Test and Measurement Forum

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.

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 ). 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,

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

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

Regards,

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

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,

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