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

Sun Jun 1 14:23:15 CDT 2008

> 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.

The location of the sensor indoors has nothing to do with the fact the
barometric pressure *is* impacted by outside air temperature. Air pressure
is the weight of a column of air above a sensor. 99.999% of the air weight
is not in your house, it's outside. Barometric pressure is VERY MUCH altered
by outside temperature. Because we're not dealing with an enclosed volume of
space, when you heat the air, molecules expand and thus there are less of
them in a given volume of space. As such, the pressure falls. When you cool
the air, air molecules contract, occupying more per unit volume and thus
weight more (pressure increase). 

In the absence of any significant airmass changes, a diurnal barometric
pressure curve will show a MINIMUM in the mid afternoon at peak heating, and
a maximum around daybreak during peak COOLING. This is for outdoors, with
the greatest impact occuring where there are the most air molecules -- in
the lower atmosphere. Changes to air temperature INSIDE your home are
generally considered negligeble given the vertical depth of air in your home
when compared to the entire atmosphere above you (yes, even with cathedral
ceilings).

All things considered equal however, you should not be seeing the barometer
rise in the afternoon, unless you are a coastal location experiencing an
afternoon seabreeze (which is accompanied by a drop in air temperature and
rise in pressure).

Evan

  _____  

From: wxqc-bounces at lists.gladstonefamily.net
[mailto:wxqc-bounces at lists.gladstonefamily.net] On Behalf Of Merton Campbell
Crockett
Sent: Sunday, June 01, 2008 2:09 PM
To: Discussion of weather data quality issues
Subject: Re: [wxqc] CWOP Information for CW4490 (C4490) in Leander, TX US

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

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