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

[Sam Drinkard]

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