Operator`s manual

SECTION 7. MEASUREMENT PROGRAMMING EXAMPLES
7-7
PROGRAM
01: P7 3 Wire Half Bridge
01: 1 Rep
02: 3 50 mV slow Range
03: 1 IN Chan
04: 1 Excite all reps w/EXchan 1
05: 4300 mV Excitation
06: 1 Loc [:Rs/R0 ]
07: 100.93 Mult
08: 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
7.11 100 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 (R
100
/R
0
-1)/100, where
R
100
and R
0
are the resistances of the PRT at
100
o
C and 0
o
C, respectively. In this PRT,
alpha is equal to 0.00392.
FIGURE 7.11-1. Full Bridge Schematic for
100 ohm PRT
The result (X) given by Instruction 6 is 1000
V
s
/V
x
(where V
s
is the measured bridge output
voltage, and V
x
is the excitation voltage) which
is:
X = 1000 (R
s
/(R
s
+R
1
)-R
3
/(R
2
+R
3
))
The resistance of the PRT (R
s
) is calculated
with the Bridge Transform Instruction 59:
R
s
= R
1
X'/(1-X')
Where
X' = X/1000 + R
3
/(R
2
+R
3
)
Thus, to obtain the value R
s
/R
0
, (R
0
= R
s
@
0
o
C) for the temperature calculating Instruction
16, the multiplier and offset used in Instruction 6
are 0.001 and R
3
/(R
2
+R
3
), respectively. The
multiplier used in Instruction 59 to obtain R
s
/R
0
is R
1
/R
0
(5000/100 = 50).
It is desired to control the temperature bath at
50
o
C 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 (V
s
) range which fills the measurement
range selected in Instruction 6. The full bridge
configuration allows the bridge to be balanced
(V
s
= 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
119.7 ohms at 50
o
C. The 120 ohm fixed
resistor balances the bridge at approximately
51
o
C. The output voltage is:
V
s
= V
x
[R
s
/(R
s
+R
1
) - R3/(R
2
+R
3
)]
= V
x
[R
s
/(R
s
+5000) - 0.023438]
The temperature range to be covered is 50
±10
o
C. At 40
o
C R
s
is approximately 115.8
ohms, or:
V
s
= -802.24x10
-6
V
x
even with an excitation voltage (V
x
) equal to
5000mV, V
s
can be measured on the ±5mV
scale (40
o
C = 115.8 Ω = -4.01mV, 60
o
C = 123.6
Ω = 3.428mV). There is a change of
approximately 4mV from the output at 40
o
C to
the output at 51
o
C, or 364µV/
o
C. With a
resolution of 0.33µV on the 5mV range, this
means that the temperature resolution is
0.0009
o
C.
The 5 ppm per
o
C temperature coefficient of the
fixed resistors was chosen so that their 0.01%
accuracy tolerance would hold over the desired
temperature range.