Nonlinear Thermistor In Half Bridge - Campbell CR510 Operator's Manual

SECTION 7. MEASUREMENT PROGRAMMING EXAMPLES
FIGURE 7.11-1. 6 227 Gypsum Blocks Connected to the CR510
PROGRAM
01: AC Half Bridge (P5)
1: 4
2: 15
3: 1
4: 1
5: 2500
6: 1
7: 1
8: 0
02: BR Transform Rf[X/(1-X)] (P59)
1: 4
2: 1
3: .1
03: Polynomial (P55)
1: 4
2: 1
3: 1
4: .15836 C0
5: 6.1445
6: -8.4189
7: 9.2493
8: -3.1685
9: .33392 C5
7.12 NONLINEAR THERMISTOR IN
HALF BRIDGE
Instruction 11, 107 Thermistor Probe, automatically
linearizes the output of a nonlinear thermistor, 107
Probe, 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 CR510 is used to measure the
7-12
CR510
Reps
2500 mV Fast Range
SE Channel
Ex Channel Option
mV Excitation
Loc [ H2O_bar_1 ]
Mult
Offset
Reps
Loc [ H2O_bar_1 ]
Multiplier (Rf)
Reps
X Loc [ H2O_bar_1 ]
F(X) Loc [ H2O_bar_1 ]
C1
C2
C3
C4
temperature of four 101 Probes (used with the CR21
but usually not the CR510). Instruction 4, Excite,
Delay, and Measure, is used because the high
source resistance of the probe requires a long input
settling time (Section 10.3.1). The excitation voltage
is 2000 mV, the same as used in the CR21. The
signal voltage is then transformed to temperature
using the Polynomial Instruction.
The manual for the 101 Probe gives the coefficients
of the 5th order polynomial used to convert the
output in millivolts to temperature (E denotes the
power of 10 by which the mantissa is multiplied):
C0
C1
C2
C3
C4
C5
The CR510 will only allow 5 significant digits to the
right or left of the decimal point to be entered from the
keyboard. The polynomial cannot be applied exactly
as given in the 101 manual. The initial millivolt
reading must be scaled if the coefficients of the higher
order terms are to be entered with the maximum
number of significant digits. If 0.001 is used as a
multiplier on the millivolt output, the coefficients are
divided by 0.001 raised to the appropriate power (i.e.,
C0=C0, C1=C1/0.001, C2=C2/.000001, etc.). With
this adjustment, the coefficients entered in
Parameters 4-9 of Instruction 55 become:
C0
C1
C2
C3
C4
C5
-53.7842
0.147974
-2.18755E-4
2.19046E-7
-1.11341E-10
2.33651E-14
-53.784
147.97
-218.76
219.05
-111.34
23.365