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Encyclopedia > Strain gauge
Typical foil strain gauge. The gauge is far more sensitive to strain in the vertical direction than in the horizontal direction. The markings outside the active area help to align the gauge during installation.

The gauge factor GF is defined as $GF=frac{Delta R/R_G}{epsilon}$ where RG is the resistance of the undeformed gauge, ΔR is the change in resistance caused by strain, and ε is strain. For metallic foil gauges, the gauge factor is usually a little over 2[2]. For a single active gauge and three dummy resistors, the output v from the bridge is $v=frac{BV cdot GF cdot epsilon}4$ where BV is the bridge excitation voltage. Strain Gauge factor (G) or strain factor is the product of strain and the quotient of change in strain gauge resistance and unstrained resistance of strain gauge. ...

Foil gauges typically have active areas of about 2-10 mm in size. With careful installation, the correct gauge, and the correct adhesive, strains up to at least 10% can be measured. For the band, see Adhesive (band). ...

## Contents

Visualization of the working concept behind the strain gauge on a beam under exaggerated bending.

Foil strain gauges are used in many situations. Different situations place different requirements on the gauge. Image File history File links Size of this preview: 388 Ã— 599 pixelsFull resolution (400 Ã— 618 pixels, file size: 51 KB, MIME type: image/png) Self made (for a university project). ... A statically determinate beam, bending under an evenly distributed load. ...

Gauges attached to a load cell would normally be expected to remain stable over a period of years, if not decades; whilst those used to measure the response in a dynamic experiment may only need remain attached to the object for a few days, be energized for less than an hour, and operate for less than a second. A single-point load cell A load cell is typically an electronic device (transducer) that is used to convert a force into an electrical signal. ...

### Variations in temperature

Variations in temperature will cause a multitude of effects. The object will change in size by thermal expansion, which will be detected as a strain by the gauge. The resistance of the gauge will change, and the resistance of the connecting wires will change.

Most strain gauges are made from a constantan alloy[3]. Various constantan alloys and Karma alloys have been designed so that the temperature effects on the resistance of the strain gauge itself cancel out the resistance change of the gauge due to the thermal expansion of the object under test. Because different materials have different amounts of thermal expansion, self-temperature compensation (STC) requires selecting a particular alloy matched to the material of the object under test.

Even with strain gauges that are not self-temperature compensated (such as isoelastic alloy), using a Wheatstone bridge arrangement it is possible to compensate for temperature changes in the specimen under test and the strain gauge. To do this in a Wheatstone bridge made of four gauges, two gauges are attached to the specimen, and two are left unattached, unstrained, and at the same temperature as the specimen and the attached gauges [4]. A Wheatstone bridge is a measuring instrument invented by Samuel Hunter Christie in 1833 and improved and popularized by Sir Charles Wheatstone in 1843. ...

Temperature effects on the lead wires can be cancelled by using a "3-wire bridge"[1] or a "4-wire Ohm circuit"[5] (also called a "4-wire Kelvin connection").

## Other gauge types

For measurements of small strain, semiconductor strain gauges, so called piezoresistors, are often preferred over foil gauges. A semiconductor gauge usually has a larger gauge factor than a foil gauge. Semiconductor gauges tend to be more expensive, more sensitive to temperature changes, and are more fragile than foil gauges. A semiconductor is a solid whose electrical conductivity is in between that of a conductor and that of an insulator, and can be controlled over a wide range, either permanently or dynamically. ... The piezoresistive effect describes the changing electrical resistance of a material due to applied mechanical stress. ... Piezoresistors are resistors made from a piezoresistive material and are usually used for measurement of mechanical stress. ...

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. Blood flow is the flow of blood in the cardiovascular system. ...

## Mechanical Types

Mechanical strain gauge used to measure the growth of a crack in a masonary foundation. This one is installed on the Hudson-Athens Lighthouse

Simple mechanical types (such as illustrated here) are used in civil engineering to measure movement of buildings, foundations, and other structures. In the illustrated example, the two halves of the device are rigidly attached to the foundation wall on opposite sides of the crack. The red reference lines are on the transparent half and the grid is on the opaque white half. Both vertical and horizontal movement can be monitored over time. In this picture, the crack can be seen to have widened by approximately 0.3mm (and no vertical movement) since the gauge was installed. Image File history File links Metadata Size of this preview: 800 Ã— 536 pixelsFull resolution (3872 Ã— 2592 pixels, file size: 4. ... Image File history File links Metadata Size of this preview: 800 Ã— 536 pixelsFull resolution (3872 Ã— 2592 pixels, file size: 4. ... The Hudson-Athens Lighthouse, sometimes called the Hudson City Light, is a lighthouse located in the Hudson River in the state of New York in the United States. ...

## References

1. ^ http://www.efunda.com/designstandards/sensors/strain_gages/strain_gage_selection_matl.cfm
2. ^ http://www.efunda.com/designstandards/sensors/strain_gages/strain_gage_sensitivity.cfm
3. ^ http://www.vishay.com/brands/measurements_group/guide/tn/tn505/505b.htm
4. ^ http://www.efunda.com/designstandards/sensors/strain_gages/strain_gage_sensitivity.cfm
5. ^ http://www.omega.com/literature/transactions/volume3/strain2.html

The piezoresistive effect describes the changing electrical resistance of a material due to applied mechanical stress. ... Resistance thermometers, also called resistance temperature detectors (RTDs), are temperature sensors that exploit the predictable change in electrical resistance of some materials with changing temperature. ...

Results from FactBites:

 The Strain Gage (1821 words) The shearing strain is the tangent of this angle. Poisson strain is defined as the negative ratio of the strain in the traverse direction (caused by the contraction of the bar's diameter) to the strain in the longitudinal direction. With an adhesive material inserted between the sensors and the strained surface, the installation is sensitive to creep due to degradation of the bond, temperature influences, and hysteresis caused by thermoelastic strain.
 Strain gauges : ELECTRICAL INSTRUMENTATION SIGNALS (1804 words) Strain gauges are frequently used in mechanical engineering research and development to measure the stresses generated by machinery. Strain gauges may be purchased as complete units, with both strain gauge elements and bridge resistors in one housing, sealed and encapsulated for protection from the elements, and equipped with mechanical fastening points for attachment to a machine or structure. Strain gauge resistance changes are typically measured in a bridge circuit, to allow for precise measurement of the small resistance changes, and to provide compensation for resistance variations due to temperature.
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