Wiki - Fine Offset Ecowitt Ambient Weather Stations (and clones)

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contributed by gszlag

updated 02 Feb 2025

ANNOUNCEMENT: The new home for the barometer wiki is www.barometer-wiki.ca/barometer

IMPORTANT: Ecowitt released a new firmware update in early 2025 that dynamically calculates SLP (sea level pressure) factoring in elevation, temperature and humidity. This new update is a radical departure from the old system where sea level pressures were somewhat crudely approximated using a simple fixed offset system. However, some Ambient and Ecowitt weather stations are not hardware capable to be upgraded to the new SLP algorithm. and must continue to use the old “fixed offset” barometer firmware.

This wiki will contain archived calibration articles regarding display consoles and gateways that cannot be updated due to hardware limitations, eg; WS2910, WS2320, GW1000 (gateway), the WH2650 and older legacy models that still have to use the old fixed offset system.

“…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.

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 Ambient, Ecowitt or clone 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 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 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.

Note: Although I may be referring to mainly Ecowitt weather stations, these tutorials also apply to the other Fine Offset manufactured brands like Ambient Weather and other re-branded (clone) equipment. Despite the different names, the hardware and barometer firmware is the same.

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):

1. Temperature: 15 °C @ 0 meters (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.
2. Pressure: 1013.25 mb @ 0 meters (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.

For what constitutes to be 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.

updated 22 Jan 2025

1. 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 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.
2. Calibrate your barometer for accuracy. Adjust the ABS offset or adjust the ABS value (if you have the “non-offset” display console) until the REL value matches the current Altimeter reading at the airport.
3. For best accuracy, re-adjust the ABS value next time the Altimeter reading at the airport = 1013.25 hPa or 29.92 inHg.

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.

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

Congratulations! You are successfully calibrated.

updated 23 Sept 2024

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 most cases, there are very few barometer calibration instructions in the manual and 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/ Don't forget to click the “Pressure difference” button.

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 may not be 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 to use a second barometer as a reference. This barometer has to be calibrated to a high standard. 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” of 35.5 between REL and ABS values 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!

updated 22 Jan 2025

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/ Don't forget to click on the rectangular “Pressure difference” button.

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, the 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!

updated 30 July 2024

Although using a local airport's METAR report comes in handy as a free 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.

If you live far away from the airport, then this introduces even more complications. Most of the time, you are never really sure if you are in the same pressure system as the airport.

And lastly, experts tell us that calibrating using one airport alone is not 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, no farther than 10–15 miles away (max 25 km).

All of these things make it difficult - sometimes impossible, 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 adjust your ABS value (station pressure) to be as accurate as possible. All the other pressure values depend on it. Other than sending your pressure sensor to an expensive calibration lab, the best way to get an accurate ABS (absolute pressure) is to compare your sensor pressure ABS reading side-by-side 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 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 for an airport to calibrate (or try to calibrate) your barometer!

For an example of a low cost, accurate barometer that can be used for calibration purposes, see our review: http://meshka.eu/Ecowitt/dokuwiki/doku.php?id=reviews#starpath_usb_baro_barometer_review

updated 09 Dec 2024

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 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 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 weather stations manufacturing processes, there is no guarantee what the actual accuracy of your barometer will be as errors can be introduced; i.e. solder drift. Unless your weather station comes with a calibration certificate, you do not know if your barometer is accurate. 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 elevation somewhere?. How do I put in the elevation for my weather station?”

SOLUTION: The manufacturer does not use elevation in its firmware. Elevation is expressed in terms of atmospheric pressure - much like a pilot reads elevation/altitude by setting pressure on his altimeter. See the articles on calibration to see how to do this initial set-up.

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.

Go to Calibration - display console or Calibration - Wi-Fi gateway. to do a thorough re-calibration.

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”.

updated 25 Sept 2024

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.

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.

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).

Station elevation or Station altitude has two different definitions. In aviation, station elevation is usually the runway elevation. For personal weather stations, station elevation refers to the elevation of the barometric sensor.

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.

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 school field trip. The airport meteorologist was not amused.

Why devote an entire wiki to just one weather sensor?

Barometers, are one of the most difficult weather sensors to set up and calibrate properly.

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.