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SECTION
7.
MEASUREMENT PROGRAMMING EXAMPLES
The relationship between temperature and PRT
resistance
is
a slightly nonlinear
one.
Instruction
16
computes
this relationship for a DIN standard
PRT where
the nominaltemperature coefficient
is
0.00385/oC. The change in nonlinearity of
a
PRT with the temperature coefficient of
0.00392/oC 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
wellwithin the accuracy
specifications of
the
PRT.
PROGRAM
01:
Po
01:
1
02:
1
03:
3
04:
1
05:
5000
06:
11
07:
.001
08:
.02344
o2:
01:
02:
03:
11
50
P16
1
11
12
.98214
0
Full Bridge
Rep
5 mV slow Range
lN Chan
Excite
all reps w/EXchan
1
mV Excitation
Loc
[:Rs/RO
]
Mult
Offset
BR
Transform
Rf[)V(1
-X)]
Rep
Loc
[:Rs/RO
]
Multiplier
(Rf)
Temperature RTD
Rep
R/Ro Loc
Rs/RO
Loc [:TEMP degO]
Mult
Offset
P59
1
03:
01:
02:
03:
Q4:
05:
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
ol the semiconductor strain gauge
necessitates
the use
of
the
50mV input range.
The sensor
is
calibrated
by
connecting
it
to
the
01:
01:
02:
03:
Q4:.
05:
06:
07:
08:
7-8
21X
and using lnstruction
6
with
a multiplier
of
and an offset of
0, noting the readings ("6
with
10
cm
of
water above
the sensor and with
334.6 cm of water above the
sensor.
The
of Instruction 6 is 1000 Vr/V* or millivolts per
volt
excitation. At
1O
cm
the reading
is
0.19963mV4/ and
at
334.6 cm
the reading
is
6.6485mV4/. The
multiplier to yield
output
in
is:
(334.6
- 10)/(6.6485-.19963) = 50.334 cm/mV
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
0,
a reading of 65.12
is
obtained. The offset for
the
actual measurements
is
thus determined
to
be72.6
-
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.
CONNECTIONS
PRESSURE
TR
FfGURE
7.12-1. Wiring Diagram for Full
Bridge Pressure Transducer
P6
1
3
1
1
5000
1
50.334
7.48
PROGRAM
Full Bridge
Rep
50 mV slow Range
lN Chan
Excite
all
reps
dEXchan
mV Excitation
Loc [:WATER cm
]
Mult
Offset
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