long measurements, but is generally not recommended for elevation measurements. This may change as satellite and GPS chip performance improves. Until then, it is best to use Google Earth or similar tools to determine the //ground elevation above sea level// of your location.

//IMPORTANT: You want to determine the elevation above sea level of your barometric sensor, not the outside weather station array.//

3. Once you have determined your ground elevation (above sea level), you must also add the extra height of your sensor above ground level. For instance, if you are on a ground floor and have the device containing the sensor on a desk, you’ll have to add the additional height (say it was 1 meter) to Google Earth’s ground elevation results.

4. Now that you have calculated your sensor elevation, you will need to determine the correct REL offset for your sensor’s elevation. This offset will be automatically added (by your station's firmware) to the absolute pressure reading in order to calculate your relative pressure.

5. In order to calculate the REL offset for your specific elevation, you need to look up the pressure difference [using an online calculator] for your elevation. See step 10 (b) for instructions.

6. In order to calibrate your barometer, you have to compare your readings with a calibrated reference. Go to https://metar-taf.com to find the ICAO code you will need to generate the latest METAR report. Make sure you choose a close-by METAR station (usually an airport). On the website, you will see a map. Hover your cursor over the closest airport (coloured dot) to your location. You will see the four-letter code for your airport in capital letters.

7. You can use a website like the Aviation Weather Center for METAR reports.
For example, to retrieve the latest METAR report for [[https://aviationweather.gov/data/metar/?id=CYZE&hours=0&decoded=yes|Gore Bay Manitoulin airport]].

To change to another airport, change the CYZE in the hyperlink above to the ICAO code for your airport. For instance, here is [[https://aviationweather.gov/data/metar/?id=EDDB&hours=0&decoded=yes|EDDB (Berlin Brandenburg airport)]]. Just change CYZE to EDDB.

Bookmark your airport for future use!

In the METAR report, you might see one or two pressures; Altimeter and SLP. We are interested in Altimeter. Outside of North America, you might only see QNH, which is a close approximation of Altimeter.

8. After you have entered your REL offset, go to your device and observe the REL value. Compare your REL value with the Altimeter value from the airport. If, for example, the REL value on your display is 1022.9 but the airport Altimeter reading shows 1019.1 on the METAR report, that tells us that our sensor’s REL pressure is indicating too high: 1022.9 – 1019.1 = 3.8 hPa higher than the airport.

9. Because it is 3.8 too high, you will need to lower it by entering an ABS offset of  -3.8 or if you have a display console, reduce the ABS value by 3.8.

Basically, we are re-calibrating and checking the accuracy of the barometer by adjusting the ABS offset due to possible shifts in the pressure sensor readings during the manufacturing process.

10(a) Alternative method (the best method!). Calculate your REL offset as in Step 10(b) below. Then use a precision calibrated barometer [make one or buy/rent one] that you can place right next to the console or device containing the barometric sensor. Adjust the ABS value or ABS offset until the ABS pressure reads exactly the same as the precision reference barometer.

10(b) Use the quick calculator. [[https://www.sensorsone.com/us-standard-atmosphere-altitude-pressure-calculator/|REL offset calculator]]: The REL offset is the pressure difference between your elevation and sea level elevation (0 meters). You will need to know the elevation of your barometric sensor to use the calculator.  Don’t forget to click on the “Pressure Difference” button.

**[[https://www.sensorsone.com/us-standard-atmosphere-altitude-pressure-calculator/|REL offset calculator]]** instructions:

  * //Click the “Pressure Difference” button.//
  * //Enter your barometer sensor elevation in the Altitude 1 box.//
  * //Enter the sea level elevation of 0 (zero) in the Altitude 2 box.//
  * //Make sure you choose your preferred units for the output or the calculator will not work.//
  * //The pressure difference should be automatically calculated. This is your REL offset.//


11. Once you have entered the REL offset or adjusted the REL value, go back up to Step 8 and Step 9 (above) in order to calibrate your ABS value using the airport’s METAR Altimeter value. When you re-adjust the ABS value (up or down), the REL value changes by the same amount.

The REL offset calculator uses the International Standard Atmosphere model, which assumes the average pressure at sea level is 1013.25 hPa or 29.92 inHg.

12. Final step. For the //best accuracy,// one should check and re-adjust the ABS value next time when Altimeter = 1013.25 hPa or 29.92 inHg at the airport.

13. For examples how to enter in your pressure settings for your elevation, see [[http://meshka.eu/Ecowitt/dokuwiki/doku.php?id=barometer#calibration_display_consoles|Calibration - display console]] or [[http://meshka.eu/Ecowitt/dokuwiki/doku.php?id=barometer#calibration_wi-fi_gateways|Calibration - Wi-Fi gateway]].

Congratulations! You are now calibrated.

==== Calibration - quickstart ====
**//updated 12 March 2025//**

Note: This is an __archived article__ for use with the older fixed offset barometer firmware.

a) Set the elevation for your weather station. You do this by calculating your REL offset for your elevation. You need to do this once only. Go to the sensorsone.com website and use their [[https://www.sensorsone.com/us-standard-atmosphere-altitude-pressure-calculator/?ab=4&v2=0&f2=m&v3=0&f3=m&f4=hPa/|Pressure Difference calculator]] to obtain your REL offset. Make sure you enter your altitude as Altitude 1 and 0 (zero) for Altitude 2. Once the calculator has determined the pressure difference, enter this number as your **REL offset**. Enter the REL offset into your display console or Wi-Fi gateway. For some weather station models, you will have to add the REL offset amount to the ABS value and enter this sum as your REL value.

b) Calibrate your barometer for accuracy. Adjust the ABS offset or adjust the ABS value (if you have an older style display console) until the REL value matches the current Altimeter reading at the airport.

//Note: If you have a display console with the older firmware, you won't be able to enter a REL offset directly. You will have to change the REL value. See the article, "Calibration - display console" further down the page for more details.//

c) For best accuracy, re-adjust the ABS value next time the Altimeter reading at the airport = 1013.25 hPa or 29.92 inHg.

TIP: You will notice that when you adjust the ABS value, the REL value changes by the same amount.

Congratulations! You are successfully calibrated.

==== Calibration — display console ====
**//updated 23 Sept 2024//**

Note: This is an __archived article__ for use with the older fixed offset barometer firmware.

Let’s assume that you have a brand new weather station. As you are going through the manual’s setup instructions, you run into a snag. The barometer calibration instructions are very short and somewhat cryptic.

//Note: In many cases, there are no barometer calibration instructions in the manual. You might find a few sentences in a support document at the manufacturer's website, but information there can be sparse.//

Rather than trying to figure out the manual, let’s see how the calibration process works by using an example:

The calibration/settings screen in the console indicates the current pressure measured by your sensor. This will be //inches mercury (inHg)// by default.

**IMPORTANT: There are currently two different types of firmware versions (the built-in software that operates your console). One version uses offsets,the other version does not. The calibration procedure is different depending on the version you have. You will need to check the barometer setting/calibration page for your model and see whether or not you have entries for ABS offset and REL offset. If so, please skip to the "Calibration - Wi-Fi gateways" section at the end of this article below as the calibration instructions will be the same as for the gateway. If you only see an ABS and REL value (no offsets) continue reading this article for setup and calibration instructions.**

You will also notice that out-of-the-box, the initial pressure values will be the same **(ABS = REL)** because your barometer hasn’t been set up yet and the factory default settings assumes your elevation is zero. These settings must be changed to your actual elevation!

We know that the pressure declines as we go higher into the atmosphere. Think Mt. Everest. The atmosphere is a lot thinner way up there than at sea level.

Assume the barometer elevation is 300 meters. Therefore, we know that at a 300-meter altitude, our atmospheric pressure should always be thinner than the pressure way down at sea level:. For the mathematically inclined : **ABS < REL (Absolute value is less than Relative value)**.

We now have to figure out how much less our pressure will be at 300 meters compared to sea level (0 meters).

There’s an online calculator for that. Press the **“Pressure difference”** button and put in your preferred units as hPa.  Don’t forget to choose your units by the answer box, otherwise the calculator won’t work.

The Pressure difference calculator can be found here: https://www.sensorsone.com/icao-standard-atmosphere-altitude-pressure-calculator/

The calculator will give an answer of about 35.5 (rounded). //This number represent the pressure drop between sea level and your barometer’s altitude// of 300 meters. Or you can think of it the other way – that pressure will increase by 35.5 from your barometer’s altitude of 300 meters down to sea level (altitude = 0)

But, there’s a question mark. How do we know if our barometer is accurate or not?. The manufacturer tells us that there could be shifts in accuracy due to the manufacturing process, so chances are that your barometer is not accurate out-of-the-box and must be calibrated before first use. Next question. How do we know what the true pressure is?

For that, we need a second barometer as a reference. This barometer has to be calibrated to a high standard. Where are we going to find such a barometer? We can buy one or perhaps make our own. The cheapest (but not the best) option is to use a close-by airport’s barometer as a reference tool to calibrate your barometer.

This is what your calibration/setting screen might look like with the factory default in Imperial units. Let’s use 28.53 inHg as a random example:

At elevation = 0 (default setting)

  * ABS = 28.53 inHg
  * REL = 28.53 inHg

Note: On our display consoles, the manufacturer expresses elevation in terms of pressure only. There will be no fields to enter an elevation.

For better accuracy, change the inHg units in the console to hPa (hectopascals). Don’t worry, you can always switch back to your preferred units after. Look in the manual for instructions to change units.
28.53 inHg is equivalent to 1000 hpa.

From the calculator we just used, we found out there should be a 35.5 hpa pressure difference between ABS and REL. We also know that ABS should be less than REL (ABS < REL).

Therefore, calculating what the REL should be very simple – //we just add 35.5 to our ABS value to get the REL value.//

Therefore, 1000 hpa(ABS) + 35.5 hpa (pressure difference)= 1035.5 hpa(REL) but how do you change the REL value on the display from 1000 hpa to 1035.5 hpa?

CAUTION: For the older style display consoles that require you to use push buttons, changing the REL and ABS value can be a bit tricky, as you have to change the REL values in the correct sequence and save their values while maintaining the “spread” of 35.5 between ABS and REL. You have to go to the console calibration screen and change the REL value from 1000 to 1035.5 by pressing the buttons below the display screen, which will change the digits one at a time. Change the REL value first. We will also need to change the ABS value as part of the calibration process.

//Note: newer display consoles can be accessed by the Ecowitt app or by your web browser, making configuration much easier as it's the same procedure as configuring the Ecowitt Wi-Fi gateway. If your display console has an Absolute offset and a Relative offset, skip to the "Calibration - Wi-Fi gateway" section at the end of this article.//

For older display consoles, the barometer firmware works in a rather non-intuitive fashion. You can change the REL value directly, or you can also change the REL value by changing the ABS value.

After changing the REL value from 1000 to 1035.5, our console now shows these ABS and REL values for a 300-meter elevation:

At elevation = 300 meters

  * ABS = 1000 hpa
  * REL = 1035.5 hpa

The ABS = 1000 hpa is an actual measurement from our barometric sensor. We don’t know if it is an accurate number, so we are going to use the airport’s pressure reading as our calibrated reference.

We need to do the following to see if our ABS reading of 1000 hpa is accurate.

If our barometric sensor is perfectly accurate, the airport should have the same reading as our REL reading on the console. The reading at the airport is called the Altimeter or Altimeter (setting). Outside of North America, where Altimeter setting is rarely used, you will be comparing to QNH.

//Note: Unlike Altimeter (setting) which uses decimal values, QNH values will be in whole integer units in a METAR report.//

Suppose the current Altimeter reading at the airport is 1036.5 mb. However, our barometer REL shows 1035.5.

This means that our REL reading is 1.0 hpa too low, and we have to increase our REL by 1.0 hpa to match the airport reading of 1036.5.

Do not change the REL value a second time! We have to move the REL up by 1.0 by increasing the ABS value by 1.0. The barometer firmware is designed so that the REL will move with ABS lock in step. If you move the ABS value, REL moves by the same amount.

Let’s increase the ABS value by 1.0 hpa (increasing ABS from 1000 to 1001).
The display now shows:

  * ABS = 1001
  * REL = 1036.5 (REL automatically increases/decreases when ABS goes up or down)

Important: Even though both ABS and REL have changed values, you will notice that the pressure difference of 35.5 stays intact, i.e., REL - ABS = 35.5. After changing the numbers on the console, make sure that the “spread” between REL and ABS stays the same when you save the settings.

SUMMARY

To calibrate a display console only requires a short number of steps.

1. Calculate the //pressure difference// between sea level elevation and your altitude.
2. //Add the pressure difference// to your current ABS value to get your REL value.
3. //Change ABS up or down// until the console REL = Altimeter reading at the airport.
4. //Double check your readings!//. Next time when Altimeter = 1013.2 at the airport, repeat Step 3 if required.

You are now calibrated!

==== Calibration — Wi-Fi gateways ====
**//updated 23 Sept 2024//**

Note: This is an __archived article__ for use with the older fixed offset barometer firmware.

EXAMPLE

**Calibrate barometer if you have a Wi-Fi gateway**

Let’s assume that you have a brand new weather station. As you are going through the manual’s barometer setup instructions, you run into a snag. The barometer calibration instructions are very short and somewhat cryptic.

Rather than trying to figure out the manual, let’s see how the calibration process works by example:

The calibration/settings screen in the gateway indicates the current pressure (ABS value) measured by your sensor. This will be inches mercury (inHg) by default.

Initially, you will see two values; ABS and REL (Absolute pressure and Relative pressure).

You will also notice that the numbers will be the same (ABS = REL) because your barometer hasn’t been set up yet and assumes your barometer’s elevation is zero (sea level). If you don't live exactly at sea level, we have to calibrate it to your specific elevation.

We know that the pressure declines as we go higher into the atmosphere. Think Mt. Everest. The atmosphere is a lot thinner way up there than at sea level.

Assume the barometer elevation is 300 meters. Therefore, we know that at a 300-meter elevation, our atmospheric pressure will be less than the pressure at sea level.

We now have to figure out the pressure difference from our elevation down to sea level.

There’s an online calculator for that. Press the “Pressure difference” button. Put in 300 meters as Altitude 1 and 0 meters (zero - for sea level) for Altitude 2. Don’t forget to choose your hPa units by the answer box, otherwise the calculator won’t work.

The Pressure difference calculator can be found here: https://www.sensorsone.com/icao-standard-atmosphere-altitude-pressure-calculator/

The calculator will give an answer of about 35.5 (rounded). This number represent the pressure difference between sea level (elevation = 0) and your barometer’s elevation of 300 meters. This is your REL offset

But, there’s a question mark remaining. How do we know if our barometer is accurate or not? The manufacturer tells us that there could be shifts in accuracy due to the manufacturing process, so chances are that your barometer must be calibrated (or certainly, checked) before first use. Next question. How do we know what the true pressure actually is?

For that, we need a second barometer as a reference. This barometer has to be calibrated to a high standard. Where are we going to find such a barometer? We can buy one or perhaps make our own. The cheapest (but not the best) option is to use a close-by airport’s barometer as a convenient tool to calibrate your barometer.

This is what your calibration/setting screen might look like with the factory default in Imperial units. Let’s use 28.53 inHg as a random pressure value:

**EXAMPLE: factory default readings.** The factory setting assumes everyone's elevation is 0 (sea level). This must be changed!

  * ABS = 28.53 inHg
  * REL = 28.53 inHg
  * ABS offset = 0
  * REL offset = 0

Note: For the gateway devices (and display consoles), the manufacturer expresses elevation in terms of pressure only. There is nowhere to enter an elevation/altitude directly.

For better accuracy, change the inHg units in the gateway to hPa (hectopascals). Don’t worry, you can always switch back to your preferred units after. Look in the manual for instructions to change units.

28.53 inHg is equivalent to 1000 hpa. When you change the units from inHg to hPa, you will see that ABS = 1000 Pa and REL = 1000 hPa. This is still the factory default setting - all we've done so far is changed the units from inHg to hPa.

We are going to use 300 meters as an example. From the on-line calculator, we found out there is a 35.5 hPa pressure difference between the ABS and REL values for our 300-meter elevation. The calculated pressure difference is our REL offset.

Therefore, manually calculating what the REL should be is straightforward – we just add 35.5 to our current ABS value to get the REL value.

Therefore, 1000 hpa(ABS) + 35.5 hpa (pressure difference)= 1035.5 hpa(REL) but how do you change the REL value on the browser user interface or app from 1000 hpa to 1035.5 hPa?

Back to our example. To change the REL value from 1000 to 1035.5 go to the calibration/setting screen in the gateway and enter the pressure difference (the one we just got from the calculator) of 35.5 into the REL offset field.

**EXAMPLE:** **Entering elevation by using a REL offset**. This what the readings should look like for a 300-meter elevation:

  * ABS = 1000 hpa (current reading)
  * REL = 1035.5 hpa
  * ABS offset = 0
  * REL offset = 35.5 (online calculated pressure difference; this number sets an elevation of 300 m)

//Note: To change the ABS value, you enter an ABS offset. To change the REL value, you enter a REL offset.//

In the example used here, ABS = 1000 hPa is the live current reading from our barometric sensor. We have no idea if ABS is an accurate number or not, so we are going to use the airport’s pressure reading as our reference true value.

We need to do the following to see if our ABS reading of 1000 hPa is accurate.

If our barometric sensor is perfectly accurate, then we should have the same REL reading as the airport. The pressure reading at the airport is called the Altimeter (setting) or Altimeter.

Suppose the current Altimeter reading at the airport is 1036.5 mb. However, our barometer REL shows 1035.5. They do not match.

This means that our REL reading is 1.0 hPa too low, and we have to increase our REL by 1.0 hPa to match the airport reading of 1036.5.

To move the REL up by 1.0, all we have to do is enter 1.0 into the ABS offset field

Let’s increase the ABS value by 1.0 hPa (increasing ABS from 1000 to 1001).

**EXAMPLE: Adjusting the barometer for accuracy:**

  * ABS = 1001 (ABS changes by the ABS offset amount of 1.0)
  * REL = 1036.5 (REL automatically increased by 1.0 when ABS increased by 1.0)
  * ABS offset = 1.0
  * REL offset = 35.5 (as calculated by the online calculator)

//IMPORTANT: Notice that two things happened when you entered the ABS offset of 1.0. The ABS value increased by 1.0 and the REL automatically increased by 1.0. The firmware automatically adds the REL offset of 35.5 to whatever the current ABS value is. If the ABS changes, then REL changes by the same amount. Note that the REL offset is fixed and does not change because the REL offset number tells the barometer that we are at a 300 meter elevation)//

SUMMARY

To calibrate a barometer in an Ecowitt GWxxxx gateway requires a short number of steps.

1. Calculate the pressure difference between sea level elevation and your altitude.
2. Enter the pressure difference as a REL offset.
3. Change the ABS offset values up or down until the console REL = Altimeter reading at the airport.
4. Double-check your readings. The next time, when Altimeter = 1013.2 at the airport, repeat Step 3 if required.

You are now calibrated!

==== Pressure Algorithms ====
**//updated 03 August 2024//**

Note: This is an __archived article__ for use with the older fixed offset barometer firmware.

The Fine Offset firmware algorithm for the barometer calculations is a very simple algorithm. It is implicitly represented to us as:

**ABS +  REL offset  = REL** (Absolute pressure + Relative pressure offset = Relative pressure).

Because the manufacturer of your weather station can’t possibly know everybody’s elevation, the REL offset is arbitrarily set to be zero at the factory. I guess they assume everyone lives at sea level (elevation = 0) unless we tell our barometer otherwise.

At sea level, the REL offset = 0 (factory default setting):

**ABS + 0 = REL therefore; ABS = REL** (default factory setting)

Therefore, out-of-the-box, you will notice that The ABS value (ABS) is exactly the same as the REL value. Unless you live exactly at sea level, the REL offset should not be left at zero! It is left to us to //figure out what our REL offset is for our own elevation.//

The ABS value starts off as the raw, uncalibrated pressure from our barometric sensor. However, most of the time, the pressure sensor is not perfectly accurate from the factory and needs to be adjusted/calibrated by applying an ABS offset. Once it is calibrated, we can refer to ABS as our station pressure:

**ABS (raw) + ABS offset = Station pressure**

The Relative value (REL) can refer to Altimeter or SLP readings. Unless you live at a very low elevation (less than 50 meters), it is recommended to set the REL value to Altimeter, so our goal is to have the REL value in our display console or gateway = Altimeter reading at the airport.

Therefore:

**Station pressure + REL offset = Altimeter or QNH**

We know that station pressure is the atmospheric pressure at our sensor elevation, and that Altimeter is the theoretical pressure of our station pressure that has been converted down (mathematically reduced) to sea level elevation. We also know that atmospheric pressure at sea level is higher than our station pressure. So how do we calculate the REL offset?

//If we live above sea level, REL offset = is the amount of pressure one must add to our station pressure in order to convert it to the equivalent sea level pressure.//

Sounds a bit confusing? Here’s an example that should help us understand these concepts a bit better:

Imagine we are standing on a vertical cliff by the sea. What would happen if we could lower our barometer down to the base of the cliff, which is at sea level (0 meters). We know that it will be a higher pressure at sea level than the top of the cliff but by how much?

Assume the cliff is 300 meters high and that we recorded the pressure on top of the cliff before lowering the barometer down. Suppose this reading was 977.7 hPa.  We then lower the barometer 300 meters down to sea level. A friend has been instructed to take a reading of the barometer when it reaches sea level. He calls back and tells us that the barometer pressure at sea level is now 1013.25 hPa. That’s a difference of 35.55 hPa higher than the top of the cliff.

We repeat the experiment the following day except the pressure has changed overnight. At the top of the cliff, the pressure has increased to 987.7 from yesterday’s reading of 977.7. You figure out that if there was a 35.55 difference yesterday, then the sea level pressure should be higher by the same amount the pressure has gone up.

This time you call your friend who is waiting patiently at the base of the cliff. You tell him before the barometer reaches the bottom of the cliff that the new reading is going to be 1023.25. Your friend calls back amazed. He reports that the barometer reads 1023.25! He asks; “How did you know…you can’t see the barometer from way up there?” “Magic?” he asks.

Hardly. Calculating sea level pressure using our very simple algorithm was just a simple addition of 35.55 to our pressure at the top of the cliff in order to estimate the sea level pressure at the base of the cliff 300 meters below. We just realized that the 35.55 difference in pressure from the top of the cliff down to sea level elevation is our REL offset. To calculate sea level pressure, all we have to do is add the REL offset of 35.55 to whatever the current pressure is at our weather station. Now that we know our REL offset, we can continue calibrating our barometer.

However, it's not very practical to lower down your barometer down to sea level to get your REL offset for your elevation or lower the barometer to sea level every time you want to take a reading of the sea level pressure. There must be an easier way.

Actually, someone has already done the math for you and made a model of the atmosphere (ISA) that tells us what the difference in pressure between your elevation and sea level should be. There’s an online calculator that will figure out the REL offset for you. To use the calculator, all you need to know is your elevation.

Although the REL calculator provides us with our required Relative offset (REL offset) number, what is the calculator actually calculating?

**REL offset = (ISA pressure at sea level) minus (ISA pressure at your elevation)**

The ISA (International Standard Atmosphere) standard pressure at sea level is held as a constant. It is = 1013.25:

Therefore, the equation becomes:

**REL offset = 1013.25(ISA pressure at sea level) – ISA pressure at your elevation**

We now need to calculate the ISA pressure at your elevation. As this involves a rather complex equation, we need a calculator to figure this out.

The calculator tells us that the pressure of a 300-meter altitude/elevation should be 977.7 hPa when the sea level pressure is 1013.25 hPa.

EXAMPLE

  - Altitude = 300 meters
  - ISA pressure at sea level (@ 0 meters) = 1013.25
  - ISA pressure @ 300 meters = 977.70

then:

**REL offset = 1013.25 (ISA pressure at sea level) – 977.7 (ISA pressure @ 300 meters) = 35.55 hPa**

How we enter this REL offset = 35.5 depends on the type of Ecowitt device(s) that you have.
If you have an Ecowitt GW series gateway, it is easy. All you have to do is enter the 35.5 directly in the calibration screen as your Relative offset.

If you have a display console, it is not as convenient. To enter these numbers, you have to cursor around using the physical buttons located beneath the display screen and change the existing values (one button press at a time). In a display console, you will only see a ABS value and a REL value. There are no fields to enter any offsets. You will have to change the REL value by the REL offset amount we just calculated.

//Note: Newer display consoles or consoles that have had upgraded firmware don't require you to use the push buttons. You can now change pressure settings either using an app or using your web browser to configure settings.//

For examples of how to adjust these values, see the "Calibration - display console" or "Calibration - Wi-Fi gateway" articles (see next two articles below.

==== Why use Altimeter or QNH instead of SLP? ====
**//updated 16 February 2024//**

Note: This is an __archived article__ for use with the older fixed offset barometer firmware.

The advice, just a short time ago, was to calibrate our Fine Offset barometers to the SLP reading at a local airport. Although this advice is valid for very low altitude weather stations that are below 50 meters, most of us live at much higher elevations. If you live greater than 50 meters, the use of SLP for calibration purposes is not recommended.

Note: The term, Altimeter (setting) is often shortened down to just “Altimeter”, not to be confused with an “altimeter” – the instrument inside an airplane cockpit. Outside of North America, QNH would be used instead of Altimeter.

We could go into a long discussion about how to calculate Altimeter, but the short explanation of Altimeter is that it is a stripped down version of SLP (sea level pressure). Strip away the effects of temperature, humidity, adjustments for high altitude stations, you are left with Altimeter. Like SLP, Altimeter is also a sea level pressure.

When I purchased my first weather station (Ambient WS-2000) back in 2019, I followed the standard barometer calibration advice to use SLP as a reference. Here was my experience using SLP to calibrate:

“Although the SLP calibration approach/method seemed to work initially, I noticed my SLP values starting to drift (compared to the airport’s SLP) especially in colder weather. Winter was the worst. I needed to constantly re-calibrate in order to keep up with the airport SLP readings. I found myself re-calibrating twice a day, only to find myself having to start all over the very next morning. Then again, on some days, the readings would come back tantalizingly close. The next day, no – it is drifting again. Very frustrating. As winter set in, these errors and discrepancies became large enough to make my barometer readings unusable. Something was not working – but what?”

I stumbled across a very old wxforum post from around 2008 which provided me with a clue. The original post mentioned that you can force a Davis VP2 LCD console (not the Vue console) to calculate an Altimeter value by putting the console into a fixed offset mode. Unfortunately, this also disabled the console’s ability to calculate SLP.

Note:Davis weather equipment is a competitor to Ambient and Ecowitt equipment.

As soon as I saw the words “fixed offset”, “disabled SLP” and “Altimeter”, I instantly recognized what the solution had to be. Definitely a eureka moment.

The problem? We had been trying to calibrate to airport SLP readings using a fixed offset. Unfortunately, SLP does not use a fixed offset – it uses variable offsets. Since our consoles and gateways only use a fixed offset, we should be doing the same as our fellow Davis colleagues had been doing for years. The solution? Use Altimeter to calibrate, not SLP!

Now, here’s the thing – just because we have conveniently used an airport’s Altimeter (setting) as a handy tool for calibration purposes, does not mean you have to actually __use__ Altimeter. After all, meteorologists use SLP, not Altimeter, for their surface analysis charts and isobar forecasts.

The lack of a SLP calculation, poses a bit of an issue for weather enthusiasts that need SLP for meteorological purposes. Our weather stations can do Altimeter, but not SLP values. So, how can we get SLP?

You can obtain, log and graph more accurate SLP and Altimeter values with a modest investment in additional hardware and software. Capable 24/7 hardware (raspberry pi microcomputer) currently costs less than $30 CAD or so. To save even more money, you can recycle an old computer and run it 24/7. It should be Linux capable, but I believe a macbook would work as well.

However, nothing beats the efficiency and power savings of a tiny raspberry pi microcomputer.. You can buy a complete ready-to-go raspberry pi kit for not much money. The software is free.

==== ABS/REL system vs ABS offset/REL offset system ====
**//updated 06 February 2025//**

Note: This is an __archived article__ for use with the older fixed offset barometer firmware.

If you purchased an Ecowitt weather station, your system will either have a display console or have a displayless Wi-Fi gateway, like the Ecowitt GWxxxx series or equivalent. There are differences between the two devices.

For setting your elevation and calibrating your barometer, the display console can use either the **ABS/REL fixed offset** system or the **ABS offset/REL offset** system.

If you have an earlier model of display console with the older firmware, you will have to enter an ABS value and a REL value. You will have to change these values manually one digit at a time. There will be a whole lot of button pushing involved. Check to see if there is newer firmware available.

//Note: The newer display consoles can be configured using an app or a web browser. These newer consoles use the ABS/REL offset system just like the gateway devices. You will be able to enter the ABS offset and REL offset settings directly. This system is far simpler, quicker to input and more intuitive.//

==== Troubleshooting ====
**//updated 09 Dec 2024//**

Note: This is an __archived article__ for use with the older fixed offset barometer firmware.

PROBLEM: //"I am trying to calibrate my barometer but where is it?. Is it outside in the array with the other sensors or is it inside the display console?. My particular weather station has a separate white remote control looking device. It is showing temperature and other things on the small LCD screen. What does this device do?"//

SOLUTION: Depending on your model, the barometer sensor can be located either inside your display console or Wi-Fi gateway or in a separate 3-in-1 remote device that contains a temperature sensor, humidity sensor and a barometric sensor. If you  look closely at the LCD screen, you will see the screen alternate readings between temperature, humidity and pressure.You can not adjust or calibrate any of these readings on the device itself Any adjustments or calibrations must be done on the display console or Wi-Fi gateway.

PROBLEM: //"I never bother to calibrate any of my weather sensors. Why should I bother with barometer calibration? Doesn't the manufacturer do that?"//

SOLUTION: In order for barometers to work properly, you have to tell it where it is - specifically, how high it is above sea level. But, you also have to make sure that your barometer is providing accurate readings. Many weather station manufacturers only rely on the stated barometer sensor chip specifications. Due to the manufacturing processes, there is no guarantee what the actual accuracy of your barometer will be as errors can be introduced; i.e. solder drift. You do not know if your barometer is accurate or not. Whether your weather station is brand new or used - you have to check and re-calibrate if necessary.

PROBLEM: //"Aren't you supposed to put in an altitude somewhere?. How do I put in the altitude for my weather station?"//

SOLUTION: In early 2025 Ecowitt started to introduce a major barometer firmware update to a limited number of its display consoles. The new update now allows you to directly enter an altitude into the set-up screen in the display console. Previously, the manufacturer (Fine Offset) did not use altitude in its firmware. Altitude/elevation was expressed in terms of atmospheric pressure - much like a pilot determines altitude by setting pressure on his altimeter. See the articles on calibration to see how to calculate pressure in order to determine the altitude or elevation of your console or gateway. Note: The new firmware update will be rolled out to the rest of the Ecowitt product line at a later date.


PROBLEM:  //“Help! I've tried to find a calibration procedure in the manual, except there is only a couple of sentences about calibration, and it did not make much sense to me. The manual said something about matching an airport's reading. My readings don't match — not even close. I think my barometer must be defective.”//

SOLUTION: Barometers are pretty robust sensors. Although it is possible that any sensor could be defective out-of-the-box, it is far more likely that the barometer just needs initial set-up and calibrating.

PROBLEM: //“The manual say's to use the airport to calibrate, but my weather station is at a completely different altitude than my airport. Shouldn't both be at the same altitude?”//

SOLUTION: In setting up their barometers, owners of weather stations are often concerned about their airport’s elevation being different from their weather station’s elevation. We assume that we can't compare our weather station with an airport at a different elevation than ours.
The manual says to compare and calibrate your barometer with a close-by airport. How can you do that if the airport is a lot higher (or lower) than your weather station?

You would be correct — you can't directly compare pressures at different elevations. If you did, high elevations would always show low pressure and low elevations would always show high pressure. It is important to note that station pressure values are reduced down to mean sea level elevation so that one can make valid pressure comparisons between weather stations that are at different elevations. Mean sea level elevation is the common denominator that is used. All the fancy algorithms and equations do is to convert your station pressure to what it would be at sea level elevation. Therefore, the isobars you see on weather maps are all the station pressures from all the weather stations that have been converted (reduced) to sea level pressure at sea level elevation, so everyone is on the same level playing field.

PROBLEM: //"I thought I had calibrated my barometer properly, but colder weather has arrived, and my readings seem to be drifting badly. Sometimes the readings appear to be OK, but most of the time I am way out."//

SOLUTION: You might be trying to match your weather station's REL value with the SLP value at the airport. SLP values go up and down not only in response to changes in pressure, but to changes in temperature. Temperatures will cause SLP values to fluctuate as SLP continually compensates for changing air density. Colder air is heavier/denser and warm air is lighter/less dense. Ambient/Ecowitt equipment do not have the necessary algorithms to compensate and correct for temperature. To obtain the best results in all weather conditions, make sure you calibrate to the airport's Altimeter setting instead of SLP.

PROBLEM: //I followed the instructions to calibrate my barometer to the Altimeter setting at my airport. However, on some days, my readings don't seem to be as accurate. After a day or two, the problem goes away. How can I fix this?//

SOLUTION: The Ecowitt weather stations uses a fixed offset (REL offset) in order to estimate Altimeter. In reality, the atmosphere isn't completely linear. The fixed offset is calculated assuming the Altimeter reading at the airport is equal to 1013.25 hPa. If the current reading at your airport is much higher or lower than 1013.25 hPa, then your readings will drift a bit. This behaviour is normal. Readings will become more accurate once again when Altimeter pressure returns closer to the average of 1013.25 hPa. If not, you will have to re-adjust your ABS (Absolute value) until your REL value matches the airport Altimeter value of 1013.25 hPa.

PROBLEM: //"As recommended, I purchased a reference, calibrated barometer, so I could properly calibrate all my barometers. I noticed that my REL values on my consoles do not match the QNH value on the reference barometer, even though the ABS values match perfectly. Why doesn't QNH match my REL values exactly?"//

SOLUTION: The manufacturer of Ambient and Ecowitt weather stations have designed their barometer firmware so that a single fixed offset (REL offset) can be used to approximate Altimeter (setting) or QNH. By choosing this simple fixed offset method, the manufacturer makes the assumption that the atmosphere is perfectly linear and that ABS (absolute pressure) and REL values (relative pressure)move lock-n-step with one another. However, REL does not really move lock-in-step with ABS. In reality, the atmosphere is not linear (it's on a curve) which requires complex algorithms to properly calculate Altimeter or QNH. Ambient and Ecowitt lack the necessary algorithms that are required, so REL values are unable to perfectly calculate Altimeter (setting) or QNH values. It is best to only calibrate absolute values/station pressure with the absolute pressure reading from a calibrated reference barometer.

PROBLEM: "I live too far from my airport and/or my airport has a different climate. How can I calibrate my weather station barometer?"

SOLUTION: Calibrating using an airport is not ideal. They use different methods to measure/calculate pressure than our personal weather stations. The best way to calibrate is to use a reference barometer side-by-side with your barometer and adjust your ABS value for accuracy. See the article, "Calibrating the best way" for more information. Please also note that Altimeter or QNH values are unaffected by local climate conditions like temperature and humidity. Altimeter and QNH use the standard atmosphere model, which uses fixed parameters determined by the model.

PROBLEM: //"I still don't get it. Just give me the calibration setting to make it work."//

SOLUTION: If all else fails and you are finding it impossible to calibrate your barometer, you can always use the "walk-away" method of calibrating as a last resort.

//Note: You are making the assumption that your barometer is perfectly accurate out-of-the-box.//

Use a close-by airport and assume it has the "true" pressure value. All you have to do is change the REL value to match the Altimeter setting or QNH value at the airport.

That's it - you are done! You just "walk-away".

==== CWOP – Citizen Weather Observation Program ====
**//updated 24 Dec 2025//**

Note: This is an __archived__ article. 

IMPORTANT: The new Ecowitt barometer algorithm calculates SLP only. However, CWOP expects users to only **upload Altimeter setting** not SLP. Therefore CWOP users using Ecowitt equipment must make alternative arrangements to upload Altimeter setting values to CWOP.  Owners of Ambient weather stations can alternatively, use the www.AmbientCWOP.com web site to upload Altimeter values to CWOP.

CWOP is one of many weather services that will accept data from personal weather stations.

//Note: In the following article, CWOP suggests a method to calibrate a barometer. It is not specific to Ecowitt (or equivalent) weather stations but I have included it here as a reference only. I have made some edits for readability purposes.//

What is CWOP?

According to Wikipedia:

//"CWOP allows volunteers with computerized weather stations to send automated surface weather observations to the National Weather Service (NWS) by way of the Meteorological Assimilation Data Ingest System (MADIS). This data is then used by the Rapid Refresh (RAP) forecast model to produce short term forecasts (3 to 12 hours into the future) of conditions across the contiguous United States. Observations are also redistributed to the public.”//

CWOP gives these calibration instructions for their members:

CWOP strongly encourages our members to use altimeter pressure from a nearby airport to calibrate the pressure being sent to CWOP.

The following calibration procedure is recommended:

Select a nearby (within 20 miles or 32 km) airport weather station (regional or larger) to provide your reference or calibrated pressure.

Wait for optimal weather conditions to conduct a series of comparisons; these conditions are:

  * High pressure is nearly overhead
  * Wind is less than 5 mph (3 m/s), preferably calm
  * Outside air temperature should relatively stable or slowly changing
  * Best time to conduct pressure comparisons is in the early afternoon; if the winds are light, then you are reasonably certain high pressure is in the area.

Edit: Unless you live very close to the airport, make sure that you and the airport are in the same pressure system/zone before doing the comparisons.

Take a series of four simultaneous pressure measurements using the altimeter pressure from the airport “METAR” report and your barometer:

Each comparison should be at least be 15 minutes apart or 1 hr apart for airports that report only hourly.

After completing the four comparisons, noting your altimeter and the reference airport pressure [differences]; sum the differences between the comparisons and divide by 4 (the number of comparisons) to get a mean difference.

If the mean difference between your station and the reference station is more than +/ 00.03 inches for altimeter comparisons, or +/ 1.0 mb; add (or subtract) the difference to correct your altimeter. Repeat the procedure until you achieve the goal of a pressure difference of less than +/ 00.03 inHg or +/ 1.0 mb.

Barometers will “drift” requiring re-calibration. Therefore, barometer comparisons (with the Altimeter setting at the airport) should be done at least annually.

SUMMARY

  * Calibrate to the airport’s Altimeter.
  * Wait for optimum weather conditions.
  * Compare your barometer’s Altimeter readings with the airport’s Altimeter (setting) at least four (4) times.
  * Calculate a mean (average) difference: take the four differences and divide by four. Apply the average difference to your barometer. Repeat until your readings are within +/- .03 inHg or +/- 1.0 hPa to the airport.
  * Re-calibrate annually.

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