Pressure calibration

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.

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

IMPORTANT: Some older display consoles have newer (upgraded) hardware with new firmware versions (the built-in software that operates your console). The new firmware will allow you to change barometer settings using an app or web browser. No more button pushing required! Adjusting barometer settings will be much easier with the new firmware's addition of absolute and relative offsets. As the procedures are quite a bit different, please skip to the Calibration - Wi-Fi gateways section at the end of this article below.

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 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 a 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 not used, you will be comparing to QNH.

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

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.

Instead of changing the REL value again, we can 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!