Hello, I'm a new owner of an Ambient WS-2902C weather station. I signed up, today, on CWOP and have been comparing my reported pressure to the altimeter setting calculated by the MADIS QC page. My readings are consistently 0.7 to 0.8 mb lower than the calculated value, and this is consistent with the difference I get using the Keisan calculator at https://keisan.casio.com/exec/system/1224575267.
I calibrated my REL pressure using Ambient's recommended method: using the pressure reported by the nearest airport. I used the altimeter setting reported by the AWOS (the "1 minute weather") rather than a published METAR. The resolution of this reading is only 0.01 in Hg, which is poorer than the 0.1 mb resolution of the reading reported by the airport station on ambientweather.net, but that report is updated only hourly, like the published METARs.
Is 0.7 mb difference too great for my station to pass MADIS QC? If so, how can I determine whether the problem is sensor error vs. incorrect calibration? I think a 0.7 mb difference is not too far outside the margin of error of the AWOS report, but is there a better source to use in performing this calibration? Or should I be using the calculated offset instead, ignoring sensor error?
Welcome to the forum.
Thank you! And thank you for the very detailed explanation. A few points:
You mentioned using the keisan calculator. This calculator does not calculate Altimeter, You need a different calculator for that.
I'm really not interested in calculating Altimeter; I was using the local AWOS Altimeter setting in lieu of SLP since I didn't know a way of getting a *current* SLP value. CW2274's link to the NOAA mesowest page solved that for me (and now I realize that I could have used the most recent published METAR as well, as KMPV's METAR does also give the SLP in mb in the standard format, at least most of the time).
Although the keisan online calculators are popular and simple to use, we have to pay attention to the temperature starting point used in the calculator. The ISA (International Standard Atmosphere) assumes the default temperature at mean sea level is 15℃ and temperature declines as you gain altitude at the standard lapse rate of .0065℃ per meter. However, the keisan calculators by default uses a non-standard temperature of 15℃ at your location/elevation/altitude. If you are calculating elevation offsets, the 15℃ needs to be changed to the ISA temperature for your particular elevation.
Wow, that's potentially a gotcha, depending on station elevation. I'm at 250 meters or about 820 feet MSL, so the keisan value would correspond to about 16.6 C at MSL using the standard lapse rate. Not sure how much difference that would make in practice, but it's definitely a deviation from standard.
Normally, I would use a ISA calculator to calculate the offset but you can still use a keisan calculator if you prefer. To calculate your fixed offset for your new Ambient weather station you need to use a different keisan calculator. It is called the keisan temperature and pressure at destination calculator. Make sure the default temperature is 15℃ at sea level and the default pressure is 1013.25 and set the altitude to 0 meters. Enter in your barometric sensor elevation and it will calculate both the ISA atmospheric pressure at your elevation (based on 1013.25 hPa at mean sea level) and the appropriate ISA temperature for your location/elevation. The difference between 1013.25 and the calculated atmospheric pressure is the elevation offset for your location.
If I understand you correctly, I put 0 m and 15 C "at present location" and 250 m "at destination", with a sea level pressure of 1013.25 hPa. Is that correct?
If so, the keisan calculator tells me my ISA temperature is 13.38 C and my atmospheric pressure is 983.57 hPa, for an offset of 29.68 hPa. Does that sound right?
For the record, my current offset in my Ambient is 29.8 hPa.
Technically, none of the Ambient Weather, Ecowitt or their clone weather stations calculate Altimeter or Sea Level Pressure(SLP) directly. Altimeter or SLP are estimated using a fixed offset.
And my WS-2902C doesn't read Altimeter at all - again, my only reason for using the AWOS altimeter setting was that it was current, and I (wrongly) thought the difference between Altimeter and SLP would be negligible.
You will notice that the difference between SLP ( Sea Level Pressure) and Altimeter changes with temperature. This difference widens when it is really hot outside or really cold outside. This is because your AWOS/ASOS station at the airport is continually applying different temperature corrections depending on outside temperatures.
This is fascinating to me but I do not understand it. Is this because of assumptions that altimeters make in translating pressure differences into altitude differences? My airplane seems to have a fairly well-calibrated altimeter, so if I use the altimeter setting given by the AWOS on the ground, the altimeter reads the correct airport elevation to within about + or - 25 feet... pretty much at any temperature over a wide range (say -20 C up to about 35 C).
Getting back to your question regarding sensor error: To calibrate your barometric sensor all you need to do is subtract the offset you just calculated from the SLP reading from METAR (or other source) and the result is what your ABS reading should be showing in your display console. If not, you will have to adjust the ABS reading to match the calculated ABS amount.
This makes sense. The reason I hesitate is that I'm not sure how much I can trust a SLP reading from an airport that's ~ 10 nm away and about 80 meters higher than my elevation, and is usually at least 5 minutes old - we've been seeing pressure changes of about 2 hPa per hour lately, not really rapid but fast enough that I'm not sure my sensor would be any more accurate after calibration than before. I really don't want to calibrate ABS unless I have a current value I can trust.
Just be careful. When you change the ABS/REL readings make sure the offset amount hasn’t changed after all the button pressing is done. Going from memory here, change ABS first to what it should be, then change the REL amount to match METAR SLP. Check again that REL minus ABS = desired offset.
IOW, first calculate the correct ABS using the SLP from the METAR and the calculated offset, then set it, then set REL and double check the offset. If I decide to do this, that's what I'll do.
There is lots of reading on this forum on this subject. Do a search on “barometric calibration” for more information.
What is your elevation? We can crunch some numbers for you and make sure you have the right offset.
Thanks again!
