[wxqc] Barometer readings

[spamfree]

Dan,

First, the dewpoint and barometer problems are likely unrelated. the dewpoint looks like it might be off because your temperature is off. However, just because your temperature doesn't match doesn't mean its wrong. It could be your microclimate. If your temperature matches pretty well during the night but gets progressively higher than surrounding stations during the day, then you may have a siting problem where your station is getting too much radiated heat from the surface it is over. A the daytime fan add-on can help the temperature sensor during the heat of the day, and it's less expensive than the fan add-on that runs 24 hours.

I'm pretty confident that the barometer issue is the sea level vs. altimeter problem. You are at elevation, and it makes this issue much more noticable. Seal level pressure and altimeter are not the same thing. They are both ways of expressing the raw sensor pressure in a way that allows comparison between stations at different altitudes. The altimeter calculation adjusts only for elevation. The sea level pressure calculation adjusts for elevation, temperature and humidity. At higher elevations the two measures can produce quite different results. For example, looking at your nearby NWS station, the altimeter is 30.01 inHG (= 1016.26 mb), and the sea level pressure is 1010.9 mb. APRS/CWOP wants stations to send the altimeter value.

The problem for Vantage Pro owners is that the Vantage Pro pressure value is a sea level pressure calculated within the console. As far as APRS/CWOP submissions go, you have 3 choices:
1) calibrate your station to a nearby reliable station's sea level pressure, and send your station's pressure to APRS/CWOP. I think this is what you're doing now. The problem with this is that your pressure will usually be off in the quality check because you're sending sea level pressure instead of altimeter.
2) calibrate your station to a nearby reliable station's altimeter pressure, and send your station's pressure to APRS/CWOP. This is better than #1, but will still cause you problems. Since the VP console uses temperature in its algorithm to calculate reported pressure, your barometer QC will match right after you calibrate it to a nearby altimeter, but as the weather changes and your average daily termperatures change, you will notice your barometer drifting off in the QC. You will find yourself needing to recalibrate your barometer several times a year to compensate for this.
3) calibrate your station to a nearby reliable station's sea level pressure, but use a weather program for APRS/CWOP submissions that calculates the altimeter value from the VP's sea level pressure. There are two programs I know of that do this: WeatherDisplay (http://www.weather-display.com), and my freeware VPLive (http://www.pensom.org/weather/vptools/vplive.html). Since the VP console calculates sea level pressure slightly differently than your nearby NWS station, you might have to do a onetime small calibration change to make your CWOP altimeter match the NWS station's altimeter.

I'm in Idaho, and my barometer QC's were pretty bad until I wrote VPLive and started using it to submit the altimeter reading CWOP wants. They've been great since then, especially when compared to the nearby NWS station, and I haven't had to recalibrate as we moved from winter temps into spring temps.

If you want to play around with these barometer issues, you can try this freeware pressure calculator made for Vantage Pro owners (http://www.pensom.org/weather/vptools/vppressurecalc.html).

Hope this helps.
Steve (CW4409)




I'm kinda new to this so bear with me if I sound like a rookie. I recently installed a Davis Vantage Pro and looking at the analysis my barometer readings are off quite a bit. I talked to the NWS in Riverton WY and they told me I could set it using the "Sea level" reading on the local airports site ( http://www.crh.noaa.gov/data/obhistory/KRKS.html ) to set my barometer. I've tried this but still get a huge error in my data. I've double checked to make sure that I have the altitude set correctly in my unit so I'm kinda at a loss. My Dewpoint readings are also off.... are they related? Thanks

Dan (CW5688)
-------------- next part --------------
An HTML attachment was scrubbed...
URL: http://pond1.gladstonefamily.net:8080/pipermail/wxqc/attachments/20060422/a6d41558/attachment.html