| Sensitivity of Strain Gage Wire Materials | ||||||||||||||||||||||||||
 | ||||||||||||||||||||||||||
|
Recall the relations among the strain measure el and the resistance R of the gage wire from the theoretical background page.  To convert the change in resistance to strain, the sensitivity factor S of the strain gage material must first be determined. The sensitivity factors of common strain gage materials are listed in the following table. Platinum and Nickel which are not used in the pure form are listed for comparison purposes only.
| ||||||||||||||||||||||||||
|
| ||||||||||||||||||||||||||
|
| ||||||||||||||||||||||||||
where  is affected by the change in wire length, cross-section area, and the piezo-resistance effect of the wire material.
Since most metal materials have the Poisson's ratio around 0.25 to 0.35, the 1 + 2 n term in the strain sensitivity factor S is expected to be 1.5 to 1.7. However, the strain sensitivity factor S itself ranges from -12.1 in Nickel up to 6.1 in Platinum. This wide variation indicates that the change in electric resistivity r, the so called piezo-resistance effect, can be quite large in some materials. | ||||||||||||||||||||||||||
| Strain Gage Wire Material Selections | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
|
Although the sensitivity factor S is usually provided by the strain gage vendors, engineers still need to choose the right gage wire materials for their applications.
| ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| Self-Temperature-Compensation |
|
|
Changes in temperature during the test duration are almost unavoidable. The mismatch of thermal expansion coefficients between gage wire, backing, and specimen induces so called apparent strain which contributes errors to the strain measurements. Although the effect of temperature can be handled by either evaluation as a part of the data or by a dummy gage in the wheatstone bridge circuit, it is more desirable to have a gage which can take care of itself, especially when the temperature gradient and/or variation are large or the bridge circuit is unavailable. There are two types of self-temperature-compensation gages: the selected-melt gage and the dual-element gage. The selected-melt gage is based on proper processing of alloys, particularly through cold working, such that the gage wire has a very low thermally induced strain (apparent strain) over a wide range of temperatures. Constantan and Karma alloy are two most common gage wire material which have self-compensation property. The dual-element gage employs two gird elements which have different thermal expansion properties. The net effect of these two elements almost cancels each other. |
