Page 90 - Instrumentation Reference Book 3E

Circuits for strain gauges 75

small unit adhering securely to a surface, gener- can arise in the leads when the junctions of dis-
ally of metal. Very small changes in electrical similar metals are at different temperatures.
resistance have then to be measured, necessitating Gauges are often mounted some distance from
close control of any possible resistances that may their associated measuring equipment, and care
be either in series or parallel to that of interest- must be taken that the long leads involved do
i.e., from leads or leakage. not introduce errors. In a simple gauge configur-
Of course it is important to ensure that a gauge ation (Figure 4.9), the two leads will be directly in
is mounted at the correct site-often best identi- series with the live gauge, and any changes in
fied by tape. It may be noted that gauges can be their resistance, for instance from temperature,
mounted on cylindrical surfaces with quite small will be indistinguishable from strain. This can be
radii, but any double curvature makes fixing very overcome by having three leads to the gauge
difficult. We have already referred to the use of (Figure 4.10); one lead is now in series with each
another gauge for temperature compensation. of two adjacent arms, SO giving compensation
The introduction of any such “dummies” must be (for equal-ratio arms) provided changes in one
thought out; it is possible that an active gauge can lead are reproduced in the other. The third lead,
be used for compensation, so doubling the signal, going to the power source, is not critical. These
if a place can be identified where the strain will are called “quarter-bridge’’ arrangements.
be equal but opposite to that at the primary site, A “half-bridge” set-up is sometimes used (Fig-
e.g., the opposite side of a bending beam. ure 4.1 1). This is when two strain gauges are both
The surface where the gauge is to be fixed must used in the measurement as explained above.
be thoroughly cleaned-probably best by abra- The third possibility is a “full-bridge’’ (Figure
sion followed by chemical degreasing. Cements 4.12), when all four arms consist of gauges at the
commonly used are cellulose nitrate up to measurement site, the four long leads being those
1OO”C, epoxy up to 2OO0C, and ceramic above connecting the power source and out-of-balance
that where special techniques must be used. Gauge detector. Their resistances are not critical so it is
manufacturers may specify a particular cement for not necessary even to ensure that changes are
use with their product. equal. As with most bridge circuits, the power
After the gauge is fixed down, its leads should source and the detector can be interchanged. We
be fastened in position and connected (by solder- have called the power source “bridge voltage,”
ing or spot-welding) to the gauge. It is most implying that the supply is at constant potential;
important for leads to be mounted securely to it can alternatively come as a constant current
withstand the vibration they may be subject to; and this has some advantages for linearity.
in practice there are more failures of leads than in Bridges can be balanced, to take up component
strain gauges themselves. tolerance, by fine adjustment of series or parallel
Unless the installation is in a friendly environ- elements in the arms. Instead, the zero can be set
ment, it must then be protected by covering with within the amplifier that commonly forms part of
wax? rubber, or some such material. The chief the detector. Changing a high resistance across a
purpose of this is to exclude moisture. Moisture strain gauge can be used to simulate a known
could cause corrosion and, also serious, an elec- strain and so calibrate all the circuit side of the
trical leakage conductance. It must be remem- measuring system. It is possible to have measure-
bered that IO8 ohms introduced in parallel with ments made in terms of the adjustment needed to
a 350-ohm gauge appears as a 3 in a million re-balance a bridge after a strain has occurred but
reduction in the latter; such a paralleling can be more often the magnitude of the out-of-balance
caused between leads or by an earth leakage signal is used as an indication.
depending on the circuit configuration and gives The larger the voltage or current applied to 2
a false indication of strain of 3 x lop6. strain gauge bridge, the higher will its sensitivity be.
The various stages of installation are illustrated The practical limit is set by self-heating in the
in Figure 4.8. gauge. If a gauge is appreciably hotter than its
substrate, temperature errors are introduced. Com-
pensation from a similar effect in another gauge
cannot be relied on because the cooling is unlikely
4.5 Circuits for strain gauges to be identical in the two cases. Self-heating varies
a lot with the details of an installation but, with
For measurement of its small resistance changes, metal substrates, can generally be ignored below 1
a strain gauge is generally connected in a Wheat- milliwatt per square millimeter of gauge area.
stone bridge. This may be energized with d.c., but We have described the basic circuitry as it con-
a.c.-at frequencies of the order of kilohertz-is cerns a single strain gauge. Tests are often made
commoner; a.c. has the advantage of avoiding involving large numbers of gauges. For these,
errors from the thermocouple potentials that there is available elaborate equipment that allows