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A strip gage consists of ten strain-sensitive
grids mounted on a common backing. This type of
gage offers a number of advantages in the study of
local strain distributions and strain gradients. As
an example, it is much easier, faster, and more
accurate to install the ten-grid strip in a single
operation than it would be to align and bond ten
individual gages for the same purpose. In addition,
the optical tooling employed in the manufacture of
the strip gage ensures that all grids are precisely
located. Grid spacing is also closer than can
usually be achieved with individual gages, thus
yielding better resolution of nonuniform strain
fields.
Overall dimensions for the complete patterns
vary with the grid and solder-tab configurations.
When necessary, some types of the gages can be cut
to produce smaller assemblies with fewer grids.
Most sizes are offered in two different versions -
with all grids oriented either parallel to, or
perpendicular to, the long axis of the strip. As
indicated in the gage listings, several types of
strip gages are designed with a common tab, or bus,
connected to all grids on one side. Since this
arrangement affects measurement accuracy, and may
not be compatible with some instrument systems, the
following information should be considered when
contemplating the use of such gages.
Common-Tab Strip Gages
Common-tab strip gages are generally not compatible
with multi-channel instruments, particularly those
incorporating individiual bridge excitation
supplies. When used with this type of
instrumentation, they will yield significantly
lower accuracy than a strip gage with electrically
independent grids. Effects of the common tab
include excessive initial unbalance of the
Wheatstone bridge circuit (possibly beyond the
balance range of the instrumentation), circulating
currents when the grids are powered simultaneously
from a common power supply, loss of leadwire
temperature compensation, and reduced accuracy in
shunt calibration. All of these effects should be
carefully considered by the user before selecting
strip gages with common tabs. Where greatest
accuracy is required, strip gages with electrically
independent grids should be employed, or common-tab
strip gages may be used with single-channel
instruments in conjunction with a switch and
balance unit.
For further information, and practical
recommendations on the use of common-tab strip
gages, request Vishay Measurements Group Tech Note
TN-516,
Errors Due to Shared Leadwires in Parallel
Strain Gage Circuits
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