[wxqc] Sensor drift

[Gary Oldham]

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.)  To quote from an earlier message on this general topic from
Steve, "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."

Here are some definitions of different barometric pressure measurements as
pertains to aviation:

QFE: 
The pressure corrected to the official airfield elevation. An altimeter set
to the particular airfield QFE reads zero when an aircraft is on the ground
(strictly the height of the altimeter above the ground). In the circuit, the
height indicated is the height above official airfield datum. 
QNH:
The pressure 'reduced' to mean sea level, assuming ISA temperature profile
from the station/airfield to MSL. An altimeter set to the airfield QNH reads
the elevation of the airfield when on the ground. 
QFF:
Barometric pressure 'reduced' to mean sea level, assuming an isothermal
atmosphere from the airfield/station to MSL, using current (screen)
temperatures. The difference between QFF and QNH can be considerable when
atmospheric conditions are significantly different from ISA: i.e. at 'hot
and high' airfields. 
QNE:
When the ISA mean sea level standard pressure of 1013.2 hPa is set on an
aircraft altimeter subscale, the height so indicated upon landing at an
airfield is known as the QNE reading. More widely, this is also the PRESSURE
ALTITUDE, which is alternatively defined as the height of any level in the
international standard atmosphere (ISA-see above), above the level
corresponding to a pressure of 1013.2 hPa. 



Gary
CW0146 

-----Original Message-----
From: wxqc-bounces at lists.gladstonefamily.net
[mailto:wxqc-bounces at lists.gladstonefamily.net] On Behalf Of dr.
Sent: Tuesday, May 23, 2006 4:49 PM
To: wxqc at lists.gladstonefamily.net
Subject: Re: [wxqc] Sensor drift

The source is sea level as all faa alt settings are that way the planes alt
reads the elevation of the field.  Davis says if you use "0" as the
elevation then the unit is suppose to read sea level alt ad to cal to a
known source such as a ASOS station.  The deming airport station is at
3414msl and I am at 4250msl  or diff of 835 ft.  Using this as a cal source
and just setting the Davis to match should not give a diff of 10 mb avg in a
24 hr time.  That is why it went to davis for repair it is boken and needs a
new sensor. Data was tracked hourly using wunderground.com for the readout
of the deming station KDMN.

So I was comparing apples to apples.

N5IHE
FAA advanced ground instructor



On 5/23/2006 2:07:43 PM, spamfree (spamfree at pensom.org) wrote:
> Hi,
> 
> Are you comparing apples and oranges? Comparing the console's Sea 
> Level Reduced pressure to an airport's altimeter value is not a proper 
> comparison unless the elevations of the two stations are at or very 
> near sea level. Since starting to use software that converts my VP2's 
> pressure to altimeter (which is what I submit to CWOP), my station 
> stays within 1 or 2 mb of the nearby ASOS airport station and I 
> haven't calibrated my barometer in over 5 months.
> 
> Either your unit was defective, or you are at elevation and are 
> comparing SLP to altimeter, which is an invalid comparison.
> 
> Steve
> 
> ======= At 2006-05-23, 12:05:00 you wrote: =======
> 
> >The following are the specs direct from the Davis web site:
> >
> >4, Wireless Vantage ProR & Vantage Pro PlusT Stations VANTAGE PRO 
> >Barometric Pressure (sensor located in console) Resolution

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