SECTION 7. MEASUREMENT PROGRAMMING EXAMPLES

PROGRAM

01:P5 AC Half Bridge

01:

6

Reps

02:15 2500 mV fast Range

03:

1

IN Chan

04:1 Excite all reps w/EXchan 1

05: 2500

mV Excitation

06:1 Loc [:H20 BARS ]

07:

1

Mult

08:

0

Offset

02:P59 BR Transform Rf[X/(1-X)]

01:

6

Reps

02:1 Loc [:H20 BARS ]

03:.1 Multiplier (Rf)

03:

P55

Polynomial

01:

6

Reps

02:1 X Loc H20 BARS

03:1 F(X) Loc [:H20 BARS ]

04:.15836 C0

05:6.1445 C1

06:-8.4189 C2

07:9.2493 C3

08:-3.1685 C4

09:.33392 C5

7.15NONLINEAR 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 CR10 is used to

measure the temperature of five 101 Probes (used with the CR21 but usually not the CR10). 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 CR10 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 CR10

7-14