The note *Derived variables <https://www.davisinstruments.com/product_documents/weather/app_notes/AN_28-derived-weather-variables.pdf>* from Davis has a pretty good discussion of this.
On Sun, Jan 10, 2021 at 5:38 PM Brent Dowell <[email protected]> wrote: > Correct Altitude is entered. > > I'm getting an average error of 3.5 millibars. > > My altitude is pretty close to the local airport, and I used a fairly calm > day to adjust my barometer to match that. Our altitude is also relatively > close, i.e. the airport is 4403ft and I'm at 4410. Althought that 4410 > could be off, I had used that for my altitude for years. > > Took another reading via gps today and have adjusted that to 4293, which > seems like it could be a significant differences. > > Thanks so much for your help in getting me to understand it. I've managed > to learn to ignore the wind errors, as I live on a hillside and don't see > anyway my wind data will ever match. if I need to do that for barometer, > I'll work on it. lol. > > Would just like to figure out what I'm doing wrong. > > > > > 1. That you have a Vantage console; > Vantage Vue Console > 2. That you are using regular LOOP packets, not the newer LOOP2 packets; > loop_request = 1 > 3. That you are using hardware record generation. > prefer_hardware > > From the wee_device command. > > BAROMETER CALIBRATION DATA: > Current barometer reading: 30.222 inHg > Altitude: 4293 feet > Dew point: 24 F > Virtual temperature: 32 F > Humidity correction factor: 1.1 > Correction ratio: 1.175 > Correction constant: +0.000 inHg > Gain: 0.000 > Offset: -16.000 > > > On Sunday, January 10, 2021 at 5:01:53 PM UTC-8 [email protected] wrote: > >> I'm assuming: >> >> 1. That you have a Vantage console; >> 2. That you are using regular LOOP packets, not the newer LOOP2 packets; >> 3. That you are using hardware record generation. >> >> If all this is true, then what's happening is that WeeWX is downloading >> altitude and temperature corrected barometric data (what pilots call QFF) >> from the console. However, CWOP wants pressure corrected only for altitude >> (not temperature; pilots call this QNH). So, WeeWX uses an algorithm to >> calculate that. This is then what is uploaded. >> >> First thing is to make sure you have the correct altitude entered in your >> console. You will also want to know whether the console is applying any >> barometric corrections. Both can be checked using the utility wee_device, >> with the --info <http://www.weewx.com/docs/hardware.htm#vantage_info> >> command. >> >> *wee_device --info* >> >> This will tell you what altitude your console thinks it's at, as well as >> any barometric calibration data. >> >> -tk >> >> >> >> On Sun, Jan 10, 2021 at 4:24 PM Brent Dowell <[email protected]> wrote: >> >>> I've been chasing my tail a bit trying to get my data that I upload to >>> CWOP to pass the madis checks, which, from what I've read is a bit >>> problematic. >>> >>> My question though is what should I have my console set as far as >>> altitude and barometer reduction. Right now I have it set for my correct >>> altitude and am using the altimeter setting. My readings are consistently >>> 'low' compared to the madis QC Check. >>> >>> It looks like to me that the values being uploaded are in fact low (raw >>> aprs data) and do not match whats on my console. So it appears that weewx >>> must be doing some altimeter calculation on my data and is sending that on >>> to cwop? I'm wondering if I'm missing a setting somewhere in my weewx >>> config to accout for that? >>> >>> Some of the research I've done on the issue talks about davis being a >>> bit of a pain because they out put the altimeter corrected data already and >>> that setting altitude to 0 and using the altimeter reduction fixes it? >>> >>> Sorry about dredging up an old issue, but I couldn't find much recent >>> information on this. >>> >>> Thanks in advance for any help. >>> >>> -- >>> You received this message because you are subscribed to the Google >>> Groups "weewx-user" group. >>> To unsubscribe from this group and stop receiving emails from it, send >>> an email to [email protected]. >>> To view this discussion on the web visit >>> https://groups.google.com/d/msgid/weewx-user/d425ef32-4da0-49e1-b2d4-fd21dffdfbaen%40googlegroups.com >>> <https://groups.google.com/d/msgid/weewx-user/d425ef32-4da0-49e1-b2d4-fd21dffdfbaen%40googlegroups.com?utm_medium=email&utm_source=footer> >>> . >>> >> -- > You received this message because you are subscribed to the Google Groups > "weewx-user" group. > To unsubscribe from this group and stop receiving emails from it, send an > email to [email protected]. > To view this discussion on the web visit > https://groups.google.com/d/msgid/weewx-user/49100cc7-7e2f-478f-9065-f0f42a92db3bn%40googlegroups.com > <https://groups.google.com/d/msgid/weewx-user/49100cc7-7e2f-478f-9065-f0f42a92db3bn%40googlegroups.com?utm_medium=email&utm_source=footer> > . > -- You received this message because you are subscribed to the Google Groups "weewx-user" group. 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