Operator`s manual

SECTION 7. MEASUREMENT PROGRAMMING EXAMPLES
7-8
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
01: P6 Full Bridge
01: 1 Rep
02: 1 5 mV slow Range
03: 3 IN Chan
04: 1 Excite all reps w/EXchan 1
05: 5000 mV Excitation
06: 11 Loc [:Rs/R0 ]
07: .001 Mult
08: .02344 Offset
02: P59 BR Transform Rf[X/(1-X)]
01: 1 Rep
02: 11 Loc [:Rs/R0 ]
03: 50 Multiplier (Rf)
03: P16 Temperature RTD
01: 1 Rep
02: 11 R/Ro Loc Rs/R0
03: 12 Loc [:TEMP degC]
04: .98214 Mult
05: 0 Offset
7.12 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
21X 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
The offset is determined after the pressure
transducer is installed in the stilling well. The
sensor is installed 65 cm below the water level
at the time of installation. The depth of water at
this time is determined to be 72.6 cm relative to
the desired reference. When programmed with
the multiplier determined above and an offset of
0, a reading of 65.12 is obtained. The offset for
the actual measurements is thus determined to
be 72.6 - 65.12 = 7.48 cm.
The lead length is approximately 10 feet, so
there is no appreciable error due to lead wire
resistance. Example 7.13 shows a means of
compensating for long lead lengths.
FIGURE 7.12-1. Wiring Diagram for Full
Bridge Pressure Transducer
PROGRAM
01: P6 Full Bridge
01: 1 Rep
02: 3 50 mV slow Range
03: 1 IN Chan
04: 1 Excite all reps w/EXchan 1
05: 5000 mV Excitation
06: 1 Loc [:WATER cm ]
07: 50.334 Mult
08: 7.48 Offset