Operator`s manual

SECTION 7. MEASUREMENT PROGRAMMING EXAMPLES
7-6
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/
o
C temperature coefficient of the
fixed resistor will limit the error due to its change
in resistance with temperature to less than
0.15
o
C over the -10 to 40
o
C temperature range.
Because the measurement is ratiometric (R
s
/R
f
),
the properties of the 10 kohm resistor do not
affect the result.
PROGRAM
01: P9 Full BR w/Compensation
01: 1 Rep
02: 3 50 mV slow EX Range
03: 3 50 mV slow BR Range
04: 1 IN Chan
05: 1 Excite all reps w/EXchan 1
06: 4200 mV Excitation
07: 1 Loc [:Rs/R0 ]
08: 1.0111 Mult
09: 0 Offset
02: P16 Temperature RTD
01: 1 Rep
02: 1 R/Ro Loc Rs/R0
03: 2 Loc [:TEMP degC]
04: 1 Mult
05: 0 Offset
7.10 100 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
Figure 7.10-1.
FIGURE 7.10-1. 3 Wire Half Bridge Used to
Measure 100 ohm PRT
As in the example in Section 7.9, the excitation
voltage is calculated to be the maximum
possible, yet allow the +50mV 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
x
) to keep the voltage drop
across the PRT less than 50mV:
0.050V > V
x
115.54/(9900+115.54); V
x
< 4.33V
The excitation voltage used is 4.3V.
The multiplier used in Instruction 7 is determined
in the same manner as in Section 7.9. In this
example, the multiplier (R
f
/R
0
) 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 R
s
calculated
with Instruction 7, is actually R
s
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 (R
0
=
100 ohms) in the ice bath would be 100.17
ohms, and the resistance at 40
o
C would be
115.71. The measured ratio R
s
/R
0
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
o
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.