Campbell Scientific 253-L and 257-L (Watermark 200) Soil Matric Potential Sensors - Manual (Page 22)

253-L and 257-L Soil Matric Potential Sensors
TABLE 7-1 shows the excitation and voltage ranges used with the CRBasic
dataloggers.
TABLE 7-1. Excitation and Voltage Ranges for
CRBasic Dataloggers
Datalogger mV excitation Full Scale Range
CR800 Series 250 ± 250 mV
CR1000 250 ± 250 mV
CR3000 200 ± 200 mV
CR5000 200 ± 200 mV
CR9000(X) 200 ± 200 mV
7.2.1.2 Resistance Calculation
Sensor resistance is calculated with a CRBasic expression. If the result of the
BRHalf() instruction is assigned to a variable called kOhms, then the
resistance would be determined with the expression:
kOhms = 1 * (kOhms/(1-kOhms))
where the 1 represents the value of the reference resistor in kOhms and can be
omitted from the expression if desired.
7.2.2 Edlog Dataloggers
7.2.2.1 Program Instruction 5
Edlog dataloggers use Instruction 5, AC Half Bridge (P5), to excite and
measure the 253 and 257. Recommended excitation voltages and input ranges
for Edlog dataloggers are listed in TABLE 7-2.
TABLE 7-2. Excitation and Voltage Ranges for Edlog Dataloggers
Datalogger mV excitation Range Code Full Scale Range
21X 500 14 ± 500 mV
CR10(X) 250 14 ± 250 mV
CR510/CR500 250 14 ± 250 mV
CR23X 200 13 ± 200 mV
CR7 500 16 ± 500 mV
7.2.2.2 Program Instruction 59
Instruction 59, Bridge Transform (P59), is used to output sensor resistance
(R
s
). The instruction takes the AC Half Bridge output (V
s
/V
x
) and computes
the sensor resistance as follows:
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