Page 84 - Mechanical Engineers' Handbook (Volume 2)

3 The Resistance Strain Gage 73

Inconel is a good flexural material in corrosive salt water environments. These materials and
others are discussed in an extremely good article on transducer flexures in Chapter 11 of
Ref. 1.

3 THE RESISTANCE STRAIN GAGE
Strain gages are used to measure the strain pattern on the surface of the flexure in bridge
transducers. In 1938, Simmons, at the California Institute of Technology, and Ruge, at the
Massachusetts Institute of Technology, discovered independently that fine wire bonded di-
rectly to a surface being studied would respond to surface strain. Ruge’s original gage was
made by unwinding a constantan wire-wound vitrified resistor, gluing a portion of this wire
with Duco cement to a celluloid bar, attaching brass shim stock as terminals, and interfacing
the completed assembly to a galvanometer. The first strain gage manufacturer established in
the United States was the Baldwin Lima Hamilton (BLH) Corporation (now part of the
Vishay Measurements Group). BLH gages operating on the principle discovered by Simmons
and Ruge were designated the SR-4 gage to include the initials of both men. The evolution
of the bonded resistance wire strain gage occurred during the early 1940s. The first practical
bridge transducer load cell was built by Baldwin Lima Hamilton in 1941. By the mid-1950s,
Baldwin Lima Hamilton remained the only major strain gage manufacturer and the foil strain
gage was beginning to appear. Subsequent work by W. P. Mason and R. N. Thurston (re-
ported in the Journal of the Acoustical Society of America, vol. 29, 1957) resulted in the
introduction of the commercial semiconductor strain gage. Continued maturation of the
bonded resistance strain gage has enabled a transducer industry centered around this tech-
nology to develop.
Other sources provide the derivation of all the equations dealing with ensuing topics in
this chapter. 2–5

3.1 Strain Gage Types and Fabrication

Paper-backed wire strain gages typically consist of a grid of resistance wire to which a paper
backing has been attached with nitrocellulose cement. The wire is manufactured by drawing
the selected alloy through progressive forming dies. To protect it during handling and as-
sembly, the wire is usually sandwiched between two thin layers of paper. Typical grid wires
are 0.02 mm in diameter.
Foil strain gages are essentially small printed circuits. Art work for a master gage pattern
is first prepared. This pattern is then photographically reduced, and multiple images are
placed on a photographic plate. A sheet of foil (typically 0.003–0.005 mm thick) of the
appropriate alloy has a light-sensitive emulsion applied, is exposed to the photographic plate,
and then undergoes a development process. Chemical etching removes all but the grid ma-
terial. The resultant grid cross section is square as opposed to round for wire. Advantages
inherent in foil gages include better strain transmission due to improved bonding of the grid
to the backing, a better thermal path for dissipation of electrically generated heat, and a grid
that can more readily be configured to minimize sensitivity to transverse strains.
The total combination of wire and foil gages span grid lengths from 0.2 mm to more
than 250 mm. Foil gages satisfy the smaller of these requirements. Both wire and foil gages
have associated with them a variety of ohmic values, such as 120, 175, 350, 1000, and so
on. Standard values which have historically evolved are 120 and 350 . These values are
carry-overs from impedance-matching requirements for galvanometers which were formally
used for strain recording. Figure 3 displays numerous configurations of wire and foil strain
gages.