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 Variable
Reluctance Transducer Operating Instructions
Product Information
Introduction
Transducer Construction
Transducer as AC Bridge Circuit
Transducer Capabilities
Installation
Calibration
Maintenance
Diaphragm Replacement
Introduction
Properly installed, operated,
and maintained, the VALIDYNE PRESSURE TRANSDUCER will give you
a lifetime of reliable service. To help ensure trouble-free operation,
this manual describes the transducer's construction and capabilities.
It also provides specific procedures for correct installation,
calibration, maintenance, diaphragm replacement, and trouble shooting.
Following these procedures will increase the unit's efficiency
and save you considerable time and expense.
Transducer
Construction & Capabilities
A typical variable reluctance
pressure transducer consists of a diaphragm of magnetically permeable
stainless steel clamped between two blocks of stainless steel.
Embedded in each block is an inductance coil on an E-shaped core.
This coil assembly, covered by an Inconel disc, has a corrosion
resistant surface. In the undeflected position, the diaphragm
is centered with equal gaps (about 0.005 inch) between it and
the legs of each E-core to provide equal reluctances for the magnetic
flux paths of each coil.
A pressure difference applied
through the pressure ports deflects the diaphragm toward the cavity
with the lower pressure, decreasing one gap and increasing the
other. As the magnetic reluctance varies with the gap and determines
the inductance value of each coil, the diaphragm deflection increases
the inductance of one coil and decreases that of the other.
Transducer
Connected In An AC Bridge Circuit
The transducer connected in an
AC bridge circuit is able to take advantage of the inductance
variations in the transducer coils. The coils form one half of
a four-arm bridge, and the center tapped secondary of the carrier
supply transformer T1 (in the carrier demodulator) forms the other
half.
The electrical output of this
bridge circuit is an AC signal whose phase depends on the direction
of diaphragm displacement. When the diaphragm is in its undeflected
(zero) position, the bridge output is at its minimum. Since the
diaphragm displacement is linear with pressure applied, the bridge
output is also linear with pressure. If the diaphragm is displaced
in the opposite direction, the bridge output is again linear with
pressure. However, the phase relationship between the bridge signal
and excitation voltage has reversed 180. The carrier demodulator
takes this AC signal input, amplifies it, demodulates (rectifies)
it, and filters it into a +/-DC voltage which represents the pressure
polarity and magnitude.
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Transducer
Capabilities
The Validyne Variable Reluctance
Pressure Transducer has a number of advantages over typical transducers.
1. Acceptance of Corrosive Liquids
and Gases -
This transducer has low symmetrical
internal volumes which allow it to accept corrosive liquids and
gases. To prevent damage, you must keep dirt particles or congealable
fluids out of these cavities.
2. Good Dynamic Response -
Low volumetric displacement (full
scale diaphragm deflection is only about 0.0013 inch) results
in good dynamic response. For the best dynamic response, you should
couple the transducer closely to any gaseous pressure source OR
make sure it is free of entrapped air or gas in liquid systems.
3. High Overload Capability -
The internal cavity walls provide
effective overload stops; this allows high overloads. High overpressure
can cause a permanent zero shift or diaphragm damage.
4. High Output Signal -
This transducer provides a high
output signal with low susceptibility to electrical noise, a recognized
advantage of carrier systems. You should operate this transducer
through a carrier demodulator designed for it.
5. Replaceable Diaphragm -
This transducer has only one pressure
sensitive element the diaphragm which you can replace if necessary.
It is essential to read the diaphragm replacement instructions
before replacing it.
The following Validyne Differential
Pressure Transducer models can be disassembled for cleaning and/or
diaphragm replacement:
DP15, DP22, DP45, DP103, DP215,
DP303, DP360, and DP363.
The following Validyne transducers
CANNOT be disassembled in the field because they feature all-welded
construction:
Absolute Pressure Transducers:
Model AP10
Differential Pressure Transducers:
Model DP10
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Transducer
Installation
Proper installation of a transducer
is vital to its operation. This section contains specific procedures
for unpacking, mounting, installing plumbing, making pressure
connections, adjusting the bleed port, and making the electrical
connections.
A. Unpacking and Checking
Your transducer is shipped with
plastic caps/plugs or adhesive stickers over the pressure ports
to prevent dirt rom entering the pressure cavities. It is best
to keep these covers on the ports until you are ready to make
pressure connections. On very low range units, the port cover
may have a small hole in it. This hole eliminates internal pressures
caused by putting on the cover. Before unpacking the unit, check
the transducer identification to make sure it has the right pressure
range. The ID number for most Validyne transducers consists of
a model number (alpha-numeric) followed by a two digit dash number
which identifies the diaphragm pressure range, connector, O-rings,
temperature range, and pressure port configuration.
Also, check the shipment for any
mating connectors, cables, or adaptor fittings you may have ordered;
they might be packaged separately.
B. Mounting
Before mounting and plumbing up
your transducer, be sure to make a system calibration. (See the
Calibration Section of these instructions.)
Transducers with 1/8-27 NPT female
ports usually can be supported by rigid pipe connections. However,
transducers connected to tubing systems should be attached to
a support or bracket to prevent strains on tubing connections.
Below are the steps for mounting your transducer correctly.
1. Since most transducers have
tapped holes in the body for mounting screws, check the data sheet
for the screw thread size.
2. If the unit is mounted by screws
into its body, make sure the mounting surface is flat so that
it does not strain the transducer body. On low range transducers,
such strains can cause small zero shifts which can become worse
by temperature changes.
3. If possible, mount the transducer
so that its diaphragm is in the vertical plane. Vertical mountings
must be rigid since any change in the mounting will cause a zero
shift because of gravity. This type of mounting has two advantages.
For one, it results in zero gravitational deflection of the diaphragm
(noticeable in very low range units, particularly with liquid
media). For another, it allows dirt particles to fall to the outer
edge of the pressure cavity where they won't hinder diaphragm
movement.
4. If you mount the transducer
above the point of pressure measurement, it will trap less dirt
and the interior will remain cleaner.
5. When you mount the transducer,
make sure to leave room for the electrical connector, and for
access to the bleed ports.
C. Plumbing Connections
Make plumbing connections that
allow you to remove the transducer, if necessary, without shutting
down the entire plant.
1. If you will be using the transducer
for gage pressure measurement (PSIG) where one port is open to
the atmosphere, install a simple shutoff valve in the line to
the transducer. Cover the open port of a differential pressure
transducer with a porous filter to prevent entry of dirt and dust;
a plastic cap with a small hole or a wad of fine wire is sufficient.
2. If you will be using a differential
pressure unit for measuring the DP across an opening or filter,
it requires more extensive valving to place it into operation,
and also to remove it without overpressure damage. In this case,
use a valve arrangement as shown in the three-valve manifold connection
diagram.
3. To pressurize the DP transducer
safely, close the drain valve and open the bypass valve. Then
open both shutoff valves to apply the pressure to both sides of
the transducer. Finally, close the bypass valve.
4. To depressurize the transducer,
open the bypass valve, close the shutoff valves, and open the
drain valve. Valve manifolds for this purpose are available from
commercial valve and fitting suppliers.
D. Pressure Connections
The 1/8-27 NPT female pressure
port accepts American Standard Tapered Pipe Thread. To prevent
damage to the transducer case, however, be sure to follow the
procedure below exactly. Otherwise, efforts to make the connector
joint fit properly can cause stress in the case material around
the pressure port.
1. Before you connect the pipe
to the transducer, make sure the pipe is free of loose internal
scale, and check the threads for cleanliness or damage. If they
are torn or nicked, clean with a die or chaser.
2. Wrap the tapered pipe thread
with two layers of 1 inch wide Teflon Pipe Thread Tape, available
at plumbing supply stores.
As you wrap, stretch the tape
tightly enough to conform to the threads. Also, wrap in the direction
of the thread (as though screwing on a fitting). The Teflon functions
as lubricant and sealant, minimizes thread galling, and makes
disassembly easier.
3. To attach the pipe or fitting,
screw it in with a small wrench until it's snug; then give it
another half turn. tansducers with ports that have straight threads
(AP10, DP22) require mating fitting adaptors with O-rings to make
a leak-free connection at the transducer.
E. Transducer Bleeding
Most Validyne transducers are
equipped with bleed ports to facilitate cleaning and filling pressure
cavities. This port is closed by a set screw machined to carry
a flat circular Teflon washer at its inner end that functions
as the bleedport seal.
1. To measure static pressure,
it is not necessary to fill the transducer cavity with air; any
entrapped air or gas will transmit the pressure to the diaphragm.
2. When there are changes in the
response to dynamic pressure, or when oscillations in liquid-filled
systems are important, make sure the pressure cavity and transducer
connections are free of gas. Acting as a pneumatic spring, the
entrapped gas can seriously decrease the frequency response of
the measuring system.
3. To remove gas from the pressure
cavity, loosen the bleed screw one or two turns with the system
pressure ON. To prevent loss of the Teflon washer, DO NOT remove
the bleed screw. After the internal gas has been expelled, the
liquid will flow out around the screw. Close the screw tightly,
and wipe up the excess liquid.
F. Electrical Connections
Normally, the electrical connector
on the transducer is a six-pin PT02A-10-6P or the equivalent.
Pins A and D are for carrier excitation, and pin B is the output
signal. Pin C is not used. The mating connector is a PT06A-10-6S
(SR) or the equivalent. Connecting the transducer to the carrier
demodulator requires a three or four conductor shielded cable.
Validyne carrier demodulators and module cases use PT02A-10-6P
connectors for the transducer input, so a cable with a PT06A-10-6S
(SR) connector at each end is required. Ideally, this cable should
contain one shielded pair for the excitation leads and another
shielded conductor for the signal lead. Separating the signal
lead from the excitation leads minimizes the capacitive coupling
between the signal and excitation leads. Unequal capacitance values
connected across each transducer coil produces an impedance imbalance
in the transducer bridge that results in an output zero offset.
In short cables (10 feet), the effect of this imbalance is slight,
but in long cables (500 feet) it can cause a large zero offset.
For transducer connections, Validyne has standardized on Belden
Type 8434 cable, a readily available four conductor cable consisting
of two shielded pairs. Although the fourth conductor is not needed
for connection to variable reluctance transducers, it is useful
with other equipment such as strain gage or LVDT transducers.
The standard Validyne cable is wired as follows: Grn, + Carrier;
Red & Blk, Signal; Wht, - Carrier.
To minimize pickup from nearby
AC power sources or lines, always use shielded cable. Also, it
is best not to run transducer cables in trays or conduits carrying
AC power. Because the 3 or 5 kHZ transducer signal is about 200
mV rms full scale, a high current 230 Vac line lying next to it
can cause an excessive 60 Hz noise signal. If you mount the transducer
so that its case is grounded through the plumbing, disconnect
the cable shield from the transducer connector. This breaks the
electrical path for AC ground currents which might circulate through
the cable shield and cause signal zero instability. Such ground
loops result from differences in potential between ground points
in the electrical systems. Grounding the shield only at the signal
conditioning end will provide a single-point ground for the instrumentation.
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Calibration
Although Validyne has factory
tested the transducer to make sure that it meets specifications,
your system transducer, cable, and carrier demodulator has not
been calibrated unless specified on the purchase order. Since
the output depends on where you set the zero, gain, and other
controls, system calibration is necessary to obtain a known relationship
between input pressure and output reading.
1. Read the calibration section
of the instruction manual for your unit before calibrating your
system. This step is necessary because the various Validyne carrier
demodulators and transducer indicators differ in their control
capabilities, ranging from simple zero and span controls to sophisticated
suppression, auto-zero, and phase controls.
2. Use a known source of pressure
of a pressure standard to calibrate your system.
3. For the greatest accuracy,
calibrate the transducer and its carrier demodulator with the
actual cabling you will be using, preferably with the transducer
at its installation location if a cable run of 100 feet or more
is involved. This will also help prove out the installation.
4. Once you have calibrated your
system control positions, lock, tape, or record them in case you
need to re-calibrate the system because of accident or human error.
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Maintenance
The transducer does not require
any special maintenance. The occasional malfunctions are nearly
always caused by one of two problems: a transducer clogged by
dirt or a diaphragm damaged by overpressure.
A. Dirt in the Transducer
The usual symptoms of dirt are
decreased output, excessive nonlinearity and hysteresis, and zero
shift. To correct this problem, it is best to remove the transducer
from its installation before cleaning it, as follows:
1. Inject Freon or some other
low-residue solvent into the cavity. To do this, remove the pressure
fittings, if any, and the bleed screw.
DO NOT LOSE THE SMALL TEFLON WASHER
ON THE END OF THE BLEED SCREW.
2. Flush the cavity with solvent
and drain it by holding the transducer so the solvent can flow
out through the pressure port. Repeat this step until the solvent
comes out clean.
3. If the dirt consists of hardened
material, you will have to disassemble the transducer for cleaning.
To do this, see the diaphragm replacement instructions.
4. When the transducer is clean,
check the system calibration before reinstalling it.
B. Damaged Diaphragm
The usual symptom of overpressure
damage to the diaphragm is a permanent zero shift. A small zero
shift, 15-20 percent of full scale, does not necessarily mean
that the diaphragm is useless. Follow these steps to check and
correct damage:
1. Rezero the system with the
carrier demodulator zero control, and check the calibration. You
will probably find that the transducer is operating correctly.
However, a heavy zero shift, over 50% of full scale, may mean
that the diaphragm is seriously damaged and must be replaced.
2. To determine the usefulness
of the diaphragm after a substantial overpressure, loosen the
body bolts and retorque to the values shown on the Torque Chart.
This permits the diaphragm to return fully to an unstressed null
position.
3. Perform a calibration test,
looking particularly at zero stability, linearity, full scale
output, and plus-to-minus full scale symmetry.
Symptoms of a badly distorted
diaphragm include unstable zero, excessive non-linearity, and/or
severe loss of full scale output sensitivity.
C. Other Problems - If neither
of the above procedures corrects the problem, malfunction is usually
caused by one of the following: pinched O-rings, improperly torqued
body bolts, or damaged electrical connections.
Check the Troubleshooting
Guide, or call Validyne, whose staff of Application Engineers
will help you solve the problem.
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Diaphragm
Replacement
A. Disassembly and Cleaning
1. Remove the transducer's body
bolts on each side with a spline head, TORX or hex wrench. Spline
or TORX wrenches are required for all models except DP22, DP303,
DP360, and DP363.
2. Separate the case halves carefully,
and remove the diaphragm.
3. Clean the diaphragm and case
halves with a low residue solvent such as Freon or alcohol. Make
sure no particles of foreign matter are left inside to interfere
with the diaphragm motion (clearance between the diaphragm and
the case is only .005" in the center), or in the O-ring grooves.
(The DP45 does not have O-rings.)
If you have to replace the diaphragm,
refer to the Diaphragm Selection Chart.
B. Reassembly
Validyne strongly recommends that
you reassemble the transducer while it is connected to a transducer
indicator or carrier demodulator. To provide a zero reference,
zero the indicator before connecting the transducer; the indicator
is especially useful during the final tightening of the body bolts
since it will show the effects of individual bolt tightening.
It is not necessary to adjust the bolts to get an absolute zero
output, although it should be within +/-20% of full scale, and
not highly sensitive to small changes in any bolt torque.
1. To reassemble, place the O-rings
in the grooves in each half, and the diaphragm between the case
halves. If the diaphragm is corrugated, face the raised portion
of the corrugations toward the negative case half.
2. Make sure the bolt holes in
the diaphragm are aligned exactly with the holes in the case halves,
and replace the body bolts on one side of the transducer; tighten
the bolts until just snug.
3. Inspect the other diaphragm
bolt holes to see that they are also aligned with the case halves,
and replace the last body bolts; tighten all bolts to the torque
values given in the Body Bolt Torque Guide. If
a torque wrench is not available, tighten the bolts as much as
possible. Make sure that the threads on the body bolts do not
catch on the holes in the diaphragm, and that the O-rings are
seated in their grooves. Otherwise, the transducer zero may shift
out of tolerance.
4. Before calibration, pressure
cycle the transducer between zero and full scale several times
to seat the diaphragm completely. Some temperature cycling would
also enhance the stabilization.
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