# Strain Gages

You can measure strain with a strain gage, which is a device with electrical resistance that varies in proportion to the amount of strain in the device, and with signal conditioning. When using a strain gage, you bond the strain gage to the device under test, apply force, and measure the strain by detecting changes in resistance (Ω). Strain gages return varying voltages in response to stress or vibrations in materials. Resistance changes in parts of the strain gage to indicate deformation of the material. Strain gages require excitation, generally voltage excitation, and linearization of the voltage measurements.

Strain measurements rarely involve quantities larger than a few microstrain (µε). Therefore, measuring strain requires accurate measurements of very small changes in resistance. For example, if a test specimen undergoes a substantial strain of 500 µε, a strain gage with a gage factor of 2 exhibits a change in electrical resistance of only 2 × (500 × 10 -6 ) = 0.1%. For 120 Ω, this is a change of only 0.12 Ω.

To measure such small changes in resistance and to compensate for temperature sensitivity, strain gages often use a Wheatstone bridge with a voltage or current excitation source, arranged in one of several bridge configurations. The gage is the collection of all of the active elements of the Wheatstone bridge.

NI-DAQmx supports measuring axial strain, bending strain, or both. While you can use some similar configuration types to measure torsional strain, NI software scaling does not support these configuration types. It is possible to use NI products to measure torsional strain, but to properly scale these configuration types you must use a custom scale with a bridge (V/V) or a custom voltage with excitation channel.

## Gage Factor

A fundamental parameter of the strain gage is its sensitivity to strain, expressed quantitatively as the gage factor (GF). Gage factor is the ratio of the fractional change in electrical resistance to the fractional change in length, or strain. The gage factor must be the same for each gage in the bridge.

The gage factor for metallic strain gages is usually around 2. You can obtain the actual gage factor of a particular strain gage from the sensor vendor or sensor documentation.

## Nominal Gage Resistance

Nominal gage resistance is the resistance of a strain gage in an unstrained position. You can obtain the nominal gage resistance of a particular gage from the sensor vendor or sensor documentation.