[wxqc] Davis VP barometer clarification

Evan Bookbinder Evan.Bookbinder at noaa.gov
Sun Jul 23 21:46:28 EDT 2006


Obviously, there is still a lot of confusion out there from what exactly 
needs to be done to get the Davis VP2 barometer correct both internally and 
in terms of transmitting to CWOP/APRS. I hope these instructions will remove 
a lot of the QC problems we're seeing.

Console:

    - Set your elevation in the console to the elevation of the console 
itself. For example, my house at ground level is 968 feet, and the console 
is sitting in my 2nd floor office next to the computer -- at a height of 980 
feet.

    - Although indoor temperature flucutations are generally minor, let the 
unit sit for a good 12-14 hours.

    - Next, adjust the console as necessary to the SEA LEVEL PRESSURE of the 
nearest official observation site (or an interpolation between two). Note 
that I said SEA LEVEL PRESSURE, as this is what is displayed on the console 
(a very useful meteorological value). How do we do this? Here's the easiest 
way I've found.

    1.) http://www.uswx.com/us/wx/
    2.) Click on your state
    3.) On the left hand side menu, click the purple "Stations" link
    4.) You will see a list of all the official weather stations in your 
state. Click on the 4 letter identifier for the observation sites closest to 
your location.
    5.) Making sure that the observation is current, you will see the 
"Pressure" item in the table, which correctly uses the Sea Level Pressure 
for that site. They display the pressure in both millibars (which I prefer 
to display on my console) or inches of mercury. Your console should be 
adjusted to THIS number (or interpolated between surrounding sites). It's 
always recommended that you pick a day with light wind, which should 
correspond to similar pressure readings in your area.
    6.) NOTE: DO NOT use the Axxxx group in a coded METAR observation, or 
the pressure given by calling into an airport observation site. These are 
altimeter settings, and not what is required on the Davis VP2 console.

    - From here on out, your console should run smoothly in tandem with the 
official observation sites. My console is generally dead on or about 0.1 mb 
different from the nearby airport site, exactly what we want to see.


CWOP/APRS:

    Now, for whatever reason, CWOP/APRS uses altimeter setting, NOT Sea 
Level Pressure as noted above. This may be due to the limitation of other 
manufacturer's instruments reporting into the network. In any event, and as 
discussed in previous threads, these are not one in the same. Altimeter 
setting is primarily used for aviation purposes, while sea level pressure 
(adjusted for temperature) is used for meteorological purposes.

    Now, theoretically, you should have to do nothing further here. The 
software you are using to send CWOP/APRS data *should* internally calculate 
the altimeter setting from your console's sea level pressure data and 
transmit it.

    Again, MAKE SURE THAT YOU ARE USING SOFTWARE THAT SENDS ALTIMETER 
SETTING DATA. If you are unclear, contact the software's author and make 
sure that they are calculating this correctly.

    VPLive works fantastic, as does WeatherLink 5.7.

  ***** For those using WeatherLink 5.6 or lower (which is a fairly common 
problem from what I've seen), PLEASE make sure that you upgrade to V5.7 
immediately. http://www.davisnet.com/support/weather/software.asp#vantagePro 
*****

    Only V5.7 correctly sends altimeter setting. V5.6 and earlier versions 
incorrectly sent the sea level pressure directly from the console.

    As a plea to Davis, in the APRS data string, it would really be nice to 
see the software version number included with the generic "DsVP" in future 
updates. This really makes troubleshooting a lot easier -- for example 
"DsVP57".


Respectfully,
Evan Bookbinder
Senior Meteorologist
NWS Kansas City/Pleasant Hill MO 



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