Air pressure measurement

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iatros
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Air pressure measurement

Post by iatros » Sun Apr 29, 2018 9:23 am

Since I got this
baro_ssmall.jpg
baro_ssmall.jpg (47.47 KiB) Viewed 4086 times
beautiful anaeroid barometer (showing air pressure at station level in mmHg) I dug a bit in the subject.
MT shows pressure values from the Meteostick, the Davis Console instead has its own pressure sensor and shows a slightly different value.
Both sensors of course measure the pressure at station level (QFE) and convert it with one of the numerous formulas in sea level pressure (QNH).
The precision of the sensor measures is 1 hPa (Console) and 0.1 hPa (Meteostick and my Galaxy S6 Edge).

Trying out different formulas to calculate the reduced air pressure at sea level showed little differences. Given the extreme variability of the air pressure QFE and the hazardous calculation of pressure QNH a precision of 1 mb seems good enough.

Davis support gave me a detailed information about how the Console is calculating QNH. Boris Pasternak (Meteostick) refused to answer the same question and instead accused me to argue with him about the best formula to use...

Anyone happens to know more about the Meteostick way to calculate QNH?
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Re: Air pressure measurement

Post by andyk1 » Sun Apr 29, 2018 11:40 pm

Really a tough one to answer since I see your MT reads as 999.2 and your analog as 718 or so. I am guessing you have some elevation to consider. If you are above sea level your analog will not take that into account. MH more than likely calculates your elevation and just guessing that is your 999.2 average. I know the formula but in inHg which will not help you.

If I knew your actual elevation I maybe could look it up for you.

Andy
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Re: Air pressure measurement

Post by iatros » Mon Apr 30, 2018 6:02 am

Hi Andy,

thanks for your reply.

It is easy to find a formula to calculate QNH for your station elevation.

There are more factors like temperature and humidity which influence the density (the "weight" and consequently the pressure) of that hypothetic column of air which extends from your station down to sea level.
Temp and Hum of your station are well known, but you have no idea about those figures at the place where your column of air touches sea level.
You have to make a lot of guesses, therefore there are quite a few different barometric formulas which more or less end up in the same QNH value.
More or less - there is always a difference of 1-3 hPa between MT (representing the MeteoStick reading) and the Davis Console (own measurement).

I was just curious if anyone could tell me the specific formula used by Pasternak in the MeteoStick.

Cheers, Martin
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Re: Air pressure measurement

Post by andyk1 » Mon Apr 30, 2018 6:21 am

iatros wrote: Mon Apr 30, 2018 6:02 am I was just curious if anyone could tell me the specific formula used by Pasternak in the MeteoStick.

Cheers, Martin
OK I understand you now and you are correct. Temp and humidity do play a factor and no I don't know what he uses. I have a meteobridge and can't get him to fix the led's on a TL-WR902AC. I think I'm the only one using one but no indicator led's to let you know it's working or connected to internet. I can ssh in manually but the settings don't stick when I reboot. Anyway... since Boris he is a business (smartbedded.com) he is not keen on letting that out I am guessing.

Re-reading your original message. Did you by chance check to see if only one, Davis or MS was used for calibration? I made that mistake once in MB and the Ambient ObserverIP not realizing I had different calibrations set because I originally started with only the ObserverIP not zeroing it out and trying to use the MB and getting two different reading. Just a thought.

Andy
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Re: Air pressure measurement

Post by iatros » Mon Apr 30, 2018 9:21 am

Davis or MS used for calibration
What do you mean?
I have 4 readings:
My Galaxy S6E (QFE),
MS(QFE and QNH),
Davis (ONH only) - and
my analog device (QFE)
which one would you use?
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Re: Air pressure measurement

Post by andyk1 » Mon Apr 30, 2018 6:22 pm

I'm a bit confuse why you would include the analog device as that is not connected to you meteostick. .
I would start with calibration of the closest device to your array so then davis is what I would use for the calibration and set all other devices back to default if I'm reading his correctly.

Andy
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Re: Air pressure measurement

Post by iatros » Wed May 02, 2018 6:41 am

andyk1 wrote: Mon Apr 30, 2018 6:22 pmI'm a bit confuse
so am I; I don't get what you try to explain me.
I have four instruments with four different readings. My simple question is, which instrument gives me the most accurate value of air pressure?
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