Airpressure BME280 needs to be "normalized"

[elythomaslumber]

Have question about air pressure measured by the BME280. I'm running a homemade weather station only including a BME280 as well as two simple commerical weather stations. To get them synchronized on air pressure value I need to "normalize" the air pressure (air presuure on sea level). In the commercial weather stations I can simply set an offset.
Is that possible in your sketch? I cannot find any feature.
Regards
Hardy

[elythomaslumber]

I have simple added an offset (+18) to the read out BME280 air pressure value. The behavoiur is very linear. Now the value follows exactly the air pressure value of my commercial devices where those are "normalized" to sea level.

[elythomaslumber]

125-63 Add conversion to sea level pressure for BME280 sensor

Great!!!

[brownrb]

Today I updated the ASCOM OC driver for mySQM+.
Just a heads up -
This quote is from the ASCOM developers guide - Pressure in an ASCOM OC driver.
This must be the pressure at the observatory altitude and not the adjusted pressure at sea level. Please check whether your pressure sensor delivers local observatory pressure or sea level pressure and, if it returns sea level pressure, adjust this to actual pressure at the observatory's altitude before returning a value to the client.

I was trying to remember why I had not implemented that in the firmware and only reported pressure at the obs (or place where mySQM+ controller is located).

Now that I put it in there before doing the ASCOM driver, its a bit too late now to rip it out.

regards
Robert

[elythomaslumber]

Hi Robert,

just found your answer after I have updated to the 125.63.

There seems to be a lot of confusion in the world of barometric pressure... :-)

I try to explain the situation but I don´t know if there should be a change in the next relases. Maybe you will think about it or with your team.

1. The old FW version does only show the measured value. Since the local professional weather station reports the normalized (SL) pressure I set my 2 consumer weather stations as well as my home made old BME280 simple weather station to a static offset value of "+18". All perfect, all values are always synchronized!
   I did this also with the mySQMPlus 122.24 FW, working... The web-server shows the corrected value and propagates also the corrected value to Weatherunderground as most other station here in the area do.

2. Now today I installed the new FW 126.63 FW.

3. The BME sensor shows 1008.xx without a static offset value.
4. Now I can see also a SL corrected value showing 1031.xx in the browser
5. Then checked all my other devices and the official value: 1026 (SL)
6. that means, the corrected value is "5"hPa higher than the corrected value in my other devices as well as officilaly shown.

That also means, if I would correct the measured value by a static value of "+18", I would receive the same value as officially shown by a weather station. But then the SL corrected pressure in the FW would than be at 1049.xx
To correct this I set the location height from 185m to 0.0m.
Now the SL corrected value is ok but the same value as the non corrected in the browser...

Hard to follow I think...

From my point of view it seems that there is a bug in the FW which corrects the measured value to the SL value. The correction should be set the 1008.xx hPa(measured at 185m height) value to 1026.xx hPa (SL) value.

Also I think that the corrected value at SL should be propagated to WeatherUnderground,... but not sure about the rules there. Most stations do in my area, some have different values much lower.

Yes confusing. Not too much important to me. For weather forecast it is more important if the pressure goes up or down and in what time frame...

Best regards,
Hardy