Operator`s manual

SECTION 7. MEASUREMENT PROGRAMMING EXAMPLES
7-11
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.
FIGURE 7.14-1. 6 Gypsum Blocks
Connected to the 21X
PROGRAM
01: P5 AC Half Bridge
01: 6 Reps
02: 14 500 mV fast Range
03: 1 IN Chan
04: 1 Excite all reps w/EXchan 1
05: 500 mV Excitation
06: 1 Loc :
07: 1 Mult
08: 0 Offset
02: P59 BR Transform Rf[X/(1-X)]
01: 6 Reps
02: 1 Loc :
03: .1 Multiplier (Rf)
03: P55 Polynomial
01: 6 Reps
02: 1 X Loc
03: 1 F(X) Loc :
04: .15836 C0
05: 6.1445 C1
06: -8.4189 C2
07: 9.2493 C3
08: -3.1685 C4
09: .33392 C5
7.15 NONLINEAR THERMISTOR IN
HALF BRIDGE (CAMPBELL
SCIENTIFIC MODEL 101)
Instruction 11, 107 Thermistor Probe,
automatically calculates temperature by
transforming the millivolt reading with a 5th order
polynomial. Instruction 55, Polynomial, can be
used to calculate temperature of any nonlinear
thermistor, provided the correlation between
temperature and probe output is known, and an
appropriate polynomial fit has been determined.
In this example, the 21X is used to measure the
temperature of 5 Campbell Scientific 101 Probes
(used with the CR21). Instruction 4, Excite,
Delay, and Measure, is used because the high
source resistance of the probe requires a long
input settling time (see Section 13.3.1). The
excitation voltage is 2000mV, the same as used
in the CR21. The signal voltage is then
transformed to temperature using the
Polynomial Instruction.