I replaced my instrument package with an Ambient 2902 and I am using a IP Observer (no console) to connect to my router. Reporting to Ambient CWOP. I am having some issues getting my barometer calibrated. Data: Local Metar at sea level 1026 hPa, barometer elevation 44.1 meters/145 ft. Local airport PAEN. If some one could give me a bit of guidance It would be very much appreciated. I have used 3 different calculators and I have thoroughly confused myself. This is a station issue and not due to other station calibrations. Station ID GW0412
I am not familiar with the ObserverIP, but according to the Ambient website, the ObserverIP uses offsets to adjust your barometer. You will see an Absolute pressure offset and a Relative pressure offset. Set these back to 0 (zero) as we'll be starting over by going back to the factory default.
As you have figured out, you use a close-by airport (when you and the airport are in the same pressure system!) to calibrate your barometer. You adjust the relative pressure offset so that your REL value is equal to the Altimeter setting at PAEN. At the time of writing Altimeter setting is currently 1023.5 hPa/30.22 inHg.
Note: it is best to switch units to hPa as the units are "finer-grained" than inHg. You can always switch back the display to inHg when calibration is complete if that is what you prefer.
Since you live above sea level, the REL adjustment (offset) will always be a positive number. The REL value (relative pressure) is an approximation of what the sea pressure would be if we moved your barometer 44.1 meters down to the sea.
Intuitively, we know that if we go up higher into the atmosphere, pressure drops. We also know that as we go down towards the sea, pressure increases. But in your case, how much does the pressure increase from your location of 44.1 meters above sea level to the sea where pressure is higher?
To find out what that number is (relative offset) we use a version of a Standard Atmosphere calculator which will give you what they call a "Pressure Difference" which is what we want; the theoretical pressure difference between 44.1 meters above sea level and the pressure at sea level under standard conditions (model of the atmosphere).
The pressure difference number from the calculator is the relative offset number you need to enter into the calibration screen of your ObserverIP. The calculator says the number is 5.3 (rounded).
Basically, the relative offset tells your barometer how high it is above sea level.
Once you've entered the relative offset, the next step is to check if your REL value ( you may have to flip to your data screen) and check if your REL value is equal to the Altimeter setting at the airport. If it is, that's great! You are calibrated!
Chances are is that the two values don't match exactly, so that's where the ABS offset comes in. You can enter a positive or negative ABS offset, depending if your REL value is higher than the Altimeter setting at the airport or lower than the airport.
For example, if the Altimeter setting at the airport is 1023.5 hPa and your barometer's REL value (relative pressure) = 1022.5 you are 1.0 Pa too low.
To fix that, all you have to do is add an adjustment of 1.0 hPa to the ABS offset (absolute pressure offset). Your REL (relative pressure) should be equal to Altimeter at the airport.
For the best accuracy, you may have to re-adjust the ABS offset to adjust the REL value again when the Altimeter setting at the airport = 1013.25hPa/ 29.92 inHg. This is because the calculator calculates the relative offset when Altimeter setting at the airport is reading 1013.25/29.92. In reality, the atmosphere does not move in a perfectly straight line - it's on a curve.
Using the above example data, your ObserverIP should be showing the following values:
Absolute pressure: 1018.2
Relative pressure: 1023.5
Absolute offset: 1.0
Relative offset : 5.3
You will notice that relative pressure will always be 5.3 higher (relative offset) than the Absolute pressure because you are 44.1 meters higher than the sea. That is - unless you move and have to recalculate your relative offset.
References:
Pressure Difference(relative offset) calculator with an elevation of 44.1 meters (using hPa):
https://www.sensorsone.com/icao-standard-atmosphere-altitude-pressure-calculator/?ab=4&v2=44.1&f2=m&v3=0&f3=m&f4=hPaThe barometer wiki - for all things barometric! How to calibrate a Wi-Fi gateway (using offsets) along with a detailed example:
http://meshka.eu/Ecowitt/dokuwiki/doku.php?id=barometer#calibration_wi-fi_gateways--
