[wxqc] Pressure (Altimeter) Calibration Tips for the UK Observers

[Dave Helms]

Handy tips for pressure calibration provided by Andy Overton/UK:  Mantra 
"trust, but verify!"

I've been carrying out some rough investigations into the calibration of 
AWS barometers as shipped from the factory.  Most manufacturers are keen 
to point out the accuracy of their instruments (typically +/- 0.5C) but 
this is rather marred by the poor calibration.  I have only had contact 
with Davis VP users so far, but I expect that my findings will be 
representative of all the manufacturers as Davis are one of the best.  
This has shown that barometers are typically out by up to 3mb either 
way.  However, they are accurate instruments once set up correctly.
 
Being one of the easiest things to recalibrate this is really something 
everyone should do.  Sources of accurate local barometric pressure in 
the UK are the Met Office Aviation Briefing Service, which gives METARS 
(you have to register but it's free) via the Home page at 
www.metoffice.gov.uk <http://www.metoffice.gov.uk> , or the BBC Weather 
Service at www.bbc.co.uk/weather <http://www.bbc.co.uk/weather> where by 
typing in a town name latest timed details, including pressure, from a 
nearby AWS are given.  Presumably other nations have something similar.  
(Note: In the US, call your local ASOS/AWOS automated recording for the 
current altimeter pressure.  Airnav.com has the phone #, example
http://www.airnav.com/airport/KMCI   provides this phone number: "WX 
ASOS: PHONE 816-243-6415")

No doubt those US CWOP members who are being data quality checked will 
have been alerted to any recalibration required but that leaves a lot of 
other people who might be carrying on in ignorance.  I know that the 
CWOP guide makes specific reference to this issue but one of my findings 
is that many people are taken in by the manufacturers' blurb, and by the 
fact that the instrument is precise to 0.1C and with no moving parts, 
into thinking that, especially when brand new, it must be working ok and 
not need checking.  Any prompting you can give to people to do a 
calibration check will probably improve data quality enormously.