Author Topic: Baro calibration.  (Read 451 times)

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Offline OldAlaskaGuy

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Baro calibration.
« on: August 12, 2021, 04:42:46 PM »
I am a bit unsure about my BARO sensor calibrations. I used this calculator https://caboose700.github.io/ws-barometer-offset-calculator/. I first set both BARO offsets to 0 and used the live data in IP Tools for the pressure calculations and METAR data from my local airport about 7 miles away. My CWOP station ID is EW5416. The array is an Ambient WS1401-IP and using IP tools as an interface. Galfert helped me several years ago with this issue.
Data used during calculation: Console elevation 42.97 meters, Current rel 1008.1 hPa, Current abs 1008.1 hPa, Nearest METAR, Kenai, Alaska Airport (ALT) 1007.5 hPa
The correction suggested was to set offset for the station BARO to +5.1 hPa, the Absolute offset to -5.7 hPa.
After calibration, the absolute pressure (altimeter) matches the airport MADIS, however on the Ambient CWOP page my ABS pressure appears to be higher than some of the other local stations. These station's data has not been flagged as bad.
http://www.findu.com/cgi-bin/wxnear.cgi?call=EW5416    https://en.allmetsat.com/metar-taf/alaska-anchorage-fairbanks.php?icao=PAEN
I just need a bit of reassurance that my data calculations are correct and that the BARO data may be incorrect from some of the other stations. There seems to be quite a disparity in ABS pressure between stations locally. As a data provider to CWOP I want to be confident that this data correct.
« Last Edit: August 12, 2021, 08:00:18 PM by Weatheroger »

Offline gszlag

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Re: Baro calibration.
« Reply #1 on: August 14, 2021, 01:46:21 PM »
I am a bit unsure about my BARO sensor calibrations. I used this calculator https://caboose700.github.io/ws-barometer-offset-calculator/. I first set both BARO offsets to 0 and used the live data in IP Tools for the pressure calculations and METAR data from my local airport about 7 miles away. My CWOP station ID is EW5416. The array is an Ambient WS1401-IP and using IP tools as an interface. Galfert helped me several years ago with this issue.
Data used during calculation: Console elevation 42.97 meters, Current rel 1008.1 hPa, Current abs 1008.1 hPa, Nearest METAR, Kenai, Alaska Airport (ALT) 1007.5 hPa
The correction suggested was to set offset for the station BARO to +5.1 hPa, the Absolute offset to -5.7 hPa.
After calibration, the absolute pressure (altimeter) matches the airport MADIS, however on the Ambient CWOP page my ABS pressure appears to be higher than some of the other local stations. These station's data has not been flagged as bad.
http://www.findu.com/cgi-bin/wxnear.cgi?call=EW5416    https://en.allmetsat.com/metar-taf/alaska-anchorage-fairbanks.php?icao=PAEN
I just need a bit of reassurance that my data calculations are correct and that the BARO data may be incorrect from some of the other stations. There seems to be quite a disparity in ABS pressure between stations locally. As a data provider to CWOP I want to be confident that this data correct.
Some things are slightly amiss. Your REL reading being equal to your ABS means you have lost your calibration settings. Here is a manual way of calibrating:

Your elevation offset/correction of 5.1 hPa is correct (determined from keisan online calculator). At your elevation ,your REL (Relative Pressure)should always be 5.1 hPA higher than your live ABS reading.

Therefore enter 5.1 as your relative pressure offset.

An ABS offset would be required if your barometric sensor is not accurate and needs a correction/offset.

If you are using CWOP you would need to set your REL to Altimeter rather than to METAR SLP. CWOP uses Altimeter only.

At your reference METAR station PAEN, currently Altimeter = 1008.5.
Now subtract the elevation offset of 5.1. The result is 1003.4 which should be your ABS.

Check your Live Data tab. What is the ABS being reported? If it shows 1003.4 you are done.

If not, then your baro sensor needs to be calibrated by adding or subtracting a ABS offset amount so that your ABS reading shows 1003.4 and REL shows 1008.5.

Compare these results to what the online calculation tool suggests. If they are the same then you've calibrated correctly.

You should not be comparing other stations ABS or comparing station pressures as each of these stations are at different elevations and will certainly have different station pressures. The point is , is that you want to compare pressures at a common denominator - sea level so you can make valid comparisons.

[edited: added closing paragraph]

In other words, if you are comparing with other CWOP stations, everyone should be using Altimeter.so use that measure to compare.
« Last Edit: August 14, 2021, 02:12:12 PM by gszlag »
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Offline OldAlaskaGuy

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Re: Baro calibration.
« Reply #2 on: August 14, 2021, 02:56:53 PM »
Thank you for your help. I have made the corrections per your suggestions and am now confident that my data is good. I am currently waiting for an email from MADIS after the QC for my station has passed.