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Electrical Pressure Elements |
Electro-mechanical pressure transducers convert the motion produced by
mechanical sensing elements into changes in electrical signals for
monitoring and controlling process pressure
Generally, the electrical output is directly proportional to the applied
pressure
Types of electrical pressure elements are:
Pressure
transducer
An instrument component which detects a fluid pressure and produces an
electrical, mechanical, or pneumatic signal related to the pressure.
In general, the complete instrument system comprises a pressure-sensing
element such as a bourdon tube,
bellows, or
diaphragm element; a device which converts motion or force produced
by the sensing element to a change of an electrical, mechanical, or
pneumatic parameter; and an indicating or recording instrument.
Frequently the instrument is used in an auto control loop to maintain a
desired pressure.
Although pneumatic and mechanical transducers are commonly used,
electrical measurement of pressure is often preferred because of a need
for long-distance transmission, higher accuracy requirements, more
favorable economics, or quicker response. Electrical pressure
transducers may be classified by the operating principle as resistive
transducers, strain gages, magnetic transducers, crystal transducers,
capacitive transducers, and resonant transducers.
In resistive pressure transducers, pressure is measured by an element
that changes its electrical resistance as a function of pressure. Many
types of resistive pressure transducers use a movable contact,
positioned by the pressure-sensing element. One form is a contact
sliding along a continuous resistor, which may be straight-wire, wire-
wound, or nonmetallic such as carbon.
Strain-gage pressure transducers might be considered to be resistive
transducers, but are usually classified separately, They convert a
physical displacement into an electrical signal. When a wire is placed
in tension, its electrical resistance increases. The change in
resistance is a measure of the displacement, hence of the pressure.
Another variety of strain gage transducer uses integrated circuit
technology. Resistors are diffused onto the surface of a silicon crystal
within the boundaries of an area which is etched to form a thin
diaphragm.
In magnetic pressure transducers, a change of pressure is converted into
change of magnetic reluctance or inductance when one part of a magnetic
circuit is moved by a pressure-sensing element—bourdon
tube, bellows, or
diaphragm.
Piezoelectric crystals produce an electric potential when placed under
stress by a pressure-sensing element. Crystal transducers offer a high
speed of response and are widely used for dynamic pressure measurements
in such applications as ballistics and engine pressures. See also
Piezoelectricity.
Capacitive pressure transducers almost invariably sense pressure by
means of a metallic diaphragm, which is also used as one plate of a
capacitor.
The resonant transducer consists of a wire or tube fixed at one end and
attached at the other (under tension) to a pressure-sensing element. The
wire is placed in a magnetic field and allowed to oscillate. As the
pressure is increased, the element increases the tension in the wire or
tube, thus raising its resonant frequency
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