If a metal conductor is stretched or compressed, the resistance changes as both length and diameter change. There is a change in the value of resistivity of the conductor also when it is strained and this property is called Piezoresistive effect. Strain gauges work on the principle of “Piezoresistive effect”. When a strain gauge is subjected to positive strain, its length increases and area of cross section decreases. As resistance R is directly proportional to its length L and inversely proportional to the area of cross section A, the resistance of the conductor increases with positive strain. But this change is greater than that due to change in dimensions. As R =( ρ L)÷A, where ρ = resistivity of the conductor, the extra change in dimensions is due to change in resistivity of the conductor when strained. This property is piezoresistive effect.
Gauge Factor: It is the defined as the ratio of per unit change in resistance to per unit change in length.
Types of strain gauges
1. Unbonded metal strain gauges 2. Bonded metal wire strain gauges 3. Bonded metal foil strain gauges 4. Bonded semiconductor strain gauges 5.Vacuum deposited metal film strain gauges 6. Diffused metal strain gauges 7. Sputter deposited thin metal strain gauge