Strain gauge

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 $GF$ is defined by the formula $GF={\frac {\Delta R/R_{G}}{\epsilon }}$ where $R_{G}$ is the resistance of the undeformed gauge, $\Delta R$ is the change in resistance caused by strain, and $\epsilon$ is strain.

For measurements of small strain, semiconductor strain gages 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.

External links

• How to measure strain - Tutorial on using strain gauges to measure strain
• [1] Strain gauges the original subject of Murphy's law