|
Electronic Pressure Element |
There two types of strain gauges
-
Bonded
Strain Gauges
-
Unbonded
Strain Gauges
Bonded
Strain Gauges
Discrete metal/silicon foil bonded or glued to a piece of insulating
material (usually some movable part of a mechanical pressure sensing
element eg. diaphragm) that will bend or flex with pressure changes
As an active sensing element strains or bends, the bonded foil will be
strained, The tension of the foil will change its resistance, Ideal for
taking dynamic measurements.
Advantages - Fast response, Low source impedance, Minimum
mechanical, motion, size and weight Works well with AC, and DC power
Disadvantages - Loss of accuracy due to hysteresis, Costly
output, measurement devices
Unbonded Strain Gauge
Consists of
insulated posts that support stretched small wires that will respond to
changes in pressure, These posts are linked to a mechanical pressure
element eg. diaphragm which deforms under pressure. The resulting
changes in tension in the wire will change the resistance indicating
changes in process pressure
Advantages - High sensitivity, Moderate accuracy, May be used in
high temperature; no adhesive required
Disadvantages - Frequent recalibration due to hysteresis, Zero
tends to shift; long term changes in wire resistivity and stress relief
Theory - The use of strain gauges is based on the fact that the
resistance of a conductor changes when the conductor is subjected to
strain. A resistance wire in it's original state, and after subjected to
a strain. The stretched wire has higher resistance as it is longer and
thinner.
The electrical resistance of a conductor is given by:
R = rl/A where: R = resistance, r = resistivity, l = length, A = cross
sectional area
The resistivity also changes as a result of the stresses within the
material of the wire, but these variations are only slight in normal
conductors and so the vast majority of the change results from the
deformation. In semi-conductive materials, this situation is reversed
such that the change in resistivity prevails.
The relationship between strain and resistance variation is almost
linear, and the constant of proportionality is known as the 'sensitivity
factor', or the 'K factor', where:
F = (dR/R)/(dl/l) where: F = strain sensitivity, R = initial resistance,
dR = change in resistance, l = initial length, dl = change in length
For a strain gauge, this constant is known as the 'strain sensitivity'
of the gauge, or the 'gauge factor', and is given the symbol 'F'.
Typical values for K (and F) lie between 2 and 4, and depend on the
material used.
Gauge Construction
Construction of electrical resistance strain gauges involves bringing
together the optimum combination of electrical resistance material and
backing plate.
For a good strain gauge, some of the most important features are listed
below:
-
Small size
and mass,
-
Ease of
production over a range of sizes,
-
Robustness,
-
Good
stability, repeatability and linearity over large strain range,
-
Good
sensitivity
-
Freedom
from (or ability to compensate for) temperature effects and other
environmental conditions,
-
Suitability for static and dynamic measurements and remote recording
-
Low cost
Wire gauges can be divided into two types
-
flat wound
and
-
wrap
around
Flat wound
strain gauge In flat wound gauges, the filament wire is zigzagged
between two pieces of paper. Wrap around strain gauge With wrap around
gauges, the wire is wrapped around a paper support. The advantage of
this is the possibility of smaller grid dimensions, the disadvantage is
that they do experience higher levels of creep.
Foil strain gauge Foil gauges are made from very thin metal strips (2-10
micrometers thick), and have very fine grids. They are essentially a
printed circuit, and therefore require the best manufacturing techniques
and careful handling to ensure good quality measurements.
It is possible to mass produce foil gauges, whereas wire gauges must
still be largely manufactured by hand.
|
