Page 88 - Mechanical Engineers' Handbook (Volume 2)

3 The Resistance Strain Gage 77

Bonding Adhesives
Resistance strain gage performance is entirely dependent on the bond attaching it to the
transducer flexure. The grid element must have the strain transmitted to it undiminished by
the bonding adhesive. The elimination of this bond is one of the principal advantages of
vacuum-deposited metallic and diffused semiconductor bridge transducers. Typical adhesives
are as follows:
Epoxy Adhesives. Epoxy adhesives are useful over a temperature range of 270 to
320 C. The two classes are either room-temperature curing or thermal setting type;
both are available with various organic fillers to optimize performance for individual
test requirements.
Phenolic Adhesives. Bakelite, or phenolic adhesive, requires high bonding pressure and
long curing cycles. It is used in some transducer applications because of long-term
stability under load. The maximum operating temperature for static loads is 180 C.
Polyimide Adhesives. Polyimide adhesives are used to install gages backed by polyimide
carriers or high-temperature epoxies. They are a one-part thermal setting resin and
are used from 200 to 400 C.

Ceramic cements (applicable from 270 to 550 C) and welding are other mounting tech-
niques.

Frequency Response
The frequency response of bridge transducers cannot be addressed without considering the
frequency response of the strain gage as well. It is assumed that the transducer is used in
such a manner that mounting variables do not influence its frequency response.
Piping in front of pressure transducer diaphragms and mounting blocks under acceler-
ometers are two examples of variables which can violate this assumption. Transducers, par-
ticularly those which measure force, pressure, and acceleration, typically are dynamically
modeled as single-degree-of-freedom systems characterized by a linear second-order differ-
ential equation with constant mass, damping, and stiffness coefficients. In reality, transducers
possess multiple resonant frequencies associated with their flexure and their case. Figure 4
presents the actual frequency response of a bridge-type accelerometer; The response indicates
this single-degree-of-freedom model to be adequate through the first major transducer res-
onance. Such devices have a frequency response usable (constant within 4% referenced to
their dc response) to one-fifth of the value of this major resonance. The strain gage itself
acts as a spatial averaging device whose frequency response is a function of both its gage
length and the sound velocity of the material on which it is mounted. Reference 6 discusses
this relationship from which Fig. 5 is extracted. Figure 5 contains curves for three different
length gages. Its abscissa must be multiplied by a specific sound velocity. For most bridge
transducers, the structural resonance of the flexure constrains its frequency response.

3.4 Electrical Aspects of Gage Operation
The resistance strain gage, which manifests a change in resistance proportional to strain,
must form part of an electrical circuit such that a current passed through the gage transforms
this change in resistance into a current, voltage, or power change to be measured. The
electrical aspects of gage operation to be considered include current in the gage, resistance
to ground, and shielding.