The 237 measures leaf wetness by determining the electrical resistance on the surface of the sensor (a wet surface is less resistant). It is primarily used to determine the percentage of time that a leaf surface is wet, versus the time it is dry.
Note: The 237 is designed for short duration ac excitation; dc excitation or continuous ac excitation may damage the sensing grid.Read More
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.
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.
|Resistance at Wet/Dry Transition||
|Operating Temperature Range||0° to 100°C|
|Short-Term Survivability Temperature Range||
-40° to +150°C
Sensor may crack when temperature drops below -40°C.
|Painting||Sensor is shipped unpainted so customer can choose appropriate surface finish to best match the application.|
|Hole Spacing||6.73 cm (2.65 in.)|
|Hole Diameter||0.54 cm (0.213 in.)|
|Dimensions||7.6 x 7.1 x 0.64 cm (3.0 x 2.75 x 0.25 in.)|
|Weight||91 g (3 oz) with 3.05 m (10 ft) cable|
Please note: The following shows notable compatibility information. It is not a comprehensive list of all compatible products.
The 237 requires one single-ended analog input and one switched excitation channel for measurement.