[wxqc] CWOP Information for CW4490 (C4490) in Leander, TX US

Sun Jun 1 14:11:49 CDT 2008

The recent version (5.7, I believe) of the WeatherLink software has the fix to send altimeter instead of the console's sea level pressure.  The fix is in the WeatherLink software itself. I think I remember seeing that they made a firmware change to the APRS data logger, but for the vast majority of VP data loggers that people have, there is nothing changed in the firmware related to this issue. 

I tested the WeatherLink software change they made and verified that it does do the altimeter conversion as they say. 

If you are using an older version of WeatherLink, or using another software program that does not properly calculate and send altimeter, then temperature definitely can have an effect on your CWOP pressure. The VP console calculates the displayed pressure value (which is sea level pressure) by taking the raw sensor pressure and applying a formula that also includes temperature and humidity as inputs.

Steve
SoftWx
  ----- Original Message ----- 
  From: Ted Lum 
  To: Discussion of weather data quality issues 
  Sent: 06/01/2008 12:31 PM
  Subject: Re: [wxqc] CWOP Information for CW4490 (C4490) in Leander, TX US

  I don't know. That thread is more than two years old and I don't know if it was ever resolved. I have not been able to find anything in the Davis release notes that say they made a change, but that does not mean they didn't. Since the pressure sensor is located in the console, NOT the station, if you see a variation with OAT then almost certainly its coming from a software calculation and not a temperature affect on the sensor its self. This is only circumstantial, but your observations seem to imply they have air density in the calculation. We should find out for sure, I don't have first hand knowledge. As a Davis owner/operator I want to know.

  Victor Engel wrote: 
    OK. But I thought the latest version of Weatherlink was sending altimeter -- and VWS as well. Do I have a wrong understanding there?

    Victor

    On Sun, Jun 1, 2008 at 12:37 PM, Ted Lum <gladstonefamily.net at tedworld.com> wrote:

      You might want to re-check this thread.

      http://server.gladstonefamily.net/pipermail/wxqc/2005-November/003212.html 

      Sam Drinkard wrote: 
Victor Engel wrote:
  I'm going to answer your question specifically and then also raise a 
more generic question.

First, it looks like your pressure reading is too low. I believe if 
your mean error exceeds 2 millibars, you will get the message. When I 
first set up my stations (C6155) in 2006, I noticed it tracked another 
stations AS425 very closely in all data points, including pressure. 
Although we tracked closely, the analysis stated our readings were too 
low, so I made an adjustment upwards. My stations has had good QC for 
pressure ever since.

I wonder, though, about something. All three of us use Davis Vantage 
Pro hardware, and all three of us were too low by similar amounts. Do 
these stations generally ship with a slight adjustment error? Could 
there be an adjustment needed at the factory? Certainly, we can't 
determine this from just three data points. I'm curious of others' 
experiences around the country, though, with this hardware.

An alternative, of course, is that our hardware is correct and the 
analysis is wrong. Were that to be the case, though, the airport would 
be too high. I think that's unlikely, since aircraft rely on accurate 
altimeter readings to navigate.

The other thing I've noticed but haven't spent time to check is that 
it appears the error in my readings may be correlated to temperature 
-- the higher the temperature the lower my reading with respect to the 
analysis. I may just be imagining that, and the magnitude of the 
effect certainly isn't large, if present. If this really is true, 
though, I wonder what would cause it.

Victor C6155

        Victor,

    I've noticed the same thing with my station, which is a Davis 
WM-II.  If I look at the long term analysis of pressure, I can see a 
trend in the summer months that *seems* to degrade the sensor, but like 
you, I've not investigated this aspect.  As most of us know, nearly all 
electronic components have some variant or change with changes in the 
component's temperature.  Perhaps it would be a starting point to 
contact Davis and ask if there is any compensation built into the 
pressure sensor, or if ambient air temp changes would in fact, cause the 
sensor to not read in a non-linear fashion.

Sam

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