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

Sun Jun 1 14:08:54 CDT 2008

Davis Instruments' Application Note 28 explains how the barometric  
pressure that is reported is derived.  The barometric pressure  
reported has been reduced to report sea level barometric pressure.  On  
page 11 at the end of the discussion of barometric pressure, a formula  
is provided for converting the reported barometric pressure to the  
altimeter reading used for CWOP reports.

I don't understand, exactly, how temperature effects the pressure  
sensor in the Davis Instruments Vantage Pro2 console but my station  
tends to report higher barometric pressures in the afternoon.  The  
console is in a room with cathedral ceilings on the west side of the  
house.  The room heats up in the afternoon and doesn't cool down until  
mid-evening.  During this period, the QC graphs show the widest  
divergence between my readings and the predicted values.

Merton Campbell Crockett

On 01 Jun 2008, at 11:31:02, Ted Lum wrote:

> 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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>
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Merton Campbell Crockett
m.c.crockett at roadrunner.com

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