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Application
Notes >> Technical Notes
Variable Reluctance
Transducer Operating Instructions
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.
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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.
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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. [See Figure: Transducer
Bridge Circuit.] 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. Transducers 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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