[wxqc] Barometer readings

[Ken Whelan]

I was also having what I believe to be the barometric pressure problem
that Steve describes below.  I went and took a look at his VP software
and I just wanted to drop a note,  if you haven't seen his VirtualVP
shareware ($30.00) and you are running a Davis Vantage Pro,  it would
probably benefit you to do so.   This little gem will let you multiplex
your serial port so you can run more than one piece of software on your
Vantage Pro(maximum 4).   Because the Davis communication is two way
communication,  this little piece of software acts as a traffic cop
routing the traffic to each piece of software so there are no
collisions.  It is really pretty cool.   One other thing it does that is
useful, it gives you health statistics on your wireless connection to
the weather station. (how many packets dropped etc).  I'd not seen
anything else that does this.   One other neat trick it does is to allow
you to connect over a network as well, converting serial to TCP/IP.   
 
Disclaimer.   I have never communicated with Steve and he doesn't know
me at all, this is not just a shameless plug,  this is truly a pretty
innovative tool and if you haven't seen it, it is worth looking at.  I
am new enough to this list, it may well have been discussed previously
but I had not seen the topic in the last month or two. 
 
Later
Ken Whelan
CW5627

________________________________

From: wxqc-bounces at lists.gladstonefamily.net
[mailto:wxqc-bounces at lists.gladstonefamily.net] On Behalf Of spamfree
Sent: Saturday, April 22, 2006 8:24 AM
To: Discussion of weather data quali
Subject: Re: [wxqc] Barometer readings


Dan,
 
First, the dewpoint and barometer problems are likely unrelated. the
dewpoint looks like it might be off because your temperature is off.
However, just because your temperature doesn't match doesn't mean its
wrong. It could be your microclimate. If your temperature matches pretty
well during the night but gets progressively higher than surrounding
stations during the day, then you may have a siting problem where your
station is getting too much radiated heat from the surface it is over. A
the daytime fan add-on can help the temperature sensor during the heat
of the day, and it's less expensive than the fan add-on that runs 24
hours.
 
I'm pretty confident that the barometer issue is the sea level vs.
altimeter problem. You are at elevation, and it makes this issue much
more noticable. Seal level pressure and altimeter are not the same
thing. They are both ways of expressing the raw sensor pressure in a way
that allows comparison between stations at different altitudes. The
altimeter calculation adjusts only for elevation. The sea level pressure
calculation adjusts for elevation, temperature and humidity. At higher
elevations the two measures can produce quite different results. For
example, looking at your nearby NWS station, the altimeter is 30.01 inHG
(= 1016.26 mb), and the sea level pressure is 1010.9 mb. APRS/CWOP wants
stations to send the altimeter value.
 
The problem for Vantage Pro owners is that the Vantage Pro pressure
value is a sea level pressure calculated within the console. As far as
APRS/CWOP submissions go, you have 3 choices:
1) calibrate your station to a nearby reliable station's sea level
pressure, and send your station's pressure to APRS/CWOP. I think this is
what you're doing now. The problem with this is that your pressure will
usually be off in the quality check because you're sending sea level
pressure instead of altimeter.
2) calibrate your station to a nearby reliable station's altimeter
pressure, and send your station's pressure to APRS/CWOP. This is better
than #1, but will still cause you problems. Since the VP console uses
temperature in its algorithm to calculate reported pressure, your
barometer QC will match right after you calibrate it to a nearby
altimeter, but as the weather changes and your average daily
termperatures change, you will notice your barometer drifting off in the
QC. You will find yourself needing to recalibrate your barometer several
times a year to compensate for this.
3) calibrate your station to a nearby reliable station's sea level
pressure, but use a weather program for APRS/CWOP submissions that
calculates the altimeter value from the VP's sea level pressure. There
are two programs I know of that do this: WeatherDisplay
(http://www.weather-display.com), and my freeware VPLive
(http://www.pensom.org/weather/vptools/vplive.html). Since the VP
console calculates sea level pressure slightly differently than your
nearby NWS station, you might have to do a onetime small calibration
change to make your CWOP altimeter match the NWS station's altimeter.
 
I'm in Idaho, and my barometer QC's were pretty bad until I wrote VPLive
and started using it to submit the altimeter reading CWOP wants. They've
been great since then, especially when compared to the nearby NWS
station, and I haven't had to recalibrate as we moved from winter temps
into spring temps.
 
If you want to play around with these barometer issues, you can try this
freeware pressure calculator made for Vantage Pro owners
(http://www.pensom.org/weather/vptools/vppressurecalc.html).
 
Hope this helps.
Steve (CW4409)
 
________________________________

I'm kinda new to this so bear with me if I sound like a rookie. I
recently installed a Davis Vantage Pro and looking at the analysis my
barometer readings are off quite a bit. I talked to the NWS in Riverton
WY and they told me I could set it using the "Sea level" reading on the
local airports site ( http://www.crh.noaa.gov/data/obhistory/KRKS.html )
to set my barometer. I've tried this but still get a huge error in my
data. I've double checked to make sure that I have the altitude set
correctly in my unit so I'm kinda at a loss. My Dewpoint readings are
also off.... are they related? Thanks
 
Dan (CW5688)
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