Equation for Relative Humidity at a given Temperatur from Relative Humidity

Hello i have a Project where i use 2 BME680 one as air intake sensor and one as air outtake sensor

Now i want to calculate if Intake Sensor is at (20Celcius and 70%) which relative humidity i get if i take the temperature of the outtake sensor (so i have a rough estimate at which relative humidity the intake air ends in the room)

Is there somebody who already did this and can maybe share the needed calculation since i kinda struggle to get the right formulas from google

I guess that could be simple:

humidity in room = humidity(intake)-humidity(outtake)

Well that would give me the difference between intake and outtake but because humidity is temperaturecompensated a formula is needed

i need the intake relative humidity to be calculated with the outtake temperature (for each sensor the BSEC Bosch Library does exactly this but i need need to calculate from intake humidity and outtake temperature, so i know which relative humidity the intake air is gonna end with at the outtake sensor)

This makes no sense to me. Please try again to explain what you are trying to do.

I suspect your best, likely ONLY, option is a multi-dimensional lookup table, and multiple interpolations. The table would be organized either as humidity at constant temperature, or temperature at constant humidity. If pressure is not constant, it gets even more complicated. I doubt the relationships can be easily boiled down to an equation, as there are too many variables, and the relationships are discontinuous, especially at saturation.

I mean how does the bsec bosch library it and any other sensor with temperaturecompensation?

I just wanna take the raw (before actual temperaturecompensation of the library) humidity data from intake sensor and pretty much compensate it with a different sensors temperature , nothing more

tho if its really needed the BME680 also has a pressure sensor, i saw those equations too but it was too complicated for me to make something out of it

i thought i just need to temperaturcompensate the (raw) intake humidity with the outtake temperature to get an estimate humidity level after intake air travels to the outtake sensor (since the air would warm up and i need that actual value after "warming up")

A 3rd, independent humidity sensor in the room might be the thing that gets the do done?

Nope also not a option, in a nutshell i wanna check if its worth to take outdoor air (intake sensor) into the room, thats why i wanna have a humidity readin which is temperature compensated with the indoor air, that why i should get an estimate what humidity ends up in the room and can look if its worth taking THAT air in

the same i do for temperature, but temperature doesnt need to be compensated with anything, thats why this part should be easy..

a 3rd sensor would be redundant because i already have an outtake sensor

Might try calculating dewpoint from intake data, then calculate humidity from outtake temp and dewpoint.

this sounds like a option but the problem remains, my IQ is not high enough to get those equations right :smiley:

Do you have google on your internets over there?

This might move things along:



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I'll need to look at it later when I have more time, but humidity will increase roughly 5% for each drop in temperature of 1 degree Celsius.

Relative humidity is not temperature compensated, however temperature is a factor.

Perhaps you want the Dew Point. Google Dew point for equations. My recollection there is an exact calculation and a less precise calculation.

Well i found this

Td = T - ((100 - RH)/5.)

where Td is dew point temperature (in degrees Celsius), T is observed temperature (in degrees Celsius), and RH is relative humidity (in percent). Apparently this relationship is fairly accurate for relative humidity values above 50%.

Tho the more accurate calculation is too complicated for me again and needs conversion to kelvin

This here also seems to be a good (and easier to understand) paper even with different accuracy ranges (see the table 1) i would probably like to use -20C to 50C https://web.archive.org/web/20200212215746im_/https://www.vaisala.com/en/system/files?file=documents/Humidity_Conversion_Formulas_B210973EN.pdf


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