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Dynamic Measurement of Soil and Leaf Water Potential With a Double Loop Peltier Type Thermocouple Psychrometer¹

ABSTRACT

Details on the construction, calibration, and performance of a three-terminal double loop thermocouple psychrometer are given. (This psychrometer design was originally conceived and constructed by Dr. W. H. Gardner, Dep. of Agronomy, Washington State University, Pullman, Wash., USA). The thermal stability of this psychrometer is about 40 times better than that of the two-terminal psychrometer (Spanner type) for ambient temperature fluctuations with a time rate of change greater than 0.2C/min. The response behavior of a fritted glass bulb and a ceramic bulb psychrometer was tested for vapor and for liquid phase water movement. For vapor phase flow the fritted glass bulb exhibited a shorter response time than the ceramic bulb psychrometer, whereas the reverse was true when water movement was predominantly in the liquid phase. Water potential measurements carried out on silty clay and silt loam soil samples were within ±0.4 bar of the porous plate extractor equilibrium values. A system that facilitates automatic and continuous in situ measurement of soil water potential using the three-terminal psychrometer is described.

¹ Contribution from the Dep. of Soil Science, Univ. of British Columbia. Part of a thesis project carried out by the senior author in partial fulfillment of the requirements for a Ph.D. degree. This work was supported by National Research Council (Canada) grant no. A2556.

² Research Assistant and Assistant Professor of Soil Science, respectively, Dep. of Soil Science, Univ. of British Columbia, Vancouver 8, B.C., Canada.

Received for publication August 21, 1972. Accepted for publication December 5, 1972.