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Strain is the deformation that occurs when an item is subjected to Stress, and is therefore expressed by a change in shape or length, for a given level of stress.

Strain has no units of measure because it is a ratio, but the term Microstrain is often used in connection with strain gauge applications, and in this case has a unit of microinches per inch. Therefore, 1000 microstrain describes a change in length of 1000/1000000 or 1/1000 per unit length for the gven level of stress.

The most common method used to detect strain is the strain gauge (or strain gage), and there are many specialist application notes available from the manufacturers of the strain gauges themselves. 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.