What is a Strain Gage?

The bonded electrical-resistance strain gage is a device for measuring the compression or tension of a metal surface subjected to stress. The strain in the surface resulting from the stress causes a very small but proportional change in the electrical resistance of the strain gage. By measuring that change in resistance of the gage, the strain in the surface can be accurately determined.

What is the Best Installation Method for High Temperature Applications?

The most common methods for attaching high temperature strain gages has been the use of high temperature ceramic cement or flame sprayed materials. The cemented gage can be used at temperatures over 1000°C (1857°F). At higher temperatures, or long term testing, the porous cement is insufficient to protect the gage system from oxidation and prevent electrical leakage to ground. Because flame sprayed techniques usually produce a denser film, a flame sprayed gage can be used for a higher temperature application, and will generally operate for a much longer time at temperature than convensional ceramic cements.

For field test applications, where conventional flame spray or ceramic cement installation cannot be applied, a weldable flame sprayed gage can be used. The gage is applied to the test component simply by spot welding along the edge of the metal carrier.

What are Self-Temperature Compensated Strain Gages

A strain gage is considered self-temperature compensated for a specific material when the output of the strain gage is 1 microstrain, or less, per degree of temperature change when the material is heated and allowed to stay mechanically strain free.

What is Apparent Strain and How is it Measured?

Apparent strain is the output curve of a strain gage measured over a temperature range for a specific material. If a strain gage is put on a plate of a specific material which is allowed to expand and contract naturally with temperature, the output caused by temperature changes on the material is the apparent strain for that material. If a 6ppm/°F strain gage is applied to a 9ppm/°F stainless steel, the output of the gage will be higher than if applied to a 6ppm/°F mild steel. However, the output curve generated by that strain gage is the apparent strain for that stainless steel, when using that particular strain gage. (ppm is parts per million)

What is the Thermal Expansion?

Thermal expansion is the amount of expansion or contraction of a specific material caused by temperature. If you heat a mild steel bar from room temperature up to 300°F, it will expand at 6ppm/°F. This may also be called the TCE or temperature coefficient of expansion. Most materials have a recorded TCE which can be easily found. The TCE is measured in a strain free condition so a complex structure or part may have a different thermal expansion than the parent material. To some industries this is still called thermal expansion.