Campbell Scientific 223-L Delmhorst Cylindrical Soil Moisture Block - Manual (Page 17)

223 Delmhorst Cylindrical Soil Moisture Block
The output from the BrHalf instruction or Instruction 5 is the ratio of signal
voltage to excitation voltage:
V
s
/V
x
= R
s
/(R
s
+ R
1
)
where, V
s
= Signal Voltage
V
x
= Excitation Voltage
R
s
= Sensor Resistance
R
1
= Fixed Bridge Resistor.
7.2.3 Calculate Sensor Resistance
The sensor resistance is calculated using an expression in CRBasic or Edlog
instruction BR Transform Rf[X/(1–X)] (P59). The expression or Edlog
instruction BR Transform Rf[X/(1–X)] (P59) takes the Half Bridge output
(V
s
/V
x
) and computes sensor resistance as follows:
R
s
= R
1
(X/(1 – X))
where, X = V
s
/V
x
The bridge transform multiplier would normally be 1000, representing the
fixed resistor (R
1
). A bridge multiplier of 1000 produces values of R
s
larger
than 6999 ohms causing the datalogger to overrange when using low
resolution. To avoid overranging, a bridge multiplier of 1 should be used to
output sensor resistance (R
s
) in terms of kohms.
7.2.4 Calculate Soil Water Potential
The datalogger program can be written to store block resistance or can
calculate water potential from a block calibration. The soil water potential
versus resistance values in TABLE 7-3 are typical values supplied by
Delmhorst Corporation.
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