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I
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SECT|ON
7.
MEASUREMENT PROGRAMMTNG EXAMPLES
voltage to
th{
excitation
voltage;this output
is
PROGRAM
converted to,Sypsum
block resistance with
Instruction
5$, Bridge
Transform.
01:
P5
AC Half Bridge
01: 6
Reps
The Campbdll Scientific
227 Soil Moisture
Block
02: 14
500 mV fast Range
uses
a Delmhorst gypsum block with a
1
kohm
03:
1
lN Chan
bridge
completion resistor (there are also
series
04:
1
Excite
all reps w/EXchan
1
capacitors
to block DC current and
degradation 05: 500
mV Excitation
due to
electr6lysis.
Using data supplied
by
06:
1
Loc
:
Delmhorst,
Campbell Scientific has
computed
07:
1
Mult
coefficients
fpr
a
5th
order polynomialto
convert
08: 0
Otfset
block resistahce to
water potential in bars.
There are
hruD
polynomials: one to optimize
the
02: P59
BB Transform
Bf[V(1-X)]
range
from -0.1 to -2 bars, and one to cover
the
01: 6
Reps
range
from -0.1 to -10 bars (the minus sign
is
02:
1
Loc
:
omitted
in
thp
output).
The -0.1
to
-2bar
03:
.1
Multiplier
(Rf)
polynomial rpquires
a multiplier of
1 in
the
Bridge
Tran$form Instruction (result in
kohms)
03: P55
Polynomial
and
the
-0.1
[o
-10 bar
polynomial requires
a
01
:
6
Reps
multiplier of
0.1 (result in 10,000s of
ohms).
The
02:
1
X Loc
multiplier is 4 scaling factor
to maintain
the
03:
1
F(X)
Loc
:
maximum ndmber of significant
digits
in
the
04:
.15836
CO
coefficients
Qf
the
polynomial.
05:
6.1445
C1
06:
-8.4189
C2
f
n
this example, we wish to
make
07:
9.2493
C3
measuremefrts on
6 gypsum blocks and
output
08:
-3.1685
C4
the
final datd in
bars.
The
soilwhere
the
09:
.33392
C5
moisture
are
to be made
is
quite
is
expected to dry beyond
the -2 bar limit
of
the
wet range pOlynomial. The dry range polynomial
is used, so
{
multiplier of
0.1 is entered
in
the
bridge
translorm instruction.
When the
wfter
potential
is
computed, it
is
written over
the resistance
value. The
potentials
are stored iri input locations
1-6
where
they may
be
accesse{ for output to Final
Storage.
lf
it
was desired
to retain the resistance values, the
potential mEasurements could be
stored
in
Locations
7)12by changing
Parameter
3
in
lnstruction
55
to
7.
EX1
Hl
1
LO1
Ht2
21X
-l
LEADS
BLOCKS
RED
LEADS TO
SINGLE_
/.\
-
ENDED
rNPUr
CHANNELS
't
- 6
-
__4 _
Y
2 ^
CLEAR
LEADS TO
GROUND
7.15
NONLINEAR THERMISTOR
IN
HALF BRIDGE (CAMPBELL
scrENTrFrc MoDEL
101)
Instruction
1
1,
107
Thermistor Probe,
automatically calculates
temperature by
transforming
the millivolt reading with a 5th order
polynomial. lnstruction 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.
F|GUFE
7.14-1. 6 Gypsum Blocks
Connected to the
21X
7-11
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