237-L

The 237 consists of a circuit board with interlacing gold-plated fingers. Condensation on the sensor lowers the resistance between the fingers, which is measured by the datalogger.

Droplets must touch two fingers simultaneously to change the sensor resistance. For this reason, the 237 is typically coated with flat latex paint, which spreads water droplets. The color and type of paint affect sensor performance. Campbell Scientific supplies the sensor unpainted allowing customers to determine the appropriate paint to apply to the sensor's surface. The appropriate pigmentation should closely emulate the properties of a typical leaf.

Calibration

The resistance of the sensor at the wet/dry transition point should be determined. A sharp change in resistance occurs in the wet-dry transition on the uncoated sensor; normally the transition is between 50 and 200 kohms. Coated sensors have a poorly defined transition which normally occurs from 20 kohms to above 1,000 kohms. For best results, the leaf wetness sensor should be field calibrated since the transition point will vary for different areas and vegetation.