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Measuring Soil Matric Potential in situ by Sensing Heat Dissipation within a Porous Body: I. Theory and Sensor Construction¹

ABSTRACT

A matric potential sensor is described that measures heat dissipation to sense the water content of a porous block in equilibrium with soil. It consists of a P-N junction diode that is surrounded by a heating coil and embedded in a porous medium.

A theoretical model is developed that describes the sensor in terms of its limiting parameters. Geometry, size, rate of heat input, and duration of heating are chosen to obtain a certain degree of accuracy and to make the measurements independent of the surrounding soil.

Construction, calibration, measurement, and temperature correction techniques are described. Since the electrical output of the sensor is linear with temperature, a single calibration curve is adequate at all temperatures within the accuracy reported.

By varying the composition of the porous body, the range and sensitivity of the sensor can be altered.

¹ Contribution from the US Salinity Lab., SWC, ARS, USDA, P. O. Box 672, Riverside, Calif. 92502, and is a part of the senior author's Ph.D. thesis, Univ. of California, Riverside.

² Physicist, Research Agricultural Engineer, and Research Soil Scientist, respectively.

Received for publication June 12, 1970. Accepted for publication October 22, 1970.