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Stress is a measure of the internal distribution of force per unit area within a body that balances and reacts to the loads applied to it.

The SI unit of measure of stress is the same as for pressure and is the Pascal (symbol Pa). In the USA, the customary unit of measure is pounds-force per square inch (symbol psi).

The most common method used to detect stress is actually to measure one of the secondary effects, namely the deformation that occurs when an item is subjected to stress. This deformation is Strain and it is measured with the strain gauge (or strain gage). There are many specialist application notes available from the manufacturers of the strain gauges themselves, so here, we shall discuss the signal processing that accompanies the strain gauge installation.

The strain gauge is a passive resistive device that has a known variation in resistance per unit strain. This relationship is known as the gauge factor (GF). To get the best performance from strain gauges, they are commonly used in a Wheatstone Bridge configuration, which has the dual advantages of signal amplification (if wired correctly !) as well as error cancellation.

Strain gauge installations are therefore described as being quarter, half or full bridge instalations, and in the case of quarter or half bridge, then bridge completion resistors need to be provided.