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

[Ted Lum]

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 
> <http://gladstonefamily.net>@tedworld.com <http://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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