Strain Gauge Basics
A strain gauge is a device used to measure the mechanical strain on an object or structure. When an object is under a lot of pressure, the material can slowly fatigue and begin to subtly bend. These bends, nearly impossible to see with the naked eye, can be early indicators that a building is aging or beginning to buckle. Strain gauges constantly measure these minute changes, giving engineers an easy way to monitor a structure's condition.
Strain Gauge Structure
A strain gauge usually consists of a long strip of metal foil attached to a sheet of flexible material.The strip is thin and long, and zig-zags back and forth between the insulating sheets to maximize its length. At each end, the gauge is attached to a Wheatstone bridge, a device that measures its resistance. The gauge is cemented onto whatever surface it is meant to measure.
How It Works
The metal foil is a resistor---it conducts electricity but also resists its flow somewhat. When the sheet is bent one way it stretches the foil slightly, making it longer and thinner. This raises its resistance, since the electricity has further to go and a more constricted path to follow. On the other hand, when the sheet is bent in the other direction it compresses the foil, making it shorter and wider and lowering the resistance. Because the foil strip is so long, even slight strains will cause fairly big changes in the resistance, which will be picked up by the Wheatstone bridge.