Campbell Scientific 227-L Delmhorst Cylindrical Soil Moisture Block - Manual (Page 14)

227 Delmhorst Cylindrical Soil Moisture Block
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 Program, if using SCWin. NOTE
The datalogger is programmed using either CRBasic or Edlog. Dataloggers
that use CRBasic include our CR800, CR850, CR1000, CR3000, CR5000, and
CR9000(X). Dataloggers that use Edlog include our CR500, CR510,
CR10(X), 21X, CR23X, and CR7. CRBasic and Edlog are included with
LoggerNet, PC400, and RTDAQ software.
The datalogger program needs to measure the sensor, calculate the sensor
resistance, and convert the transform resistance to potential in bars. Example
programs are provided in Section 7.2.4, Programming Examples.
7.2.1 Excite and Measure the 227
The sensor is excited and measured using the BrHalf instruction in CRBasic or
Edlog instruction AC Half Bridge (P5). Recommended excitation voltages
and input ranges are given in TABLE 7-2.
7.2.2 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
) shown in FIGURE 7-1. 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.
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