Lab Room | Apparatus Map | Strain Gauges | Types of Strain Gauges | Electrical Strain Gauges

Electrical Strain Gauges

Resistance
Capacitance
Photoelectric
Semiconductor

Resistance Strain Gauges

The resistance of an electrically conductive material changes with dimensional changes which take place when the conductor is deformed elastically. When such a material is stretched, the conductors become longer and narrower, which causes an increase in resistance. This change in resistance is then converted to an absolute voltage by a wheatstone bridge. The resulting value is linearly related to strain by a constant called the gauge factor.

This is the type of strain gauge you will be using in the laboratory.

Capacitance Strain Gauges

Capacitance devices, which depend on geometric features, can be used to measure strain. The capacitance of a simple parallel plate capacitor is proportional to:

where:

C is the capacitance,

a is the plate area,

k is the dielectric constant, and

t is the separation between plates.

The capacitance can be varied by changing the plate area, a, or the gap, t. The electrical properties of the materials used to form the capacitor are relatively unimportant, so capacitance strain gauge materials can be chosen to meet the mechanical requirements. This allows the gauges to be more rugged, providing a significant advantage over resistance strain gauges.

Photoelectric Strain Gauges

An extensometer (an apparatus with mechanical levers attached to the specimen) is used to amplify the movement of a specimen. A beam of light is passed through a variable slit, actuated by the extensometer, and directed to a photoelectric cell. As the gap opening changes, the amount of light reaching the cell varies, causing a varying intensity in the current generated by the cell.

Semiconductor Strain Gauges

In piezoelectric materials, such as crystalline quartz, a change in the electronic charge across the faces of the crystal occurs when the material is mechanically stressed. The piezoresistive effect is defined as the change in resistance of a material due to an applied stress and this term is used commonly in connection with semiconducting materials. The resistivity of a semiconductor is inversely proportional to the product of the electronic charge, the number of charge carriers, and their average mobility. The effect of applied stress is to change both the number and average mobility of the charge carriers. By choosing the correct crystallographic orientation and dopant type, both positive and negative gauge factors may be obtained. Silicon is now almost universally used for the manufacture of semiconductor strain gauges.

Last Updated: January 16, 2000, beam@bits.me.berkeley.edu
Copyright © 1993-1995, 2000, Pamela A. Eibeck and Brandon Muramatsu
Original WWW Conversion by Winston Wang, 1994
WWW ReConversion by Brandon Muramtasu, 2000