Campbell Scientific CR7 Measurement and Control System - Manual (Page 77)

SECTION 7. MEASUREMENT PROGRAMMING EXAMPLES
7-9
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 2 differential input channels.
PROGRAM
01: P 7 3 Wire Half Bridge
01: 1 Rep
02: 4 50 mV slow Range
03: 1 IN Card
04: 1 IN Chan
05: 1 EX Card
06: 1 EX Chan
07: 1 Meas/EX
08: 4300 mV Excitation
09: 1 Loc [:Rs/Ro ]
10: 100.93 Mult
11: 0 Offset
02: P 16 Temperature RTD
01: 1 Rep
02: 1 R/Ro Loc Rs/Ro
03: 2 Loc [:TEMP degC]
04: 1 Mult
05: 0 Offset
7.13 100 OHM PRT IN 4 WIRE FULL
BRIDGE
This example describes obtaining the
temperature from a 100 ohm PRT in a 4 wire
full bridge (Instruction 6). The temperature
being measured is in a constant temperature
bath and is to be used as the input for a control
algorithm. The PRT in this case does not
adhere to the DIN standard (alpha = 0.00385)
used in the temperature calculating Instruction
16. Alpha is defined as (R100/R0-1)/100 where
R100 and R0 are the resistances of the PRT at
100 oC and 0 oC, respectively. In this PRT
alpha is equal to 0.00392.
FIGURE 7.13-1. Full Bridge Schematic For
100 Ohm PRT
The result given by Instruction 6 (X) is 1000
Vs/Vx (where Vs is the measured bridge output
voltage and Vx is the excitation voltage) which
is:
X = 1000 (Rs/(Rs+R1)-R3/(R2+R3))
The resistance of the PRT (Rs) is calculated
with the Bridge Transform Instruction 59:
Rs = R1 X'/(1-X')
Where
X' = X/1000 + R3/(R2+R3)
Thus, to obtain the value Rs/R0, (R0 = Rs @
0oC) for the temperature calculating Instruction
16, the multiplier and offset used in Instruction 6
are 0.001 and R3/(R2+R3), respectively. The
multiplier used in Instruction 59 to obtain Rs/R0
is R1/R0 (5000/100 = 50).
It is desired to control the temperature bath at
50oC with as little variation as possible. High
resolution is desired so the control algorithm will
be able to respond to minute changes in
temperature. The highest resolution is obtained
when the temperature range results in an output
voltage (Vs) range which fills the measurement
range selected in Instruction 6. The full bridge
configuration allows the bridge to be balanced
(Vs = 0V) at or near the control temperature.
Thus, the output voltage can go both positive
and negative as the bath temperature changes,
allowing the full use of the measurement range.
The resistance of the PRT is approximately
119.6 ohms at 50 oC. The 120 ohm fixed
resistor balances the bridge at approximately 51
oC. The output voltage is:
Vs = Vx [Rs/(Rs+R1) - R3/(R2+R3)]
= Vx [Rs/(Rs+5000) - 0.023438]
The temperature range to be covered is 50
+5oC. At 45 oC Rs is approximately 117.6
ohms, or:
Vs = -458.448x10-6 Vx
Vs can be measured on the ±1500 µV scale.
Setting Vs equal to -1500 µV and solving for Vx
results in Vx = 3.272 V. Vx is entered as 3270
mV in Parameter 8 of Instruction 6.