Campbell Scientific CR7 Measurement and Control System - Manual (Page 82)

SECTION 7. MEASUREMENT PROGRAMMING EXAMPLES
7-14
7.17 NONLINEAR THERMISTOR IN
HALF BRIDGE (CAMPBELL
SCIENTIFIC MODEL 101)
Instruction 11, 107 Thermistor Probe,
automatically linearizes the output of the
nonlinear thermistor in the 107 Probe by
transforming the millivolt reading with a 5th
order polynomial. Instruction 55, Polynomial,
can be used to linearize the output 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 CR7 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 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 -53.7842
C1 0.147974
C2 -2.18755E-4
C3 2.19046E-7
C4 -1.11341E-10
C5 2.33651E-14
The CR7 will only allow 5 significant digits to the
right or left of the decimal point to be entered
from the key board. The polynomial can not 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 -53.784
C1 147.97
C2 -218.76
C3 219.05
C4 -111.34
C5 23.365
FIGURE 7.17-1. 101 Thermistor Probes
Connected to CR7
PROGRAM
01: P4 Excite,Delay,Volt(SE)
01: 5 Reps
02: 8 5000 mV slow Range
03: 1 IN Card
04: 1 IN Chan
05: 1 EX Card
06: 1 EX Chan
07: 5 Meas/EX
08: 10 Delay (units .01sec)
09: 2000 mV Excitation
10: 1 Loc [:101 T #1 ]
11: 0.001 Mult
12: 0 Offset
02: P55 Polynomial
01: 5 Reps
02: 1 X Loc 101 T #1
03: 1 F(X) Loc [:101 T #1 ]
04: -53.784 C0
05: 147.97 C1
06: -218.76 C2
07: 219.05 C3
08: -111.34 C4
09: 23.365 C5