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[Solved] What is the calculation for absoulute humidity?  

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SetMin
(@setmin)
Trusted Member

What is the calculation for the two selectable absoulute humidity measurements?

Thanks 👍 

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Posted : 09/01/2019 10:37 am
Pierre Lannoy
(@pierre-lannoy)
Member Admin

Hello. I'm sorry, I doesn't really understand you question. Could you explain me what you're looking for?

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Posted : 14/01/2019 8:35 am SetMin liked
SetMin
(@setmin)
Trusted Member

My netatmo measure relative humidity and temperature.

To create the absolute humidity there must be any kind of calculation in the plugin.

What is the calculcation to display the absolute humidity?

 

Thx

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Posted : 14/01/2019 1:15 pm
Pierre Lannoy
(@pierre-lannoy)
Member Admin

You want to know the formulae used by Weather Station, is that correct?

If so, here it is for partial absolute humidity:

pAH = 0.622 * pVap / (p - pVap)

where pVap is the partial vapor pressure

and for saturation absolute humidity:

sAH = 0.622 * sVap / (p - sVap)

where sVap is the saturation vapor pressure

Note: you can check all the formulas used in /live-weather-station/includes/traits/CommonUtilities.php

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Posted : 14/01/2019 1:43 pm SetMin liked
SetMin
(@setmin)
Trusted Member

Thx.

Why is the formula so short compared to the formula used here?
https://carnotcycle.wordpress.com/2012/08/04/how-to-convert-relative-humidity-to-absolute-humidity/

 

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Posted : 14/01/2019 2:19 pm
Pierre Lannoy
(@pierre-lannoy)
Member Admin

Because it includes the computation of vapor pressure. In Weather Station, the 2 computations are done separately. 

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Posted : 14/01/2019 2:31 pm
SetMin
(@setmin)
Trusted Member

Hi @pierre-lannoy

Is the result of the two formulas then the same plus/minus some percent?

Or something completely different?

Thx

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Posted : 15/01/2019 9:58 am
Pierre Lannoy
(@pierre-lannoy)
Member Admin

Hello.

Unfortunately it will be different because its formula is wrong. You can not compute absolute humidity in g/m³ if you don't involve the air density. To compute air density, you must use current atmospheric pressure. The computation you point out doesn't use pressure and, so, can not be in g/m³...

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Posted : 15/01/2019 5:04 pm SetMin liked
SetMin
(@setmin)
Trusted Member

Hi Piere,

The text on the site says "...requires an added algorithm that generates saturation vapor pressure as a function of temperature".

I understand this like that the pressure IS  in the forumula.

best regards

SetMin

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Posted : 15/01/2019 5:10 pm
Pierre Lannoy
(@pierre-lannoy)
Member Admin

Ah, yes, you're right, I did not see that.  😇 

The vapor pressure is in the formula, not the atmospheric pressure (the pressure measured at the station level)...

If you like Weather Station, please consider to make a review to help make it known. That would be the best way to thank me...

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Posted : 15/01/2019 5:18 pm

  
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