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Strain Gauge 101: How a Strain Gauge Works

Having read the previous blog entry, we can say we understand a little bit about strain. Now we’d like to look at how a strain gauge is used to measure the amount of strain on an object. Strain gauges are frequently used in mechanical engineering research and development to measure the stresses generated by machinery.

 

The most basic/common strain gauge construction involves affixing a metallic foil, usually in a zigzag pattern (this is the slinky from last week’s blog ) with an active area of about 2-10mm2, to a flexible backing which is in turn affixed to the object being measured (image). When the object is loaded, it deforms and the foil pattern deforms as well. This deformation of the foil pattern causes its electrical resistance to change. That change in electrical resistance is what we can measure, usually using a Wheatstone bridge , by applying a known voltage to the input leads of the gauge and then reading the output voltage to determine the change in resistance.

 

Strain gauges operate on the principle that the changes to the foil due to strain will cause changes to the electrical resistance in a defined way. When the foil is stretched (tension application), it will get longer and narrower (image ). This increases its electrical resistance. In the case of a compression application of load, the foil will get wider and shorter with the deformation of the object the strain gauge is attached to (image ). This would decrease the electrical resistance. Using a zigzag pattern creates a multiplicatively larger change in resistance than using a single straight  piece of foil, because it foil is essentially being deformed in multiple places instead of one. This makes it easier to measure small changes and makes the measurement more accurate. Why are we measuring such small changes? Because forces great enough to induce greater resistance changes would likely cause permanent deformation to either the test specimen or the strain gauge thereby “overloading” or ruining the device.

 

So, to sum up: The metallic foil (slinky) is attached to the object being stressed. When load is applied, the object and, consequently, the foil deform. The deformation in the foil causes a change in the electrical resistance which we can then measure. Through mathematical calculations, we can then determine how much force was used to produce that change in electrical output. And that, in a nutshell, is how a strain gauge works.

 

As always, if you have any questions related to this material, our support staff at Cooper Instruments is available to help. Contact them by calling (800) 344-3921 or emailing sales@cooperinstruments.com.

 

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