[wxqc] Barometer Errors

[spamfree]

Hi,

After looking at your data and site info, and plugging some numbers into the freeware VP pressure calculator I wrote a few months ago (http://www.pensom.org/weather/vptools/vppressurecalc.html) your barometer drift does seem to fit this explanation. As an example, if your station is at 1246 ft elevation, and we assume a 60% humidity in all cases, then with a mean temperature (avg between temp now and temp 12 hours ago) of 57 F, and a console pressure of 1020.0 mb, the altimeter value you should be sending to CWOP is 1020.0 mb. But at a mean temp of 77 F, a console pressure of 1020.0 mb translates to an altimeter value of 1021.8 mb. In other words, when it was cooler, you would match the nearby stations. But when the temperatures get much warmer, you would be underreporting your pressure by 1.8 mb relative to the nearby stations.

You can set your console elevation to zero, and recalibrate your pressure to match a reliable, nearby altimeter. Setting the elevation to zero turns off the sea level reduction calculation that goes on in the console. There are two drawbacks with this though. First, if you have a solar sensor, there are other solar radiation related calculations that go on in the console that need a correct elevation setting to give correct results (a non-issue if you don't have the solar sensor). Second, this method assumes that the offset between sensor pressure and altimeter is a static value at all pressures. This is not the case given the way the altimeter formula works. I don't know offhand whether this would be a significant error for your elevation though.

Best regards,
Steve
  
======= At 2006-04-16, 22:10:31 you wrote: =======

>Hi,
>
>If the times his barometer drifts off has a correlation with wide temperature swings, and he is at altitude (say above 500ft), then the VP pressure issue could explain it. The VP calculates a sea level reduced pressure from the sensor pressure. Temperature and humidity is part of that calculation which is done in the console. This is what WeatherLink sends to APRS/CWOP. But CWOP wants altimeter, which is the sensor pressure adjusted only for elevation. There is a difference between these two pressure representations, and the result is that many VP stations have CWOP pressures values that drift in and out of the expected norm. There are a couple of weather programs that calculate the correct altimeter value from the VP pressure for submission to APRS/CWOP, but WeatherLink currently isn't one of them. There has been talk that Davis may do this in a future version.
>
>Best regards,
>Steve
>  
>