What is a strain gage?
A strain gage or extensometer is a sensor, which measures strain, pressure, load, torque, position, etc. and it is based on the piezoresistive effect, which is the property that materials have to change the nominal value of their resistance when they are stressed and deformed in the direction of the mechanical axes.stress that deforms the strain gage will generate a variation in its electrical resistance.

strain gage

Types of strain gauges
The gauge adheres to the object whose deformation is to be studied using an adhesive, such as cyanoacrylate. As the object is deformed, so does the sheet, thus causing a variation in its electrical resistance. The materials that are usually used to make gauges are metal alloys, such as constantan, nichrome, or semiconductor elements, such as silicon and germanium.

Strain gauges can be classified into different types:

By resistance
By capacitance
Metal gauges
The metal gauges are made up of a very thin and fine base, to which a very fine metallic thread is attached, which can be wound or folded. The two terminals where the wire ends are attached to the transducers, for its manufacture various metallic conductors are used, such as the constantan, karma, isoelestic and platinum alloys.

metal gauge

The metal gauges can be of:
Metallic thread
They are attached to a base with constant measurements. They present errors when tension states exist and are the simplest. They consist of a protective film, support, a measuring wire, and the connection terminals.

wire gauge

Metallic film
This class of gauges has a manufacturing characteristic similar to printed circuits that have flexible bases. They are developed by the means of creating plates using photographs, called the photoengraving method. They consist of a protective film, support, a connection pad, and wide areas to reduce the effect of transverse stresses.

metal film gauge

Deposited metal:
They are applied directly to the surface by two methods: evaporation or chemical bombardment.

Gauges for resistance
This type of gauge is an electrical conductor that, when deformed, increases its resistance, since the conductors become longer and thinner.

resistance gauge

Capacitance gauges
These are associated with geometric characteristics and are used to measure stress and deformation. The electrical properties of the materials used for deformation have negligible electrical properties, so the materials of the capacitance gauges can be calibrated according to mechanical requirements. This allows them to have better calibrations compared to those of the electrical type.

Photoelectric gauge
By using an extensometer we can amplify the movement of the specimen, while a ray of light is passed through a variable opening, acting with the extensometer and directly with a photoelectric cell. As the gauge changes its opening, so does the amount of light that reaches the cell, which means that the intensity of the energy generated by the cell presents a variation, which we can measure, and from which we can obtain the deformation.

photoelectric gauge

Semiconductor gauges
They are made of silicon or other semiconductor material. Their resistive change is less linear than metal and they have a great dependence on temperature. They are used in the manufacture of integrated pressure sensors where they are implanted in the micro diaphragm to measure pressure.