237-L Leaf Wetness Sensor

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Leaf Wetness Sensors / 237-L

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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.

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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Detailed Description

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 data logger.

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.

For more information on the effects of paint color and sensor angle on sensors of this type, see Gillespie, T.J., & Kidd, G.E., 1978. Sensing duration of leaf moisture resistance using electrical impedance grids. Canadian Journal of Plant Science 58: 179-187.

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 kohm. Coated sensors have a poorly defined transition, which normally occurs from 20 kohm to above 1,000 kohm. For best results, the leaf wetness sensor should be field calibrated because the transition point will vary for different areas and vegetation.

Similar Products

LWS Leaf-Wetness Sensor

Compatibility

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

Data Loggers

	Compatible	Note
CR1000 (retired)
CR1000X (retired)
CR300 (retired)
CR3000 (retired)
CR310
CR350
CR6
CR800 (retired)
CR850 (retired)

Additional Compatibility Information

Data Logger Considerations

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

Specifications

Resistance at Wet/Dry Transition	Normally 50 and 200 kohm (uncoated sensor) Normally 20 to 1000 kohm (coated sensor)
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

FAQs for

Number of FAQs related to 237-L: 13

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What is the distance between the two electrodes in the 237-L?

Approximately 0.9 mm.
Is there a difference in the wetness duration between the 237-L and the leaf because of the difference in thermal conductivity?

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.
What are the power requirements of the 237-L?

Variable, but always negligible. The theoretical maximum for each measurement is 5000 mV at 5 µA for less than 3 ms.
What type of paint should be used on the surface of the 237-L?

Paint only with a flat latex paint. Usually, a high-quality, white, flat latex paint is used with a tiny amount of pigment.
Can the 237-L be used as a conductivity sensor?

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.
How should the 237-L be mounted?

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.
Can the 237-L generate a specific moisture content?

No. The sensor signal can only be interpreted as either wet or dry.
Is the 237-L really a sensor?
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.
Can a leaf wetness sensor be added to an ET107?

No.
What is the difference between the LWS and the 237-L leaf wetness sensors?

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.

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