Pressure Transducer - 4 Wire Full Bridge - Campbell CR510 Operator's Manual

SECTION 7. MEASUREMENT PROGRAMMING EXAMPLES
Even with an excitation voltage (V
mV, V can be measured on the +2.5 mV scale
s
(40 C = 115.8 ohms = -2.006 mV, 60 C = 123.6
ohms = 1.714 mV). There is a change of
approximately 2 mV from the output at 40 C to the
output at 51 C, or 181 µV/ C. With a resolution of
0.33 µV on the 2.5 mV range, this means that the
temperature resolution is 0.0018 C.
The 5 ppm per 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 slightly nonlinear one. Instruction
16 computes this relationship for a DIN
standard PRT where the nominal temperature
coefficient is 0.00385/ C. The change in
nonlinearity of a PRT with the temperature
coefficient of 0.00392/ 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: Full Bridge (P6)
1: 1 Reps
2: 21 2.5 mV 60 Hz Rejection
Range
3: 1 DIFF Channel
4: 1 Excite all reps w/Exchan 1
5: 2500 mV Excitation
6: 11 Loc [ Rs_Ro
7: .001 Mult
8: .02344 Offset
02: BR Transform Rf[X/(1-X)] (P59)
1: 1 Reps
2: 11 Loc [ Rs_Ro
3: 50 Multiplier (Rf)
03: Temperature RTD (P16)
1: 1 Reps
2: 11 R/Ro Loc [ Rs_Ro
3: 12 Loc [ Temp_C
4: .98214 Mult
5: 0 Offset
7-8
) equal to 2500
7.9 PRESSURE TRANSDUCER - 4 WIRE
x
FULL BRIDGE
This example describes a measurement made
with a Druck PDCR 1230 depth measurement
pressure transducer. The pressure transducer
was ordered for use with 5 volt 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 gage necessitates the
use of the 25 mV input range. The sensor is
calibrated by connecting it to the CR510 and using
Instruction 6, an excitation voltage of 2500 mV, 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
or millivolts per volt excitation. At 10 cm the
reading is 0.19963 mV/V and at 334.6 cm the
reading is 6.6485 mV/V. The multiplier to yield
output in cm is:
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.
]
01:
]
]
(334.6 - 10)/(6.6485-.19963) =
50.334 cm/mV/V
PROGRAM
Full Bridge (P6)
1: 1 Reps
2: 23 25 mV 60 Hz Rejection
Range
3: 1 DIFF Channel
4: 1 Excite all reps w/Exchan 1
5: 2500 mV Excitation
6: 1 Loc [ HT_cm
7: 50.334 Mult
8: 7.48 Offset
/V
s x
]