Campbell 237, Product Manual

Page: 13 / 19

237 Leaf Wetness Sensor
7
TABLE 7-2. Measurement Instructions, Parameters, Results
Datalogger
Measurement
Instruction
Excitation
(mV)
Input
Range
Code
Integration/
f
N1
Multiplier Offset Result
CR6,
CR1000X
BrHalf () 2500
mV200 or
mV200C 1
15000 1 0 Vs/Vx
CR300 BrHalf () 2500 mV2500 4000 Hz 1 0 Vs/Vx
CR200(X) ExDelSE () 2500 n/a 500 µs 0.0004 0 Vs/Vx
CR800,
CR1000
BrHalf () 2500
mV25 or
mV25C 1
250 µs 1 0 Vs/Vx
CR3000 BrHalf () 5000
mV50 or
mV50C 1
250 µs 1 0 Vs/Vx
1 The mV200C, mV25C, and mV50C input range codes check for an open connection indicating that no sensor is connected
or a cable has been cleanly cut. If an open connection occurs, NAN will be returned. A floating voltage can occur that
falsely indicates the presence of a missing sensor when the mV200C, mV25C, and mV50C input range codes are not used.
7.4.2 Calculating Sensor Resistance
The CRBasic program should include an expression that calculates sensor
resistance. With reference to FIGURE 7-2 , sensor resistance (Rs), expressed in
kΩ, is calculated as follows:
Rs = R
2
/ (Vs/Vx) – R
2
– R
1
Therefore,
Rs (kΩ) = 1/(Vs/Vx) – 101
Except for the CR200(X), Campbell Scientific also suggests including the
following after the resistance calculation:
If Rs > 10000 Then
Rs = 10000
EndIf
The high resolution of Campbell Scientific dataloggers can pick up very small
signals, which can result in an erratic signal when the sensor is dry. The
wet/dry transition is less than 1000 kΩ , and therefore, 10,000 kΩ gives plenty
of range outside of what one would consider to be dry.
8. Operation
An exact range of measurements is impossible to give since the 237 is field
calibrated. The sensor is not precise and the quality of water bridging the
electrodes varies. As demonstrated in Appendix B , Example Programs
(p. B-1)
, a
common practice is to measure grid resistance in kΩ using a one bin histogram
to calculate at what fraction of the output interval the sensor is wet (Section
8.1 ,
Calculating Wet Time Fraction
(p. 8)
). If resistance meets the threshold
( ≤150 kΩ in the example program), 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.