Campbell Scientific 237-L Leaf Wetness Sensor - Manual (Page 11)

Model 237 Leaf Wetness Sensor
5.2 Plant Pathology Application Programming
An exact range of measurements is impossible to give since the 237 is field
calibrated. The manufacture of the sensor is not precise and the quality of
water bridging the electrodes varies. As demonstrated in program examples in
Section 4, a common practice is to measures grid resistance in terms of kOhms
using a 1 bin histogram to calculate at what fraction of the output interval the
sensor is wet. If resistance is ≤ 150 kΩ, the grid is considered wet. Since the
output interval is 60 minutes, if the histogram fraction equals 0.33, the leaf was
wet for 20 minutes during that hour.
5.3 Sensor Deployment
The sensor is not supplied with a mounting bracket. Gillespie and Kidd 1 found
that sensor orientation affects performance. As with surface preparation,
orientation varies across applications and users. A common practice is to
mount the sensor such that is receives minimal direct sunlight at mid-day
during the growing season. Gillespie and Kidd favor a 60 degree tilt on a north
facing sensor such that water runs away from the cable connection to minimize
puddling on the electrodes. Figure 2 shows a simple-to-construct mounting
bracket.
FIGURE 2. Mounting the 237 Sensor
5.4 Calibration
A wet / dry threshold of 150 kΩ is used in the programming examples in
Section 4. While this threshold may work well, refining the threshold for a
specific sensor and installation is recommended. A sharp change in resistance
occurs at the threshold on uncoated sensors. A less defined threshold occurs
with coated sensors. The threshold of uncoated sensors is normally between 50
and 200 kΩ. The threshold of the coated sensor is normally between 20 and
1,000 kΩ.
For best results, the sensor should be field calibrated. The transition point will
vary for different areas, vegetation, and water quality. Place the sensor in
vegetation, the wetness of which is to be monitored. Observe the vegetation
until it reaches the desired wetness. When the vegetation is at the desired
"wetness", the measured resistance can be used as a threshold. Sensitivity of
the sensor is changed by contaminants such as fingerprints and smudges.
Before painting and calibrating the sensor, clean it gently with alcohol.
7