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Application
Notes >> Technical
Notes
Operating Instructions
for Validyne P55 DC Output Transducers
Introduction:
P55 DC Output Pressure Transducers combine a rugged and reliable variable
reluctance pressure sensor with solid state electronics to provide a
DC-DC transducer capable of operating from an un-regulated power supply
in industrial environments. Differential and gage versions feature symmetrical
pressure cavities with only corrosion-resistant materials and O-Ring
cavity seals in contact with the pressure media. Absolute versions feature
all-welded stainless steel construction. Both versions provide internal
adjustments for zero and span.
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Unpacking:
Transducers are shipped with plastic caps, plugs or adhesive stickers
over the pressure ports. These prevent dirt from entering the pressure
cavities. It is recommended that the covers be left on the ports until
making pressure connections. On very low range units, the port covers
may have a small hole. This is done to eliminate internal pressures
caused by installing the covers. Be sure to check the shipping carton
thoroughly for any accessory items - pressure fitting adapters, mating
connectors, etc. that may have been ordered. A pressure fitting adapter
with an o-ring seal is provided with absolute transducers.
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Installation:
Proper installation of the transducer is important but not difficult.
This section offers guidelines for mounting, pressure connections, plumbing,
liquid filling and electrical connections.
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Mounting:
An integral plate with four holes is provided for mounting on a flat
surface. The differential version with 1/8 inch pipe thread ports
may be mounted with ridgid pipe connections. To prevent strain on
the transducer body, the integral plate may be mounted to a flat surface,
otherwise ambient temperature changes may cause output shifts. Mounting
the transducer above the point of measurement will minimize the accumulation
of condensate or dirt in the transducer cavity. Also be sure that
the unit is mounted with adequate clearance for electrical connections
and bleed port access.
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Plumbing:
Plumbing should allow transducer removal and re-installation without
shutdown of the system. If the transducer is used for gage pressure
measurements, where one port is left open to atmosphere, then a simple
shut-off valve can be installed in the line to the transducer. The
open port (gage reference) should be covered by a porous filter to
prevent access of dirt and dust. A plastic pipe plug with a small
hole works well for this purpose. Non-porous plugs are not recommended
as the volume of air in the reference cavity will change the transducer
response and cause temperature error.
A differential pressure
transducer, such as that used to measure the pressure drop across
an orifice, requires more extensive valving. A three-valve
manifold is used to allow the transducer to be removed safely
while the system is under pressure. A bypass valve connects the +
and - pressure ports of the transducer. Two isolation valves are used
between each port and the plumbing where the measurement is made.
To pressurize the transducer after installation, first open the bypass
valve. This allows the system pressure to be applied equally to both
ports when the isolation valve is opened. Next, open each isolation
valve. Finally, close the bypass valve so that the differential pressure
to be measured will be applied to the transducer.
To remove the transducer
from service, first open the bypass valve. Next, close each isolation
valve, in turn. Bleed the system pressure out of the manifold using
the transducer bleed ports or an auxiliary drain valve connected to
the manifold. The transducer may now be removed from service.
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Pressure Connections:
Pressure connections are made via ports in each cavity. Before making
connections to the transducer, be sure the connecting fitting is free
of loose internal scale. Check the threads for cleanliness or damage.
If the thread is torn or nicked, it may be cleaned up with a thread
file or chaser. For pipe thread connections, wrap the threads in two
layers of teflon tape, stretching tightly so that the tape conforms
to the threads. The teflon acts as a lubricant and sealant, minimizes
galling, and makes disassembly easier. When attaching the pipe or
fitting, screw it in with a small wrench until snug, then give it
another half turn. For transducers having the 5/16-24 port, a mating
adapter fitting with an O-Ring seal is required. These transducers
are supplied with a 5/16-24 to 1/8 inch NPT male adapter fitting.
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Liquid Filling:
A bleed port for each pressure cavity facilitates cleaning or liquid
filling of the cavities. The bleed port is sealed with a set screw
machined to carry a glass-filled teflon gasket at its inner end. For
static pressure measurements it is generally not necessary to fill
the sensor cavity with the liquid media as any entrapped air will
transmit the pressure to the sensing diaphragm of the transducer.
For dynamic pressure measurements, however, the presence of entrapped
gas will severly limit the frequency response of the transducer. To
remove entrapped gas from the sensor cavity, loosen the bleed screw
on or two turns with a 5/64 inch hex wrench. Do not attempt to remove
the bleed screw as this may be dangerous in high pressure systems
and usually results in the loss of the bleed screw seal. As the gas
is expelled from the sensor cavity and bubble-free liquid flows out
from around the bleed screw threads, tighten the the bleed screw to
seal the cavity.
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Electrical Connections:
The standard electrical connector is a six pin PT02A-10-6P (Bendix
or equal). (The model P55 also has a pigtail lead option.) The internal
circuitry is electrically isolated from the transducer case. If desired,
the cable shield can be connected to the transducer case through the
connector shell. If the transducer is grounded through its mounting
or plumbing connections, the cable shield should be left floating
at the transducer end to eliminate noise from ground loop currents
circulating through the cable shield. (See Voltage
Output Wiring Diagram or Current Output Wiring
Diagram)
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Calibration:
A supplied from the factory, DC transducers have been calibrated for
the full scale pressure and output voltages specified on the purchase
order. If desired, the transducer can be adjusted for other calibrations
using the zero and span adjustments. For model P55, the zero and span
adjustments are accessible from the outside, and are located near the
electrical connector. The zero and span adjustments are used only to
trim the existing calibration, however, and cannot be used to make changes
greater than about 10%.
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Calibration Equipment Required:
DC Power Supply
Digital Voltmeter, 0 to 10.00
Vdc or Ammeter, 0-25 mA
Calibrated Pressure Source
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Calibration Procedure:
A. Connect the transducer to calibrated pressure source commensurate
with the range of the transducer. For absolute transducers, use a
vacuum pump for zero psia input and full scale ranges less than atmospheric
pressure.
B. Connect the DC power
supply to electrical connector terminals A (+) and D (-) for voltage
oputput versions. Connect power supply to terminals E and D for current
output versions. Or to correct pigtail leads (see discussion of electrical
connections, above). Set the supply voltage for the actual voltage
to be used at the installation. Check the data sheet for the acceptable
power supply voltage range.
C. Connect the voltmeter
to the electrical connector at terminal B (+) and C (-) for voltage
output versions. For current output versions, connect the ammeter
in series with terminal D and the power supply return. Or to correct
pigtail leads (see discussion of electrical connections, above).
D. Apply zero pressure, and adjust
the zero potentiometer on the transducer so that the voltmeter reads
0.000 +/- 0.001 Vdc or 4.000 +/-0.001 mA
E. Apply full scale pressure to the
transducer and adjust the span pot for a voltmeter reading of 5.00
+/-0.010 Vdc. or 20.000+/-0.001 mA
F. Repeat steps E and F , if necessary.
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