Interface for WATERMARK sensor or other moisture probes



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The SMX is an interface module for sensors that measure moisture content of soil or other substrates by way of their AC electrical resistance. It forestalls corrosion of the sensor by using AC excitation and by galvanically isolating the sensor electrodes from unwanted underground DC currents. It provides three output modes, voltage, current or frequency, allowing an interface to a wide range of general purpose data logging equipment, handheld multimeters, and SCADA systems.

The SMX is available in two forms. The SMX-oem is a circuit board, suitable for wiring into custom systems that have their own enclosure. The SMX-encapsulated is potted in industrial epoxy with 6 wire leads, suitable for outdoor deployment.

The SMX is commonly used with Irrometer WATERMARK™ sensors. When buried at root depth in good contact with the soil, the WATERMARK reaches equilibrium with the local conditions and exhibits an AC electrical resistance that is inversely proportional to soil moisture. (It also depends slightly on temperature.) The SMX converts this AC resistance to either an output frequency, voltage, or current, that can be read by a data logger and converted to standard moisture pressure units of kilopascal (= centibar). The documentation illustrates the steps.

The SMX has had more unusual applications too, apart from soil moisture. Some of these are: sensing moisture in airplane wing deicer bags; moisture in baby diapers; salt moisture deposits on automobile underbodies, depth of water in wave action, sump pump controls, hydroponics, science exhibits, education on sensing conductivity and depth.

Specifications and Documentation

SMX manual


2229 5th Street, Berkeley, CA 94710 • (510) 848-5725 •