Campbell CR23X, Operator'S Manual

Page: 96 / 374

SECTION 7. MEASUREMENT PROGRAMMING EXAMPLES
7-8
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
±
10mV
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 364
µ
V/
o
C. With a
resolution of 0.33
µ
V on the
±
10mV 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.
The relationship between temperature and PRT
resistance is a slightly nonlinear one. Instruction
16 computes this relationship for a DIN standard
PRT where the nominal temperature coefficient
is 0.00385/
o
C. The change in nonlinearity of a
PRT with the temperature coefficient of
0.00392/
o
C is minute compared with the slope
change. Entering a slope correction factor of
0.00385/0.00392 = 0.98214 as the multiplier in
Instruction 16 results in a calculated
temperature which is well within the accuracy
specifications of the PRT.
PROGRAM
1: Full Bridge (P6)
1: 1 Reps
2: 21 10 mV, 60 Hz Reject, Slow
Range
3: 3 DIFF Channel
4: 1 Excite all reps w/Exchan 1
5: 5000 mV Excitation
6: 1 Loc [ Rs_Ro ]
7: .001 Mult
8: .02344 Offset
2: BR Transform Rf[X/(1-X)] (P59)
1: 1 Reps
2: 1 Loc [ Rs_Ro ]
3: 50 Multiplier (Rf)
3: Temperature RTD (P16)
1: 1 Reps
2: 1 R/R0 Loc [ Rs_Ro ]
3: 2 Loc [ TEMP_degC ]
4: .98214 Mult
5: 0.0 Offset
7.11 PRESSURE TRANSDUCER - 4
WIRE FULL BRIDGE
This example describes a measurement made
with a Druck PDCR 10/D depth measurement
pressure transducer. The pressure transducer
was ordered with passive temperature
compensation for use with positive or negative
excitation and has a range of 5 psi or about 3.5
meters of water. The transducer is used to
measure the depth of water in a stilling well.
Instruction 6, 4 Wire Full Bridge, is used to
measure the pressure transducer. The high
output of the semiconductor strain gauge
necessitates the use of the
±
50mV input range.
The sensor is calibrated by connecting it to the
CR23X and using Instruction 6 with a multiplier
of 1 and an offset of 0, noting the readings
(
6
Mode) with 10 cm of water above the
sensor and with 334.6 cm of water above the
sensor. The output of Instruction 6 is 1000 V
s
/V
x
or millivolts per volt excitation. At 10 cm the
reading is 0.19963mV/V and at 334.6 cm the
reading is 6.6485mV/V. The multiplier to yield
output in cm is:
(334.6 - 10)/(6.6485-.19963) = 50.334 cm/mV/V