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Calibrating relative (corrected sea level) pressure
elisatems:
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?
CW2274:
--- Quote from: elisatems on December 20, 2021, 04:01:53 PM ---Is 0.7 mb difference too great for my station to pass MADIS QC?
--- End quote ---
No. That said, if you are calibrating to the altimeter setting, your equipment is incapable of calculating it. You can certainly be close, but you should be using SLP to be more consistent.
elisatems:
--- Quote from: CW2274 on December 20, 2021, 06:33:47 PM ---
--- Quote from: elisatems on December 20, 2021, 04:01:53 PM ---Is 0.7 mb difference too great for my station to pass MADIS QC?
--- End quote ---
No. That said, if you are calibrating to the altimeter setting, your equipment is incapable of calculating it. You can certainly be close, but you should be using SLP to be more consistent.
--- End quote ---
I'm not certain what you mean. The Ambient 2902C cannot calculate SLP? Well, yes, it just uses the offset that I give it when I enter the relative pressure. Or it's incapable of calculating the altimeter setting? How much difference is there between SLP and altimeter setting? The airport only reports an altimeter setting via the AWOS, and that's the only *current* value I can get from it. It might report SLP on ambientweather.net, but that reading is nearly always at least an hour old. Where would you recommend getting a reliable (and current) SLP reading?
CW2274:
Like I said, you can not calculate the altimeter setting. The difference can be negligible, or not. What AWOS four letter ID are you using?
elisatems:
--- Quote from: CW2274 on December 20, 2021, 07:00:10 PM ---Like I said, you can not calculate the altimeter setting. The difference can be negligible, or not. What AWOS four letter ID are you using?
--- End quote ---
KMPV. Again, what source should I be using? Although KMPV is on ambientweather.net and an SLP reading (at least, I assume that's what it is) is given there, I've never seen a current reading from it there. It's always at least an hour old. And that station is about 10 nm from my PWS anyway.
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