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Author Topic: Barometer Calibration  (Read 7991 times)

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Offline Stuntman

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Barometer Calibration
« on: December 01, 2010, 11:48:06 PM »
I want to calibrate my barometer but am not sure if I am doing this right..

I have a known good barometric number from a nearby airport that is updated constantly... the airport lies at an elevation of 1513'.  My station is at an elevation of 2050'..
I thought I had read that pressure drops about 1% per 250' of elevation gained.. if that was the case I'd need to lower my barometer just over 2% from the reading at the airport??

So, if the airport was 30.02inHg I would set mine to 29.38inHg?

30.02 * .9785

Is this right???????

Thanks!

Offline niko

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Re: Barometer Calibration
« Reply #1 on: December 02, 2010, 12:34:28 AM »
It depends, normally everyone reports baro as the equivalent sea level value regardless of their actual elevation ASL, in other words unless the barometer is at sea level the reported value will be higher than the measured value (unless it's below sea level in Death Valley in which case it will be lower than the measured value). If your airport number is a sea level number then that's the same number you would want to display, if it's not a sea level number then you would have to do some calculation.

What airport (so I can look at its data)?

Offline Stuntman

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Re: Barometer Calibration
« Reply #2 on: December 02, 2010, 02:32:35 AM »
Thank you for the reply!  I don't know what value the airport is reporting.. I listened to an automated message that I called from the airport.. 
The airport is Hemet-Ryan, in Hemet, California.. This is what I found...

Latitude: 33 44' 2" N (deg min sec), 33.7340 (decimal), 3344.04N (LORAN)
Longitude: 117 1' 21" W (deg min sec), -117.0225 (decimal), 11701.35W (LORAN)
Elevation: 461 metres (1512 feet) -- validated against 460 metres (1510 feet) from NED Contiguous U. S. 1/3W arc second elevation data
Location: Hemet-Ryan Airport, CA, United States of America
County: Riverside, CA
Forecast Offices: San Diego (SGX), Phoenix (PSR)
Automated weather phone (AWOS-3): 951-925-6886

The other question is how to configure Weather-Display and CWOP with the Davis Vantage Pro 2 once I get the number on the console correct???


Offline niko

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Re: Barometer Calibration
« Reply #3 on: December 02, 2010, 02:57:24 AM »
Hmmm, is March relatively close? They seem to have more comprehensive weather, search for KRIV in http://www.runwayfinder.com/ and click on March, KRIV, they are showing altimeter: 30.050198 at 7:55pm PST which is a sea level value.

Update: Looks like Hemet uses the weather from March/KRIV and doesn't have its own official station.
« Last Edit: December 02, 2010, 03:07:14 AM by niko »

Offline Stuntman

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Re: Barometer Calibration
« Reply #4 on: December 02, 2010, 04:44:53 AM »
March ARB is about 30 miles to my West and probably 500 lower in elevation.

So, should I calibrate the Davis to show that pressure?  Does my elevation not matter?


Also.. on Weather-Display, under the CWOP barometer section.. there is some stuff there about 'Send Altimeter Pressure' and 'Use this pressure for WD'

Does CWOP want the altimeter pressure then, which I'm assuming is not the same as the sea level pressure I'd be calibrating the unit to?

Sure appreciate the help!

Offline NorCal Dan

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Re: Barometer Calibration
« Reply #5 on: December 02, 2010, 04:53:39 AM »
http://www.globalair.com/airport/apt.weather.aspx?aptcode=hmt


It looks like Hemet is using KRIV for their wx conditions.  Here's the metar report:

Quote
METAR text:    KRIV 020455Z AUTO 26003KT 10SM CLR 06/M10 A3006 RMK AO2 SLP185 T00631104
Conditions at:    KRIV (RIVERSIDE/MARCH , CA, US) observed 0455 UTC 02 December 2010
Temperature:    6.3C (43F)
Dewpoint:    -10.4C (13F) [RH = 29%]
Pressure (altimeter):    30.06 inches Hg (1018.0 mb)
[Sea-level pressure: 1018.5 mb]
Winds:    from the W (260 degrees) at 3 MPH (3 knots; 1.6 m/s)
Visibility:    10 or more miles (16+ km)
Ceiling:    at least 12,000 feet AGL
Clouds:    sky clear below 12,000 feet AGL
Weather:    automated observation with no human augmentation;
there may or may not be significant weather present at this time

Offline Stuntman

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Re: Barometer Calibration
« Reply #6 on: December 02, 2010, 05:09:05 AM »
I assume that the barometric pressure I see on my Davis VP is the sea level pressure then... so if I get that to match the pressure at March ARB then the VP will calculate the altimeter pressure based on the altitude I entered at setup?  Then I tell WD to use that altimeter pressure value to send to CWOP?  WD uses the sea level pressure coming from the VP then (unless that second box is ticked in the CWOP settings)?

Right?


Offline niko

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Re: Barometer Calibration
« Reply #7 on: December 02, 2010, 05:27:20 PM »
All I can tell you about CWOP is that there are a lot of posts discussing which is the correct baro reading to send  :lol:

Back to something I might know about though, if you set the altitude correctly on your console it should read close to the value that March is reporting, if not then there's something strange happening.

Offline Stuntman

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Re: Barometer Calibration
« Reply #8 on: December 02, 2010, 07:33:34 PM »
Man, I didn't know I was getting into so much when I came up with the thought.. "I wonder if my barometer is right?"
 :D

I'll go into the Davis VP and set the Baro to the sea level pressure reported at March Air Base, and then look to see if the Altimeter Pressure that WD reports to CWOP is within reason.. that value should, if I understand this at all, be lower than the Altimeter Pressure reported at March, as that value takes your station elevation into account in determining the value..

We'll see!

Thanks for the help!!!!!!!!!!!!!!!!
 :occasion5:

Offline KeithBC

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Re: Barometer Calibration
« Reply #9 on: December 02, 2010, 11:32:57 PM »
1. Forget elevation.  As Niko said, almost all barometer readings are corrected to sea level.  The reason is so that you can see how the weather systems are moving across the country without always having to account for elevation differences.

2. When the station you are calibrating from is a distance away, you want correct for the distance, not the altitude.  Find a current weather map that shows isobars.  From the map figure out the pressure gradient in millibars per mile between you and the station.  Then multiply the pressure gradient (mb/mi) by the distance in miles, and you have the number of milibars difference between you and the station.  Add or subtract that number, as appropriate, from the station's reported pressure and that is the pressure you should set.

 

cumulus