What is a Strain Gauge?
Strain gauges measure the amount of stress or strain placed on an object. Arthur C. Ruge and Edward E. Simmons invented them in 1938. The most common type of strain gauge in use today consists of an insulating, flexible backing that supports a metallic foil pattern. The gauge is placed on the object with an adhesive. As the object comes under strain or gets deformed, the foil deforms and causes the electrical resistance to change. The change in resistance is then measured with a Wheatstone bridge.
How does a Strain Gauge Work?
Strain gauges use the electrical conductance’s dependence on the conductor’s geometry to measure the strain placed on an object. When an electrical conductor is stretched, it becomes longer and narrower and the electrical resistance increases as well. When a conductor is compressed, the resistance decreases across the material’s surface as it becomes shorter and broader. A strain gauge uses this principle when affixed to an object in order to measure the amount of strain being placed on the object.
Strain Gauge Usage
Strain gauges are read by applying excitation voltage to input leads on the gauge network. The reading is taken from the output leads. Gauge outputs are given in millivots, with the input voltages being between 5 to 12 volts depending on the network architecture. Strain gauges are most commonly used when making pressure sensors that comprise of silicon, metal or thick film, and bonded foil. Their uses vary as some are only used for one-time tests, while others are used on load cells that are designed to last for several years.
Specialized Strain Gauge Types
A foil gauge is not suitable for measuring small amounts of strain. As a result, a semiconductor strain gauge is used since it has a greater gauge factor than the traditional foil type. These types are more expensive and become less accurate when exposed to extreme temperature changes. A mercury-in-rubber strain gauge that contains liquid mercury enclosed in a rubber tube is used in biology. The gauge is placed around a body part such as a leg or toe. As the body swells, the tube stretches and the device’s electrical resistance increases. Finally, strain gauges have been employed in fiber optics using a capacitive strain gauge. This gauge type measures the level of mechanical deformation along a fiber line in order to provide readings.
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