Campbell Scientific 253-L and 257-L (Watermark 200) Soil Matric Potential Sensors - Manual (Page 21)

253-L and 257-L Soil Matric Potential Sensors
shield wire that is not connected to the sensor. The white lead connects to the
high end of a multiplexer channel, the black lead to the low end of the
multiplexer channel, and the clear lead to a multiplexer ground channel. A
1000 ohm resistor at the datalogger wiring panel is used to complete the half
bridge circuitry.
FIGURE 7-2. 253 wiring example
7.2 Programming
This section describes using CRBasic or Edlog to program the
datalogger. See Section 4.2, Use SCwin to Program Datalogger
and Generate Wiring, if using Short Cut. NOTE
The 253 and 257 sensors are measured with an AC Half Bridge measurement
followed by a sensor resistance calculation.
This section will distinguish between CRBasic dataloggers and Edlog
dataloggers. CRBasic dataloggers refer to the CR800, CR850, CR1000,
CR3000, CR5000, and CR9000(X). Edlog dataloggers are the CR10(X),
CR510, CR500, CR23X, CR7, and 21X.
7.2.1 CRBasic Dataloggers
7.2.1.1 BRHalf Instruction
CRBasic dataloggers use the BRHalf() instruction with the RevEx argument set
to True to excite and measure the 253 and 257. The result of the BRHalf()
instruction is the ratio of the measured voltage divided by the excitation
voltage.
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