Campbell Scientific 237-L Leaf Wetness Sensor - Manual (Page 6)

Model 237 Leaf Wetness Sensor
The black outer jacket of the cable is Santoprene® rubber. This
compound was chosen for its resistance to temperature extremes,
moisture, and UV degradation. However, this jacket will support
combustion in air. It is rated as slow burning when tested
according to U.L. 94 H.B. and will pass FMVSS302. Local fire
codes may preclude its use inside buildings. NOTE
3. Programming
Refer to programming examples in Section 4 for suggested implementation of
measurement and processing concepts.
3.1 Measurement of Vs / Vx
The base measurement of the 237 sensor is Vs/Vx where Vs is the voltage
measured and Vx is the excitation voltage supplied by the datalogger. Vs/Vx is
measured by the datalogger with the instructions and parameters listed in
Table 2.
TABLE 2. Measurement Instructions, Parameters, Results
Datalogger
Measurement
Instruction Excitation
(mV) Input
Range Integration/
Delay
Multiplier Offset Result
CR510, CR10(X) P5 AC Half Bridge 2500 ±25 mV fast 1 0 Vs/Vx
CR7 P5 AC Half Bridge 5000 ±50 mV fast 1 0 Vs/Vx
CR200(X) ExDelSE () 2500 n/a 500 µs 0.0004 0 Vs/Vx
CR800, CR1000 BrHalf () 2500 ±25 mV 250 µs 1 0 Vs/Vx
CR3000, CR9000X BrHalf () 5000 ±50 mV 250 µs 1 0 Vs/Vx
3.2 Calculating Sensor Resistance
With reference to Figure 1, 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.
2