As its name implies, a strain gage detects strain changes -- on everything from plane wings in a testing environment to parts of the human body. Most strain gages measure changes in electrical resistance that occur when an object undergoes strain.
The Electronics Behind the Measurements
Stress is a force exerted on an object, while strain is the deformation an object undergoes under stress. Strain gages are sensitive enough to identify minute deformations the eye can't see. If you built a typical strain gage, you would attach metal foil or wire to flexible backing material and affix that on the object you want to monitor. When that object deforms, the foil or wire does the same, causing its resistance to increase. If the object stretches the wire or foil when compression occurs, resistance decreases.
Strain Gages at Work
People use strain gages for a variety of creative purposes. For instance, a company called Sensimed developed a tiny strain gage that detects tiny pressure changes in a glaucoma patient's eyes. Engineers performing a force balance test in a wind tunnel can subject airplane wings to multiple levels of force and measure them accurately using strain gages. These devices also help companies stress test new products before releasing them.
Alternative Measurement Methods
There are strain gages that measure strain using acoustical, mechanical, optical and other methods. Because cost, complexity and other factors limit their widespread use, gages that detect resistance changes are still the most common. Optical sensors, for example, measure deformation, but they are delicate and best suited for laboratory work. Mechanical strain gages also work, but they are bulky and don't provide high resolutions.