[wxqc] Sensor drift

[dr.]

You are close, If you adjust the pressure using temp then you get pressure
alt.  This is the effective alt and is used by pilots to see if they have
adequate runway length to reach a Indicated air speed that will allow them
to take off.  Airports Auto Terminal Reporting Systems (ATIS) IS NOT
CORRECTED FOR TEMP!  It is again the setting to use in the Koltsman window
of the Altimeter so when you land your altimeter reads the field Alt which
is the amount the field is above the ocean or normally called MSL Mean Sea
Level.

On a davis VP and you set the alt of your station to "0", then the console
as you said does not do a calculation using temp in the equation.  This is
direct from Davis as of V5 of their software/firmware combination. You are
correct, no mater what Davis says this still will not work and is posted as
such on the CWOP web site.  Maybe some day they will get the idea and modify
the firmware but alas, I have been told this will not happen!  Stubborn
bunch aren't they!!!!!!!

Ms. N5IHE



> -----Original Message-----
> From: wxqc-bounces at lists.gladstonefamily.net 
> [mailto:wxqc-bounces at lists.gladstonefamily.net] On Behalf Of spamfree
> Sent: Tuesday, May 23, 2006 11:50 PM
> To: Discussion of weather data quali
> Subject: Re: RE: [wxqc] Sensor drift
> 
> 
> Hi,
> 
> What he (and Davis) are saying is that you should set your 
> console elevation to 0 ft (this effectively turns off the 
> internal sea level reduction algorithm so only sensor/station 
> pressure is displayed) and then calibrate the barometer to 
> match a nearby airport. The calibration sets a fixed 
> barometer offset that is added or subtraced from the sensor 
> pressure to obtain the displayed pressure. Based on my 
> understanding of pressure, I think this is not good advice, 
> especially at higher altitudes. Here's why:
>  
> Let's say I have a VP2 console with a perfect barometer, and 
> I have it sitting right next to the barometer at an ASOS 
> station at an airport at 4000 ft. elevation. I set the 
> elevation on my console to 0 ft., and since it's perfect, it 
> shows a sensor pressure of 860 mb which perfectly matches the 
> sensor pressure of the airport's ASOS barometer. A week later 
> I see that the sensor pressure now shows 890 mb which again 
> perfectly matches the ASOS barometer's sensor pressure. 
> 
> Now that I am sure my barometer sitting next to the ASOS 
> barometer is perfect, I decide to take Davis' advice and make 
> it show altimeter. Although the ASOS barometer has a sensor 
> pressure of 890 mb, the airport's current altimeter value 
> derived from that sensor pressure and elevation show 1029.74 
> mb (see 
> http://www.srh.noaa.gov/elp/wxcalc/altimetersetting.shtml for 
> calculation). So, I set my perfect console to 1029.74 to 
> match the airport's altimeter setting. Internally, the VP2 
> sets a calibration offset of +39.74 mb (i.e. 890 + 39.74 = 1029.74).
> 
> A week later I return to the airport. The pressure has since 
> fallen, and the ASOS sensor pressure is now back down to 860 
> mb. The derived altimeter value for the airport is now 995.96 
> mb. Internally, my perfect VP2 console also has come up with 
> a sensor pressure of 860 mb. But, much to my amazement, my 
> perfect console is displaying a pressure of 899.74 mb (860 + 
> 39.74 = 899.74). Somehow my perfect console is off by over 6 
> mb. It doesn't match the airport's altimeter that I 
> calibrated to, and now my CWOP quality check has a big red X. Hmmmm!
> 
> If you have a VP station, and you report data to CWOP, and 
> you are not near sea level, then if you set the console's 
> elevation, and submit the console's sea level pressure you 
> will drift off the surrounding stations that report true 
> altimeter as temperature changes. If you set the console's 
> elevation to 0 ft and calibrate to a nearby station you will 
> drift off the surrounding stations that report altimeter as 
> the pressure changes. The only way you'll stay where you 
> should be is to use the accepted pressure formulas to convert 
> the VP's SLP or sensor pressure to an altimeter value and 
> submit that. Currently WeatherDisplay, VPLive, and 
> PC-WeatherStation do that. I don't know of any other software 
> that does.
> 
> Steve
>   
> ======= At 2006-05-23, 17:30:36 you wrote: =======
> 
> >Actually, either Davis misspoke or your misunderstood them.  If you 
> >don't apply an altitude correction to the barometer - either via the 
> >console or via software (which will then only reflect the 
> correction in 
> >the software and not on the console), you will display "station 
> >pressure" - the actual barometric pressure at your station.  
> Use of an 
> >altitude entry allows for a correction to sea level, which 
> is what is 
> >reported in media sources and by NWS.  Altimiter pressure is 
> calculated 
> >slightly differently as Steve has stated (and is indeed what CWOP 
> >ideally would like to have reported, but at present, only VPLive and 
> >Weather Display can report altimeter pressure to my
> >knowledge.) 
> 
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