237-L Leaf Wetness Sensor
Sensitive yet Durable
Compatible with most Campbell Scientific data loggers
weather applications supported water applications supported energy applications supported gas flux & turbulence applications supported infrastructure applications supported soil applications supported

Overview

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. The sensing grid may become damaged by dc excitation or continuous ac excitation.

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Benefits and Features

  • Sensor is shipped unpainted so customer can choose appropriate surface finish that best fits the application
  • Imitates characteristics of a leaf

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Specifications

Resistance at Wet/Dry Transition
  • Normally 50 and 200 kohm (uncoated sensor)
  • Normally 20 to 1000 kohm (coated sensor)
Operational Temperature Range 0° to 100°C
Short-Term Survivability Temperature Range -40° to +150°C
Sensor may crack when temperature drops below -40°C.
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

Compatibility

Please note: The following shows notable compatibility information. It is not a comprehensive list of all compatible products.

Data Loggers

Compatible Note
21X (retired)
CR10 (retired)
CR1000
CR1000X
CR10X (retired)
CR200X (retired)
CR206X (retired)
CR23X (retired)
CR295X (retired)
CR300
CR3000
CR310
CR500 (retired)
CR5000 (retired)
CR510 (retired)
CR6
CR800
CR850
CR9000 (retired)
CR9000X

Additional Compatibility Information

Data Logger Considerations

The 237 requires one single-ended analog input and one switched excitation channel for measurement.

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FAQs for

Number of FAQs related to 237-L: 13

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  1. Variable, but always negligible. The theoretical maximum for each measurement is 5000 mV at 5 µA for less than 3 ms.

  2. Yes. The thermal characteristics of the 237-L are probably different from those of any surrounding objects, including leaves. Consequently, the 237-L will dry at a rate different from surrounding objects, including leaves.

  3. Paint only with a flat latex paint. Usually, a high-quality, white, flat latex paint is used with a tiny amount of pigment.

  4. The 237-L is not designed to be used as a conductivity sensor. To our knowledge, a few people have tried this but have been unsuccessful. 

  5. No. The sensor signal can only be interpreted as either wet or dry.

  6. The mounting method used depends on the application. In plant canopies, consider mounting the 237-L so that it receives the least amount of solar radiation at noon. This means tipping the sensor, electrodes up, so that its sensing surface is parallel to the plane of the ecliptic. Tipping the sensor also minimizes the chance of water puddling on its surface.

    On non-living surfaces, such as a man-made structure, consider mounting the 237-L flat against a flat surface on the shady side. This will cause the 237-L’s thermal characteristics to be more similar to those of the surface being studied.

  7. Only in the most basic sense. The signal output from the 237-L can only be interpreted as an indication of the presence of a conductive material bridging the two electrodes on its surface. If the circuit is open (infinite resistance or zero conductance), there is no conductive material.  If the circuit is closed, there is conductive material.

    The primary use of the 237-L is to indicate the presence of free water on the surface of surrounding objects. The thermal characteristics of the 237-L are probably different from those of any surrounding objects, including leaves. Consequently, the 237-L will dry at a rate different from surrounding objects, including leaves. Data from the 237-L are only interchangeable from measurement site to measurement site if the following are true:

    • Each 237-L is prepared and maintained in the same way.
    • Each 237-L is mounted in nearly identical environments.

    Plant disease researchers found that if a 237-L sensor was placed in a plant canopy at a consistent position, with a consistent coating of a spreading material on its surface (that is, paint), they could estimate when free water was in the plant canopy. From this discovery, they were able to formulate disease emergence models. The resulting models tolerate significant deviation in moisture-presence data. Even so, use of a different spreading material, or difference in sensor positioning, may invalidate the data.

  8. Both leaf wetness sensors are compatible with all Campbell Scientific data loggers. However, the 237-L is often used in large legacy networks that rely on the 237-L for data continuity. The 237-L does require painting and field calibration, whereas the LWS does not.