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 Theory
of Operation - VR Pressure
Transducers
Features
Variable Reluctance Sensing Technology
Sensitivity to Very Low Pressures
Changeable Ranges
Frequency Response
Features
- High Sensitivity to Low Pressures
- Ranges as Low as 0.1 In H2O
Full Scale
- Changeable Sensing Diaphragms
- Rugged Construction
- Gas or Liquid Media
- Fast Dynamic Response
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Variable Reluctance
Pressure Sensing Technology
The ability to measure very low
pressures accurately, to provide changeable transducer ranges,
high frequency response and rugged durability all depend on manufacturing
a pressure transducer with no linkages or other connections to
the sensing element. Here is how it is done.
A variable reluctance pressure
transducer is perhaps best described as an inductive half-bridge,
and consists of a pressure sensing diaphragm and two coils. The
coils are wired in series and are mounted so their axes are normal
to the plane of the diaphragm. Clamped tightly between the coil
housings, the diaphragm is free to move in response to differential
pressure.
The coils are supplied with an
AC excitation, typically 5 Vrms at 3 or 5 Khz. The coils are matched
so that their impedances are approximately equal. When a differential
pressure is applied to the sensor, the diaphragm deflects away
from one coil and towards the opposite. The diaphragm material
is magentically permeable, and its presence nearer the one coil
increases the magnetic flux density around the coil. The stronger
magnetic field of the coil, in turn, causes its inductance to
increase, which increases the impedance of one coil. At the same
time, the opposite coil is decreasing its impedance. The change
in coil impedances brings the half-bridge out of balance, and
a small AC signal appears on the signal line.
The change in coil impedance is
directly proportional to the position of the diaphragm, so the
amplitude of the signal is directly proportional to the applied
pressure. The phase of the signal with respect to the excitation
is determined by the direction of movement of the diaphragm. The
output of a variable reluctance circuit at its full scale pressure
is 20 mV/V or more. This is about ten times more output than is
typical for strain gage transducers.
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Sensitivity
to Very Low Pressures
Because the diaphragm need move
only one or two thousandths of an inch to produce a full scale
output, the thickness and area of the diaphragm determines the
full scale pressure range. A large diaphragm made of thin foil
will respond to extremely low pressures; Validyne offers standard
full scale ranges as low as 0.1 In H2O. Conversely, a thick diaphragm
with a small area responds to very high pressures and Validyne
offers one model having a full scale of up to 10,000 Psid.
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Changeable
Ranges
Becasue the diaphragm is the only
moving part, Validyne transducers are designed so that the user
may change the pressure range by substitution of diaphragms. The
coil housings are clamped together with bolts and may be disassembled
in the field. A series of diaphragms are available to allow a
wide variety of pressures to be sensed with a single transducer.
One Validyne model has 23 different diaphragms available covering
full scale pressures from 3.5 In H2O to 3200 psi.
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Frequency
Response & Durability
The diaphragm is the only moving
part of a variable reluctance pressure sensor; there are no linkages
or other items that must move in response to pressure. In addition,
the diaphragm must move only one or two thousanths of an inch
over its full pressure range. This means that the diaphragm must
be accelerated and moved but a small distance to reflect a change
in pressure. This results in a high natural frequency. There are
no internal fluids or hydraulic constraints inside the sensor,
which enchances its high natural frequency response.
Similarly, there are no delicate
mechanisms inside the variable reluctance sensor to be damaged
by shock or impact. The transducer may be dropped or otherwise
mishandled without damage. In the event of an overpressure, the
only parts likely to be damaged, the diaphragms, o-rings, etc,
may be easily replaced.
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