SECTION 7. MEASUREMENT PROGRAMMING EXAMPLES

FIGURE 7.10-1. 3 Wire Half Bridge Used to Measure 100 ohm PRT
7.10100 OHM PRT IN 3 WIRE HALF BRIDGE

The temperature measurement requirements in this example are the same as in Section 7.9. In this case, a three wire half bridge, Instruction 7, is used to measure the resistance of the PRT.

The diagram of the PRT circuit is shown in Fig. 7.10-1.

As in the example in Section 7.9, the excitation voltage is calculated to be the maximum possible, yet allow the +25 mV measurement range. The 10 kohm resistor has a tolerance of

±1%; thus, the lowest resistance to expect from it is 9.9 kohms. We calculate the maximum excitation voltage (Vx) to keep the voltage drop across the PRT less than 25 mV:

0.025V > Vx 115.54/(9900+115.54);

Vx < 2.17 V

The excitation voltage used is 2.1 V.

The multiplier used in Instruction 7 is determined in the same manner as in Section

7.9.In this example, the multiplier (Rf/R0) is assumed to be 100.93.

The 3 wire half bridge compensates for lead wire resistance by assuming that the resistance of wire A is the same as the resistance of wire

B.The maximum difference expected in wire resistance is 2%, but is more likely to be on the

order of 1%. The resistance of Rs calculated with Instruction 7, is actually Rs plus the difference in resistance of wires A and B. The average resistance of 22 AWG wire is 16.5

ohms per 1000 feet, which would give each 500 foot lead wire a nominal resistance of 8.3 ohms. Two percent of 8.3 ohms is 0.17 ohms.

Assuming that the greater resistance is in wire B, the resistance measured for the PRT (R0 = 100 ohms) in the ice bath would be 100.17 ohms, and the resistance at 40°C would be

115.71.The measured ratio Rs/R0 is 1.1551; the actual ratio is 115.54/100 = 1.1554. The temperature computed by Instruction 16 from the measured ratio would be about 0.1°C lower than the actual temperature of the PRT. This source of error does not exist in the example in Section 7.9, where a 4 wire half bridge is used to measure PRT resistance.

The advantages of the 3 wire half bridge are that it only requires 3 lead wires going to the sensor and takes 2 single-ended input channels, whereas the 4 wire half bridge requires 4 wires and 2 differential channels.

PROGRAM

01:P7 3 Wire Half Bridge

01:

1

Rep

02:23 25 mV 60 Hz rejection Range

03:

1

IN Chan

04:1 Excite all reps w/EXchan 1

05:

2100

mV Excitation

06:

1

Loc [:Rs/Ro ]

07:

100.93

Mult

08:

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

7-8