237 Leaf Wetness Sensor
8
8.1 Calculating Wet Time Fraction
Fraction of time wet are common data derived from 237 measurements.
Calculating time fraction requires a wetness threshold. Refer to Section
(p. 8)
, for more information on determining the threshold.
Fraction of time wet is calculated in all current Campbell Scientific
dataloggers, except the CR200(X), by using the
Histogram()
instruction with a
single bin and closed form. The bin select value for the histogram is the
Variable
containing sensor resistance (Rs). The lower limit of the histogram is
zero, and the upper limit is the wet / dry threshold. This will give the fraction
of the output interval that the sensor is wet. A fraction of time wet of 0.33
when the output interval is one hour means that the sensor was wet for 20
minutes during that hour.
Refer to Appendix
, for information on calculating
fraction of time wet with the CR200(X).
8.2 Calibration
A wet / dry threshold of 150 kΩ is used in
(p.
B-1)
.
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.
The transition point varies for different areas, vegetation, and water quality.
Therefore, for best results, the sensor should be field calibrated using the
following procedure:
1. Clean the sensor gently with alcohol. Contaminants change the sensitivity
of the sensor.
2. Place the sensor in the vegetation where the wetness is to be monitored.
3. When the vegetation is at the desired wetness, measure the resistance and
use that resistance as the threshold.
