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Figure A-1 illustrates a simple Strain Gauge Transducer. A
strain gauge measures the external force (pressure) applied
to a fine wire. The fine wire is usually arranged in the form
of a grid. The pressure change causes a resistance change due
to the distortion of the wire. The value of the pressure can
be found by measuring the change in resistance of the wire
grid. Equation E-1 shows the pressure to resistance
relationship.
Equation E-1
where:
R = resistance of the wire grid in ohms
K = resistivity constant for the particular type of wire grid
L = length of wire grid
A = cross sectional area of wire grid
Figure A-1
As the wire grid is distorted by elastic
deformation, its length is increased, and its cross-sectional
area decreases. These changes cause an increase in the
resistance of the wire of the strain gauge. This change in
resistance is used as the variable resistance in a bridge
circuit that provides an electrical signal for indication of
pressure. Figure A-2 illustrates a strain gauge pressure
transducer.
Figure A-2
An increase in pressure at the inlet of the
bellows causes the bellows to expand. The expansion of the
bellows moves a flexible beam to which a strain gauge has
been attached. The movement of the beam causes the resistance
of the strain gauge to change. The temperature compensating
gauge compensates for the heat produced by current flowing
through the fine wire of the strain gauge. Strain gauges,
which are nothing more than resistors, are used with bridge
circuits as shown in Figure A-3.
Figure A-3
Alternating current is provided by an exciter
that is used in place of a battery to eliminate the need for
a galvanometer. When a change in resistance in the strain
gauge causes an unbalanced condition, an error signal enters
the amplifier and actuates the balancing motor. The balancing
motor moves the slider along the slidewire, restoring the
bridge to a balanced condition. The slider’s position is
noted on a scale marked in units of pressure.
Other resistance-type transducers combine a
bellows or a bourdon tube with a variable resistor, as shown
in Figure A-4. As pressure changes, the bellows will either
expand or contract. This expansion and contraction causes the
attached slider to move along the slidewire, increasing or
decreasing the resistance, and thereby indicating an increase
or decrease in pressure.
Figure A-4
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