Pico Technical Support and Discussion Forum

GrahamP · Guest

I need to record temperature and humidity in an Environmental Cabinet. The cabinet has a Temperature control range of –40 to +150 and a Humidity control from 0 – 95% RH.

Most humidity sensors I have seen are only suitable for temperatures up to 85 deg C. I was thinking of using a Pico Logger and wet and dry thermocouples, but how can I convert them to RH in real time.

matthew · Posted: Mon Feb 17, 2003 5:34 pm Post subject:

Hi Graham,

If you are talking, 40 to 150degC then our humidity sensor won't be able to cope with this either.

I can't remember the equations for relative humidity now (its been a while Rolling Eyes

). The correct terminology is Wet Bulb Temperature, try typing it into a search engine; I'm sure there will be a fair bit of information available.
Let me know if you find any good info.

Kind Regards,

Guest · Posted: Tue Feb 18, 2003 8:35 am Post subject:

Hi Matt

Thanks for that, but what I was trying to find out was is it possible, in the pico log software, to use 2 channels, the dry and wet bulb temperatures, use them both into an equation with the result of an extra derived channel of RH in real time?

regards
Graham

matthew · Posted: Wed Feb 19, 2003 8:35 am Post subject:

Yes, no problem ... if you know the equation! Smile

Regards,

GrahamP · Guest

Hi

Well Mat, never wanting to be out done, and always liking a challenge, here you are. But will it work in PicoLog!!!!

C2 = Dry bulb temperature in Deg C
C3 = Wet bulb temperature in Deg C
Assuming a barometric pressure of 1013 mbar

Relative Humidity =

100*((POWER(2.71828,(((16.78*C3)-116.9)/(C3+237.3)))-(0.00066*((1+(0.00115*C3)))*104*(C2-C3)))/POWER(2.71828,(((16.78*C2)-116.9)/(C2+237.3))))

Thanks to Qwest.net for the formula.

If you put this formula into Excel, you can check the answers at:

http://www.users.qwest.net/~daveoc/demo/wetbulb.htm

Regards
Graham

matthew · Posted: Fri Feb 28, 2003 11:46 am Post subject:

Hi Graham,

The equation shouldn't be a problem for PicoLog once you've converted it to our notation (as detailed in our help files).

Best Regards,

GrahamP · Guest

Hi
Sorry but I used powers instead of exponents in the equation. I have tested this one and it does work

%RH= 100*((EXP(((16.78*C3)-116.9)/(C3+237.3))-(0.00066*((1+(0.00115*C3)))*101.325*EXP(0.0001184*C4)*(C2-C3)))/EXP(((16.78*C2)-116.9)/(C2+237.3)))

Where:
C3 is the wet bulb temperature in degrees centegrade.
C2 is the dry bulb temperature in degrees centegrade.
C4 equal to the height above sea level in metres.
(101.325*EXP(0.0001184*C4) (in red) is the air pressure in kPa. This section can be replaced by the barometric pressure if known.

Regards
Graham

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