A NOTE ON THE PRESSURE PLATE-MEMBRANE APPARATUS

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Abstract
Results are given to show that the moisture content of a soil in a pressure plate apparatus decreases indefinitely when a dry gas is used and gaseous diffusion through the membrane is continuous. For any soil sample there is only one water vapor pressure of the applied gas which can be in thermodynamic equilibrium with the sample. This vapor pressure depends on the osmotic pressure of the soil soln. and the pressure of the gas applied. From the literature, it would appear that generally the moisture content of the sample is detd. for a given pressure when the outflow of liquid water through the membrane has become negligible. This condition might be called a "time equilibrium," which will vary with the time of pressure appln. The condition under which the humidity of the gas leaving the apparatus is the same as that entering might be called "hydraulic equilibrium." Except for multisoil apparatus, this equilibrium will be thermodynamically true and the relative humidity involved will be the one in equilibrium with both applied pressure and osmotic pressure. True hydrostatic equilibrium can be reached only with an ideal membrane that is permeable to water but completely impermeable to gases. Under the conditions of hydraulic equilibrium and ideally of hydrostatic equilibrium, it is possible to relate a given moisture content to a given applied pressure with no ambiguity. The requirements for hydraulic equilibrium are outlined and examples of deviations from these requirements are given. It is pointed out that the difference between "time" and "hydraulic" equilibrium has not yet been investigated, but depending on the subsequent use made of the results, the difference may not justify changes in existing procedures. It is suggested that precise exptl. conditions be included when results for pressure plate-membrane apparatus are reported.

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