Campbell Scientific CR10X Measurement and Control System - Manual (Page 99)

SECTION 7. MEASUREMENT PROGRAMMING EXAMPLES
7-15
PROGRAM
01: AC Half Bridge (P5)
1:
6 Reps
2: 15
±2500 mV Fast Range
3: 1 SE Channel
4: 1 Ex Channel Option
5: 2500 mV Excitation
6: 1 Loc [ H2O_bar#1 ]
7: 1 Mult
8: 0 Offset
02: BR Transform Rf[X/(1-X)] (P59)
1:
6 Reps
2: 1 Loc [ H2O_bar#1 ]
3: .1 Multiplier (Rf)
03: Polynomial (P55)
1:
6 Reps
2: 1 X Loc [ H2O_bar#1 ]
3: 1 F(X) Loc [ H2O_bar#1 ]
4: .15836 C0
5: 6.1445 C1
6: -8.4189 C2
7: 9.2493 C3
8: -3.1685 C4
9: .33392 C5
7.15 NONLINEAR THERMISTOR IN
HALF BRIDGE (MODEL 101 PROBE)
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 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 CR10X is used to
measure the temperature of five 101 Probes (used
with the CR21 but usually not the CR10X).
Instruction 4, Excite, Delay, and Measure, is used
because the high source resistance of the probe
requires a long input settling time (Section 12.3.1).
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 CR10X 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 -53.784
C1 147.97
C2 -218.76
C3 219.05
C4 -111.34
C5 23.365
FIGURE 7.15-1. 101 Thermistor Probes Connected to CR10X