Ohm Prt In 3 Wire Half Bridge - Campbell CR510 Operator's Manual

SECTION 7. MEASUREMENT PROGRAMMING EXAMPLES
7.7 100 OHM PRT IN 3 WIRE HALF
BRIDGE
The temperature measurement requirements in
this example are the same as in Section 7.8. 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.
7.7-1.
As in the example in Section 7.8, 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 (V
across the PRT less than 25 mV:
0.025V > V
x
V
The excitation voltage used is 2.1 V.
The multiplier used in Instruction 7 is
determined in the same manner as in Section
7.8. In this example, the multiplier (R
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
CR510
FIGURE 7.7-1. 3 Wire Half Bridge Used to Measure 100 ohm PRT
7-6
) to keep the voltage drop
x
115.54/(9900+115.54);
< 2.17 V
x
/R ) is
f 0
resistance is 2%, but is more likely to be on the
order of 1%. The resistance of R
with Instruction 7, is actually R
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 (R
100 ohms) in the ice bath would be 100.17
ohms, and the resistance at 40 C would be
115.71. The measured ratio R
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 7.8, 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.
A terminal input module (Model 3WHB10K) can
be used to complete the circuit in Figure 7.7-1.
calculated
s
plus the
s
=
0
/R is 1.1551;
s 0