Gauge Construction
The large resistance variation characteristic of a potentiometer contrasts sharply with the minute resistance changes characteristic of a resistance strain gauge. Types of resistance strain gauges used as transducer elements include the unbonded metallic-filament strain gauge, the bonded metallic foil strain gauge and the bonded piezoresistive or semiconductor gauge.
Unbonded Strain Gauges
Unbonded strain gauge elements are made of one or more filaments of resistance wire stretched between supporting insulators. The supports can be attached directly to an elastic member used as a sensing element or can be fastened independently using a rigid insulator to couple the elastic member to the filaments of the resistance wire. The displacement (strain) of the sensing element causes a change in the filament length. The change in length results in changes in resistance. Because they are fragile, transducers that use unbonded gauges are becoming less popular.
Foil Gauges
A typical foil gauge consists of a foil made from a resistance alloy, such as Constantan, bonded to an epoxy backing film. The foil is either die-cut or etched to produce a grid pattern sensitive to strain along one axis.
Concepts associated with strain measurement
Strain
Normal Strain is defined as a fractional change or deformation per unit length in a body resulting from an applied force. Strain is either positive or negative. Positive strain is described an tensile; negative strain is described as compressive. The ratio of the change to the resting length is a dimensionless quantity that can be described as a percent strain. Generally, the ratio is expressed as microstrain defined in parts per million. For example:
0.001 inch/inch = 0.1% strain = 1000 micro-strain.
Stress
Stress is a term used to describe the relationship between the loading of a material with the material's ability to carry the load. Stress is defined as "normal stress" if the applied force is uniformly distributed over the area of a plane. Stress is defined as "shear stress" if the load is applied parallel to the plane.
Because stress cannot be measured directly, it must be evaluated using measurable parameters, such as strain, that vary in response to loading.
Gauge Factor (Gf)
Gauge factor is defined as the fractional change in resistance to the fractional change in length (strain) along the axis of a strain gauge. It is a dimensionless quantity that applies to the changes in the strain gauge as a whole. Typical gauge factors are close to 2.0. Gauge factors for special strain gauges can be significantly larger or even negative. The equation for gauge factor is:
Gf= (delta R/R)/(delta L/L) or delta R = R x Gf x strain
This implies that when 10 microstrain is applied to a gauge with R=120 ohms and Gf = 2.0, resistance will be changed by 0.0024 ohms.
Generally provided by the strain gauge manufacturer, the gauge factor (Gf) specification is valid only for a specific excitation voltage and ambient temperature. As a result, Gf cannot always be used directly to calibrate a measurement.
A sensitivity factor is sometimes specified. The sensitivity factor equals the gauge factor multiplied by a correction based on current measurement conditions. If the current conditions are the same as those under which the gauge factor was specified, the sensitivity factor and the gauge factor are identical.
