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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 Unfortunately, this response may confuse some readers: Although the answer to the question of whether there is any salt whose SATURATED solution will give "an higher relative humidity to obtain lower suction"--"is unfortunately no", more dilute samples of any of the salts will of course solve the problem. The only practical problem is to use a large enough volume of solution, and with adequate enough mixing (for example by bubbling air through the solution), to assure that the concentration at the air-water interface remains essentially constant, despite evaporation of small amounts of water during the calibration. This problem COMPLETELY disappears when you use a SATURATED solution (again,with adequate mixing). Len Ornstein At 11:17 AM +0000 1/27/01, owner-sowacs@aqua.ccwr.ac.za wrote: >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.ÉK) and M the molar weight of water : 18 g/mole. So, >numerically, if you choose kPa as unity, the above equation (1) becomes at >20ÉC : > > (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 > Magn»sium Chloride, MgCl2-6H2O, 33%, -149994 > Potassium Carbonate, K2CO3-2H2O, 44%, -111072 > Magn»sium 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, Charg» de recherches au CNRS >Laboratoire d'»tude 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] >Envoy» : 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>