SECTION 7. MEASUREMENT PROGRAMMING EXAMPLES

FIGURE 7.9-1. Wiring Diagram for PRT in 4 Wire Half Bridge

The result of Instruction 9 when the first differential measurement (V1) is not made on the 2.5 V range is equivalent to Rs/Rf. Instruction 16 computes the temperature (°C) for a DIN 43760 standard PRT from the ratio of the PRT resistance at the temperature being measured to its resistance at 0°C (Rs/R0). Thus, a multiplier of Rf/R0 is used in Instruction 9 to obtain the desired intermediate, Rs/R0 (=Rs/Rf x Rf/Ro). If Rs and R0 were each exactly 100 ohms, the multiplier would be 1. However, neither resistance is likely to be exact. The correct multiplier is found by connecting the PRT to the CR10 and entering Instruction 9 with a multiplier of 1. The PRT is then placed in an ice bath (@ 0°C; Rs=R0), and the result of the bridge measurement is read using the *6 Mode. The reading is Rs/Rf, which is equal to Ro/Rf since Rs=Ro. The correct value of the multiplier, Rf/R0, is the reciprocal of this reading. The initial reading assumed for this example was 0.9890. The correct multiplier is: Rf/R0 = 1/0.9890 = 1.0111.

The fixed 100 ohm resistor must be thermally stable. Its precision is not important because the exact resistance is incorporated, along with that of the PRT, into the calibrated multiplier.

The 10 ppm/°C temperature coefficient of the fixed resistor will limit the error due to its change in resistance with temperature to less than 0.15°C over the specified temperature range. Because the measurement is ratiometric (Rs/Rf), the properties of the 10 kohm resistor do not affect the result.

PROGRAM

01:P9 Full BR w/Compensation

01:

1

Rep

02:23 25 mV 60 Hz rejection EX Range

03:23 25 mV 60 Hz rejection BR

 

 

Range

04:

1

IN Chan

05:1 Excite all reps w/EXchan 1

06:

2100

mV Excitation

07:

1

Loc [:Rs/Ro ]

08:1.0111 Mult (Rf/Ro)

09:

0

Offset

02:

P16

Temperature RTD

01:

1

Rep

02:1 R/Ro Loc Rs/Ro

03:2 Loc [:TEMP C ]

04:

1

Mult

05:

0

Offset

7-7