Campbell Scientific CR7 Measurement and Control System - Manual (Page 81)

SECTION 7. MEASUREMENT PROGRAMMING EXAMPLES
7-13
02: P34 Z=X+F
01: 1 X Loc mm RAW
02: 266 F
03: 2 Z Loc [:mm CORECT]
7.16 227 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 mid-
bridge 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. 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 -15 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 -15 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 12 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 1-12
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 13-24 by changing
parameter 3 in Instruction 55 to 13.
FIGURE 7.16-1. 12 Gypsum Blocks
Connected to the CR7
The first 6 blocks are excited by excitation
channel 1 and the last 6 by channel 2. Thus, 6
is entered for the number of measurements per
excitation channel in Parameter 7 of Instruction
5.
PROGRAM
01: P5 AC Half Bridge
01: 12 Reps
02: 16 500 mV fast Range
03: 1 IN Card
04: 1 IN Chan
05: 1 EX Card
06: 6 EX Chan
07: 6 Meas/EX
08: 500 mV Excitation
09: 1 Loc [:POTEN #1 ]
10: 1 Mult
11: 0 Offset
02: P59 BR Transform Rf[X/(1-X)]
01: 12 Reps
02: 1 Loc [:POTEN #1 ]
03: .1 Multiplier (Rf)
03: P55 Polynomial
01: 12 Reps
02: 1 X Loc POTEN #1
03: 1 F(X) Loc [:POTEN #1 ]
04: .15836 C0
05: 6.1445 C1
06: -8.4139 C2
07: 9.2493 C3
08: -3.1685 C4
09: .33392 C5