Barometer calculation

[wysiwyg]

Hello !

I just setup a pressure sensor on my orange pi (with the plan to make it work with weewx in the future).

My sensor is a MS5611, it reports absolute pressure, so some calculation have to be done to get the barometer value.

I try to check if I can do it by myself or if I shall rely on weewx calculation for that.

Here is what I found so far:

- weewx wiki https://github.com/weewx/weewx/wiki/Barometer,-pressure,-and-altimeter

it is stated here that barometer depends also on Temp, Temp 12h ago and even humidity.

- weewx code: if I understood correctly the content of formulas code, it's more simple. only use current temp, I didnt see temp -12h or humidity.

- I found a documennt on the web that explain how davis station calculate barometer (this time with temp, temp -12h and humidity)

- I tried to find how official weather organisation (like meteofrance for my country) calculate barometer, but no success til now.

- on some forum I found a simple formula using only altitude ( baro = pressure + 1mbar/8.36meter )

The MS5611 is a very accurate sensor, it can reach +/-1.5mbar accuracy (max value, at 25°C which is pretty true if used inside house).

So I would like to use the much accurate algorythm possible "at home". 

1st step could be to use weewx formula but then ? 

I wonder if the davis formula is really much better (as I was not able to confirm with any "official" weather organisation) or if some other algorythm could be more reliable ?

if possible I would like to make the calculation on my sensor side, so I can deliver the value to other than weewx ( in case I want to build my own TFT display one day).

PS: it's slightly confusing that the wiki talk about temp -12h and humidity while it's not used in weewx, (still If I understood correctly) , it may worth to display the weewx formula in the wiki

Best regards,

[wysiwyg]

also found this:

https://barani.biz/apps/sea-level-pressure/

and this:

http://barani.biz/apps/sea-level-pressure-advanced/?

1st one seems same complexity as weewx but slightly different result... (very slightly,  with same data input, I found 0.016mbar difference)

 

[wysiwyg]

ok, I found some more informations.

The formula here:

https://barani.biz/apps/sea-level-pressure/

seems to be a so called "international barometric leveling formula"

at least, it's stated with this name in french wikipedia

https://fr.wikipedia.org/wiki/Formule_du_nivellement_barom%C3%A9trique

I was not able to find it as clear in english wiki

https://en.wikipedia.org/wiki/Barometric_formula

[Thomas Keffer]

As explained in the weewx Wiki, barometer depends on the temperature 12 hours ago, but altimeter does not.

If your station emits pressure, that is, the raw gauge pressure, the others should be calculated by StdWXCalculate. Is that not happening?

-tk

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[wysiwyg]

Hello Tom,

My question is more related to understand theoritical calculations   :-)

Weewx works perfectly !

 but I try to understand what is done in stdWXCalculate.

I may have misunderstand, but my understanding is that calculation is:

barometer = pressure /  exp (-elev_meter / (t_K * 29.263))

is this correct?

t_K is current temp in Kelvin or ?

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[Thomas Keffer]

That's pretty much correct. See the function wxformulas.sealevel_pressure_Metric()

-tk

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[wysiwyg]

Thanks for confirmation!

Does this formula has a name or source?

The more I read, the more it seems complex to understand how to calculate barometer from pressure :-)
I mean there are many models/algorithm more or less complex

I think also that whatever the formula we talk about 0.0x mbar difference but for science knowledge it's still interesting.