Characteristics of Strain Gauges

Understanding the Variables Involved in Selecting Strain Sensors

Oct 18, 2008

Choosing the right strain gauge for a measurement application requires selecting gauges with the right characteristics for the job.

When selecting a strain gauge for a measurement application, the characteristics of the selected sensor must match the characteristics of the system or object being measured. Strain gauges are available in many different sizes and configurations, so understanding the variables will result in proper gauge selection.

Alloy

The alloy is the type of material used for the strain gauge wire. A variety of alloys are available for strain gauges, depending on the specific application. Constantan is an all-purpose alloy, which is applicable for most general use applications. Annealed constantan is useful for high-strain applications. Isoelastic alloys are appropriate for dynamic strain applications or fatigue cycling. The Karma alloy is most applicable for long-term monitoring, as it maintains performance well over time.

Gauge Factor

The gauge factor is the the sensitivity of the strain gauge, or the relationship between resistance and strain. The gauge factor depends on the specific strain gauge wire material. The majority of strain gauges have a gauge factor around 2. Isoelastic alloys have a gauge factor around 3.6.

Gauge length is the length of the gauge matrix in the direction of strain. Larger gauge lengths are more resistant to localized temperature and strain effects, and are easier to handle due to their larger size. Smaller gauge lengths are appropriate for confined spaces or to measure very localized strains. Gauges with large gauge lengths are often less expensive than gauges with small gauge lengths.

Resistance

Resistance refers to the electrical resistance of the gauge wire, measured in ohms. Common strain gauge resistances are 350 ohms and 120 ohms. If a half-bridge or quarter-bridge circuit is used, the other arms of the bridge must be filled with dummy or active gauges with the same resistance.

Rosette

A rosette is a configuration of two or more individual gauges in one package. Rosettes can include two gauges to measure strain in two perpendicular axes. A rosette with three gauges, each 45 degrees apart, is used to calculate shear stresses. Three-gauge rosettes can also be configured so the three gauges are stacked on top of each other. This allows a gauge to fit into a smaller space, or to measure localized strains.

Self Temperature Compensation (S-T-C)

S-T-C is the ability of the gauge to resist environmental thermal changes. The S-T-C number is related to the coefficient of thermal expansion of the strain gauge materials.

Selecting the right strain gauge for a specific application will result in more consistent and accurate measurements throughout the monitoring process. For more information on strain gauges, please read the article An Introduction to Strain Gauges. For more information on strain gauge installation methods, please read the article Strain Gauge Installation Methods.

Sources

The copyright of the article Characteristics of Strain Gauges in Mechanical Engineering is owned by Susan Kristoff. Permission to republish Characteristics of Strain Gauges in print or online must be granted by the author in writing.