237 Leaf Wetness Sensor
3
and you will be prompted to send the program just created to the
datalogger.
7. If the sensor is connected to the datalogger, as shown in the wiring
diagram in step 4, check the output of the sensor in the datalogger support
software data display to make sure it is making reasonable measurements.
5. Overview
The 237 is often used for plant pathology applications because plant diseases
are often associated with wet leaves. Duration of wetness and air temperature
during wetness are inputs to many disease models.
The sensor emulates a leaf, thereby approximating the wetness state of
surrounding foliage. To determine leaf wetness, the 237 uses 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 colo
u
r and type of paint affect sensor performance. Campbell Scientific
supplies the sensor unpainted because individual modifications will vary with
the application. The paper listed in Section
(p. 9)
, describes the
effects of paint colo
u
r and sensor angle on sensors of this type.
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
kΩ
. Coated
sensors have a poorly defined transition which normally occurs from 20
kΩ
to
above 1,000
kΩ
. For best results, the leaf wetness sensor should be field
calibrated since the transition point will vary for different areas and vegetation.
Features:
•
Sensor is shipped unpainted so customer can choose appropriate
surface finish that best fits their application
•
Imitates characteristics of a leaf
•
Compatible with Campbell Scientific CRBasic dataloggers:
CR200(X) series, CR300 series, CR6 series, CR800 series, CR1000X,
CR1000, CR3000, CR5000, and CR9000(X)
6. Specifications
Resistance at Wet/Dry
Transition:
normally 50 and 200 k
Ω
(uncoated sensor);
normally 20 to 1000 k
Ω
(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
