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NOTE: To get off this list, send email to majordomo@aqua.ccwr.ac.za with the body of the message containing the line: unsubscribe sowacs The procedure with this "jar" that you mentionned consists in creating given controlled relative humididities using SATURATED saline solutions. The fact that, in this case, you realize a system with 3 components (air, water and a chemical) and three phases (solid, liquid and gas) makes the variance equal to 0 and, thus, fix the relative humidity h hich remains only slightly temperature dependent. The relation between h and the corresponding suction Psi is given by the well-known Kelvin's law which can be written : (1) Psi = -RT Ln(h) / M if Psi is expressed as a pressure in Pascals. R is the ideal gas constant (8.32 J/mole.ƒK) and M the molar weight of water : 18 g/mole. So, numerically, if you choose kPa as unity, the above equation (1) becomes at 20ƒC : (2) Psi [kPa] = 135294 Ln (h) If we take the case of potassium sulfate (K2SO4), the relative humidity will be fixed at approximatively 97% and, applying (2), it's easy to verify that this will effectively lead to a suction of -4120 kPa as you said. The problem you raised is now : is there a salt that would give an higher relative humidity to obtain lower suction ? And for me, as far as I now, the answer is unfortunately no ! The following salts are commonly used to calibrate hygrometers : Salt, Formula, h(%), Psi[kPa] Lithium Chloride, LiCl-nH2O,12%, -286855 MagnÈsium Chloride, MgCl2-6H2O, 33%, -149994 Potassium Carbonate, K2CO3-2H2O, 44%, -111072 MagnÈsium Nitrate, Mg(NO3)2-6H2O, 55%, -80883 Sodium Nitrite, NaNO2, 66%, -56216 Sodium Chloride, NaCl, 76%, -37129 Ammonium Sulfate,(NH4)2SO4, 81%, -28509 Potassium Nitrate, KNO3, 93%, -9818 Potassium Sulfate, K2SO4, 97%, -4121 (Data from the french norm NF X 15-014) You see that 97% is the highest relative humidity listed there and, for myself, I haven't ever heard about a salt "wetter" than that ! Nevertheless, don't be too sorry : applying (2) in the reverse mode, you will notice that -400kPa, your lowest boundary, will correspond to 99.7% RH will is very difficult to handle because even small temperature variations could create condensation problems. A solution, sometimes used to obtain sorption isotherms for concrete materials, is to use instead ABSOLUTE water vapor pressures. In this case, you work under vacuum conditions and you have to adjust the overall pressure to the suction you want to obtain. Another one, simpler from my point of view, would be to use a reference material with a well know retention curve : any ideas dear Sowacsers and Sowacseresses ? Hope it's clarifying your question... Bien cordialement. ______________________________________________________ Dr. Jean-Paul Laurent, ChargÈ de recherches au CNRS Laboratoire d'Ètude des Transferts en Hydrologie et Environnement. (LTHE, UMR 5564 CNRS-INPG-UJF-IRD) BP53, F-38041 Grenoble-Cedex 09, FRANCE Tel. +33 (0) 4 76 82 50 59, Fax. +33 (0) 4 76 82 52 .86 Page Web : http://www.lthe.hmg.inpg.fr/~laurent ______________________________________________________ -----Message d'origine----- De : owner-sowacs@aqua.ccwr.ac.za [mailto:owner-sowacs@aqua.ccwr.ac.za] EnvoyÈ : jeudi 25 janvier 2001 10:59 Objet : 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. </smaller></fontfamily> <fontfamily><param>Times New Roman</param><smaller>Elizabeth A. Garven </smaller></fontfamily> <fontfamily><param>Times New Roman</param><smaller>125B-105th Street East Saskatoon, Saskatchewan CANADA Phone: (306) 955-3218 <<mailto:eag-okc@attcanada.ca>eag-okc@attcanada.ca </smaller></fontfamily>