Re: Thermal Conductivity Sensors

[Glenn Brown <gbrown@okstate.edu>]

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Elizabeth,

The relationship between the humidity above a solution and the thermodynamic
potential of the solution is given by

ht = (RT/Mg) ln (H)

where ht = the thermodynamic potential (in cm of water)
R = ideal gas constant
T = absolute temperature
M = molar weight of water
g = acceleration of gravity
H = relative humidity produce by the solution

the units of ht are a function of the units on the right hand side.

You can use any solution you want.  Some have used various concentrations of
sulfuric acid solutions, but they are a little bit of a hassle since the
solutions must be refreshed as water evaporates or condenses changing the
concentration with it.  An article in Ecology 41:233 (1960) published a long
list of salts, whose saturated solutions produce a range of humilities.
Saturated salt solutions have the advantage in that as long as the solid
salt is present, the solution will maintain a constant humidity as water is
exchanged with the atmosphere.

Two words of caution.
  1.  Your need a constant temperature within 1 degree C.
  2.  It takes a long time to reach equilibrium via vapor transport.  Be
prepared to wait one week or even a month.  You can however speed up the
process by placing the solution and sample inside a vacuum chamber.  (Vapor
diffusion coefficients are inversely proportional to the total pressure.)

Here are the solutions I use and their H and ht at 22.5 C

Salt             H                ht (cm of H20)
ZnCl2            0.100     -3.20 x 10^6
MgCl2         0.328     -1.55 x 10^6
K2CO3         0.436     -1.14 x 10^6
Mg(NO3)2     0.542     -8.51 x 10^5
NaNO2         0.649     -6.00 x 10^5
NaCl             0.758     -3.85 x 10^5
KCl             0.850     -2.26 x 10^5
KNO3         0.931     -9.93 x 10^4
K(H2PO4)2     0.963     -5.24 x 10^4
K2Cr207     0.980     -2.82 x 10^4

Good luck.

Glenn Brown
Associate Professor
Biosystems Engineering
Oklahoma State University
Stillwater, OK  74078
(405) 744-8425
gbrown@okstate.edu



-----Original Message-----
From: owner-sowacs@aqua.ccwr.ac.za <owner-sowacs@aqua.ccwr.ac.za>
To: gbrown@ceat.okstate.edu <gbrown@ceat.okstate.edu>
Date: Thursday, January 25, 2001 9:23 AM
Subject: Thermal Conductivity Sensors

Bruce:
>
>I am going to be calibrating some of the Campbell Scientific 229 sensors.
> I am going to use pressure from 0-400 kPa but I would like to calibrate
>at a higher kPa.  I read somewhere that letting the sensors "hang" in a
>jar containing a solution of potassium sulfate at 20 degrees Celcius will
>give an equivalent total suction of 4120 kPa.  Do you know of a solution
>(and temperature) that might give an equivalent total suction between 400
>and 4120 kPa?  I would really appreciate the input.
>
Elizabeth A. Garven
>
125B-105th Street
>East
>
>Saskatoon, Saskatchewan
>
>CANADA
>
>Phone: (306) 955-3218
>
><<mailto:eag-okc@attcanada.ca>eag-okc@attcanada.ca