Strain gauge
From Free net encyclopedia
Image:Strain gauge.svg A strain gauge (alternatively: strain gage) is a device used to measure deformation (strain) of an object. Invented by Edward E. Simmons in 1938, the most common type of strain gauge consists of an insulating flexible backing which supports a metallic foil pattern. The gauge is attached to the object by a suitable adhesive. As the object is deformed, the foil is deformed, causing its electrical resistance to change. This resistance change, usually measured using a Wheatstone bridge, is related to the strain by the quantity known as the gauge factor.
The gauge factor <math>GF</math> is defined as <math>GF=\frac{\Delta R/R_G}{\epsilon}</math> where <math>R_G</math> is the resistance of the undeformed gauge, <math>\Delta R</math> is the change in resistance caused by strain, and <math>\epsilon</math> is strain. For metallic foil gauges, the gauge factor is usually a little over 2. For a single active gauge and three dummy resistors, the output <math>v</math> from the bridge is <math>v=\frac{BV.GF.\epsilon}4</math> where <math>BV</math> is the bridge excitation voltage.
Foil gauges typically have active areas 2-10 mm in size. With careful installation, the correct gauge, and the correct adhesive, strains up to at least 10% can be measured.
Contents |
Gauges in practice
Foil strain gauges are used in many situations. Different situations place different requirements on the gauge.
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 energised for less than an hour, and operate for less than a second. Gauge manufacturers, eg Vishay or TML provide guidance.
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.
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 to the specimen, and two are left unattached, unstrained, and at the same temperature as the specimen and the attached gauges.
Other types of gauge
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.
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
External links
- Strain Gauge Tutorial
- 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
- Strain Gauge->Computer Tutorial
