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A strain gage (alternatively: strain gauge) is a device used to measure deformation (strain) of an object. Invented by Edward Simmons in 1938, the most common type of strain gage consists of a flexible backing which supports a metallic foil pattern etched onto the backing. As the object is deformed, the foil pattern is deformed, causing its electrical resistance to change due to the piezoresistive effect. This resistance change, usually measured using a Wheatstone bridge circuit, can be used to calculate the exact amount of deformation by means of the quantity known as the gage factor.

The gage factor of a strain gage relates strain to change in electrical resistance. The gage factor ${\displaystyle GF}$ is defined by the formula ${\displaystyle GF={\frac {\Delta R/R_{G}}{\epsilon }}}$ where ${\displaystyle R_{G}}$ is the resistance of the undeformed gauge, ${\displaystyle \Delta R}$ is the change in resistance caused by strain, and ${\displaystyle \epsilon }$ is strain.

For measurements of small strain, semiconductor strain gages, so called piezoresistors, are often preferred over foil gages. A semiconductor gage usually has a larger gage factor than a foil gauge. Semiconductor gages tend to be more expensive, more sensitive to temperature changes, and are more fragile than foil gages.

In biological measurements, especially blood flow / tissue swelling, a variant called mercury-in-rubber strain gauge is used. This kind of strain gauge consists of a small amount of liquid mercury enclosed in a small rubber tube, which is applied around e.g. a toe or leg. Swelling of the body part results in stretching of the tube, making it both longer and thinner, which increases electrical resistance.

See also

• Load cell
• piezoresistive effect
• piezoresistor

External links

• How to measure strain - Tutorial on using strain gauges to measure strain
• Fastest man on earth Strain gauges was the original topic of Murphy's law

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