Campbell Scientific CR23X Micrologger - Manual (Page 100)

SECTION 7. MEASUREMENT PROGRAMMING EXAMPLES
7-12
2: BR Transform Rf[X/(1-X)] (P59)
1: 6 Reps
2: 1 Loc [ Vs_Vx_1 ]
3: .1 Multiplier (Rf)
3: Polynomial (P55)
1: 6 Reps
2: 1 X Loc [ Vs_Vx_1 ]
3: 1 F(X) Loc [ Vs_Vx_1 ]
4: .15836 C0
5: 6.1445 C1
6: -8.4189 C2
7: 9.2493 C3
8: -3.1685 C4
9: .33392 C5
7.14 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 CR23X 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 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 CR23X 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
CR23X
FIGURE 7.14-1. 101 Thermistor Probes
Connected to CR23X
PROGRAM
1: Excite-Delay (SE) (P4)
1: 5 Reps
2: 24 1000 mV, 60 Hz Reject,
Slow Range
3: 1 SE Channel
4: 1 Excite all reps w/Exchan 1
5: 10 Delay (units 0.01 sec)
6: 400 mV Excitation
7: 1 Loc [ V_1 ]
8: 1.0 Mult
9: 0.0 Offset
2: Polynomial (P55)
1: 1 Reps
2: 1 X Loc [ V_1 ]
3: 1 F(X) Loc [ V_1 ]
4: -53.784 C0
5: 147.97 C1
6: -218.76 C2
7: 219.05 C3
8: -111.34 C4
9: 23.365 C5