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Using an Electro-Optical Switch to Measure Soil Water Suction

Pacific Northwest Laboratory, Richland, WA 99352

*Corresponding author.

ABSTRACT

A simple and inexpensive electro-optical switch (infrared light-emitting diode; IR LED) was used to show that soil water suctions can be measured accurately by employing nylon filter disks with different pore sizes. Nylon filter disks that become translucent when water is absorbed were shown to open or close an electro-optical switch at specific soil water suctions associated with the air-entry suction of a disk, thereby demonstrating the principle of a digital tensiometer. Sensors constructed by inserting a nylon filter disk in a switch were shown to operate across a suction range of 0.004 to 2.4 MPa and were tested in a silt loam soil under drying and wetting conditions. The switch-sensor's performance was compared with a second type of sensor (CdS photoresistor with LED) made with a layered fritted-glass disk instead of nylon. This second sensor was able to continuously detect changes of water content from air dry to saturated. We concluded, however, that the first type of sensor was superior for precisely measuring soil water suction.

Pacific Northwest Laboratory is operated by Battelle Memorial Inst. for the U.S. Dep. of Energy under Contract DE-AC06-76RLO 1830. This research was supported by the Subsurface Science Program of the Office of Health and Environmental Research, U.S. Dep. of Energy.

Received for publication February 25, 1991.