Calibration of barometric pressure settings

Calibration of barometric pressure requires some additional understanding, which we will provide here. Also provided is a step by step procedure for calibrating correctly.

Absolute barometric pressure, can be calibrated at manufacturing time by comparing with a precise instrument that measures pressure at the same location. In practice, sometimes small adjustments of a few hPa may be needed. The relative pressure represents what the air pressure would indicate if your station was at sea level and depends on the altitude of your console and cannot be known in advance. This is why it needs an adjustment.

There are different manners in which to handle this adjustment. We will outline a reliable procedure below, which requires adjusting both pressures. The method assumes that you have an official airport sufficiently nearby to act as a reliable reference. Usually distances of up to 25 miles work reliably, but this is not always true and depends on geography. We start by assuming that your station’s absolute pressure reading is correct and needs no offset correction.

The procedure below assumes you are starting from the console’s factory setting. With those settings, ABS and REL should, at this time, be displaying the same value. We also assume, for the moment, that ABS pressure is 100% correct.

1.      For this procedure we will get the most precise results if our display for pressure is in hPa units. Even if you do not want to use those units eventually, set the console to use them for now.

2.      Determine the altitude, or elevation above sea level, of your station’s console. This measurement is necessary to account for the difference in air pressure caused by the elevation of your console. Elevation above sea level reduces the absolute pressure measured by your sensor. Determine this altitude using a GPS, or look it up using a tool such as this web site: https://www.freemaptools.com/elevation-finder.htm. You can input your location’s GPS coordinates, or manipulate the map to your location. Click on “Estimate Elevation” and observe the result. For an example we will use a console location at 42 ft. above sea level.

3.      This tool will provide the ground level elevation at your location, so you will need to add the right amount for how high above ground level your console is. If you are on a ground floor and have the console on a desk, you’ll have to add something like 3-4 ft. If you are using a GPS system that tells you elevation, make sure it is right next to the console and you’ll be able to read the correct elevation right from the GPS results without further adjustment.

4.      With the correct altitude/elevation in hand you will need to determine the correct offset. To be added to the absolute pressure reading in order to compute relative pressure (sea level equivalent). Correction tables can be found on-line in many places. One example is the table found at the web site at https://novalynx.com/manuals/bp-elevation-correction-tables.pdf.
Locate your elevation in the first column and read the correction in the third column. This table, however is rather coarse, making it hard to be precise. An alternative is an on-line calculator such as the one found here:
http://www.csgnetwork.com/barcorrecthcalc.html
For our example of 42 ft. above sea level we input 42 ft. of elevation and a standard pressure of 1013.25 hPa/mb and press calculate. We find an “absolute barometer value” that should be -1.5626061222588443 hPa lower than at sea level. The inverse (because relative pressure is higher than absolute pressure) of this number will be our “REL PRESS OFFSET” value. Use the settings procedure to increase REL by +1.6 (nearest rounded value we can input).

5.      Now we need a reliable reference for pressure at sea level. Locate the official identifier for the nearest airport. Refer to “World Airport Codes” at https://www.world-airport-codes.com or a similar reference. Enter your location or nearby airport name, and press “Search.” Select the correct airport from your search results and click on it. For example, search for “Mountain View” and click on “Moffet Field.”

6.      From the resulting page find the ICAO code, if listed. Otherwise use the IATA code. For the example above, you would find IATA code “NUQ.”

7.   Now go to a web site like AVIATION WEATHER CENTER (for US locations) at https://www.aviationweather.gov/metar?gis=off and enter the code you found in step 2, and click “Decoded” (to make the next step easier) before requesting the METAR information. For the example we would enter “KNUQ” and find a result output like: “30.09 inches Hg (1019.0 mb) [Sea level pressure: 1019.1 mb]”

8.      Go to the calibration settings page and observe the “REL Barometer value (this is the value we just adjusted in step 4 above). Compare the REL value with the value from the airport. IN our example, the REL display was 1022.9 where we expected 1019.1. This then tells us that our displayed REL pressure is 1022.9 – 1019.1 = 3.8 hPa different from the reference source.

9.      Since we assumed the absolute pressure measured was correct, and we presumably calculated the elevation related offset correctly, we must conclude that the absolute pressure was not correct after all. It appears to be 3.8 too high, so we’ll now enter a correction of -3.8 in the settings for “ABS Barometer” until it reads a value 3.8 hPa lower than before. This kind of correction is entirely normal as during manufacturing small shifts in the pressure sensor readings can be introduced.

10.   For a more precise procedure, locate a very precise barometer that you can place right next to the console, you would adjust “ABS Barometer” until the ABS pressure reads identical. You would then still adjust “REL barometer” until it displays the value from the reference airport. This procedure would also produce the correct relative pressure, but due to a precise calibration of the absolute pressure, it too is correct.

The first procedure above generally works quite well, but for stations at fairly high altitudes (e.g. 5,000 ft. or higher) it may be more incorrect than at lower altitudes. In such cases comparisons with other known correct, and nearby at similar altitude, stations may help.

Now that calibration is complete, feel free to change the pressure units to whatever you like.

Note:   Airport METAR data is often only updated every 10, 15 or even 30 minutes. If you use the information in the procedure above, you may be looking at pressure data that is out of date by as much as the update interval. To get best results observe several times and figure out the update interval and then use two values for the procedure: one taken immediately after an update, another taken about halfway through the interval.

Note:   It is also a good idea to observe some more after the calibration procedure is complete to make sure the numbers are correct.

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