Derivation of the humidity value ?

[Bernhard Lauss <lauss@berkeley.edu>]

Dear Marat,

I have been thinking a little about our interpretation
problem with the humidity content versus capture yield,
and this prompted that I have to ask you to explain
the way the humidity is calculated on your CHUPS display.



Please correct my view where it is wrong:

The Pura device is a dew-point transmitter which has a current
output signal proportional to the dew point.

The company provided a calibration curve: current versus dew point

Your program somewhere calculates from the dew point temperature
a partial water vapor pressure [mbar] and this is equivalent
to the moisture concentration in ppm / 10^6.




Now I have the following problem:

There are several ways of getting the water vapor pressure from
the dew point temperature, and I think this is a very important part
I do not yet understand how you do it.

1) Is it correct to take the water vapor pressure over ice
   or over liquid water for our application.
   the resulting saturation pressure differ by a factor ~2
   The table which we got, I think from the PNPI chemists, only
   quotes the pressure over ice.


2) What formula do you use for calculating the saturation pressure
   and what coefficients ?

A good example of what different formulas are accurate for different
temperature ranges is given at
http://hurri.kean.edu/~yoh/calculations/satvap/satvap.html

The comparison plot there shows that they differ quite a lot,
the Goff-Gratch Equation seems to me to be state-of-the-art
to be used for MuCAP conditions.


You can download from the Michell company
website a humidity calculator which calculates the humidity nicely:
http://www.michell.co.uk/downloads/index.html


Please explain how you derive from the measured current
the dew point and the partial pressure and concentration.
I did not find an elog on that, maybe you ave it explained
already somewhere, please post it.


			Thanks a lot

 			  Bernhard



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Bernhard Lauss                 E-Mail: lauss@socrates.berkeley.edu
Physics Department
366 LeConte Hall
University of California @ Berkeley
Berkeley, CA 94720                             Tel: (510)-642 4057
United States                                  Fax: (510)-642 9811
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