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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. 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 eg 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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