Campbell CR23X, Operator'S Manual

Page: 99 / 374

SECTION 7. MEASUREMENT PROGRAMMING EXAMPLES
7-11
CR23X
FIGURE 7.12-2. 6 Wire Full Bridge
Connection for Load Cell
PROGRAM
1: Full Bridge w/mv Excit (P9)
1: 1 Reps
2: 25 5000 mV, 60 Hz Reject,
Fast, Ex Range
3: 21 10 mV, 60 Hz Reject, Slow,
Br Range
4: 1 DIFF Channel
5: 1 Excite all reps w/Exchan 1
6: 3300 mV Excitation
7: 1 Loc [ RAW_MEAS ]
8: 46.583 Mult
9: 0.0 Offset
2: Z=X+F (P34)
1: 1 X Loc [ RAW_MEAS ]
2: 266 F
3: 2 Z Loc [ MEAS_OFFS ]
7.13 227 GYPSUM SOIL MOISTURE
BLOCK
Soil moisture is measured with a gypsum block
by relating the change in moisture to the change
in resistance of the block. An AC Half Bridge
(Instruction 5) is used to determine the
resistance of the gypsum block. Rapid reversal
of the excitation voltage inhibits polarization of
the sensor. Polarization creates an error in the
output so the fast integration time is used. The
output of Instruction 5 is the ratio of the output
voltage to the excitation voltage; this output is
converted to gypsum block resistance with
Instruction 59, Bridge Transform.
The Campbell Scientific 227 Soil Moisture Block
uses a Delmhorst gypsum block with a 1 kohm
bridge completion resistor (there are also series
capacitors to block DC current and degradation
due to electrolysis. Using data supplied by
Delmhorst, Campbell Scientific has computed
coefficients for a 5th order polynomial to convert
block resistance to water potential in bars.
There are two polynomials: one to optimize the
range from -0.1 to -2 bars, and one to cover the
range from -0.1 to -10 bars (the minus sign is
omitted in the output). The -0.1 to -2 bar
polynomial requires a multiplier of 1 in the
Bridge Transform Instruction (result in kohms)
and the -0.1 to -10 bar polynomial requires a
multiplier of 0.1 (result in 10,000s of ohms). The
multiplier is a scaling factor to maintain the
maximum number of significant digits in the
coefficients of the polynomial.
In this example, we wish to make
measurements on 6 gypsum blocks and output
the final data in bars. The soil where the
moisture measurements are to be made is quite
wet at the time the data logging is initiated, but
is expected to dry beyond the -2 bar limit of the
wet range polynomial. The dry range polynomial
is used, so a multiplier of 0.1 is entered in the
bridge transform instruction.
When the water potential is computed, it is
written over the resistance value. The potentials
are stored in input locations 1-6 where they may
be accessed for output to Final Storage. If it
was desired to retain the resistance values, the
potential measurements could be stored in
Locations 7-12 by changing Parameter 3 in
Instruction 55 to 7.
CR23X
FIGURE 7.13-1. 6 Gypsum Blocks
Connected to the CR23X
PROGRAM
1: AC Half Bridge (P5)
1: 6 Reps
2: 14 1000 mV, Fast Range
3: 1 SE Channel
4: 1 Excite all reps w/Exchan 1
5: 1000 mV Excitation
6: 1 Loc [ Vs_Vx_1 ]
7: 1.0 Mult
8: 0.0 Offset