Strain Gage: Sensitivity
 Strain Gage: Sensitivity
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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.

 Material Sensitivity (S) Platinum (Pt 100%) 6.1 Platinum-Iridium (Pt 95%, Ir 5%) 5.1 Platinum-Tungsten (Pt 92%, W 8%) 4.0 Isoelastic (Fe 55.5%, Ni 36% Cr 8%, Mn 0.5%) * 3.6 Constantan / Advance / Copel (Ni 45%, Cu 55%) * 2.1 Nichrome V (Ni 80%, Cr 20%) * 2.1 Karma (Ni 74%, Cr 20%, Al 3%, Fe 3%) * 2.0 Armour D (Fe 70%, Cr 20%, Al 10%) * 2.0 Monel (Ni 67%, Cu 33%) * 1.9 Manganin (Cu 84%, Mn 12%, Ni 4%) * 0.47 Nickel (Ni 100%) -12.1 * Isoelastic, Constantan, Advance, Copel, Nichrome V, Karma, Armour D, Monel, and Manganin are all trade names owned by the respective owners.

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.

 Pros and Cons of common strain gage wire materials: • Constantan (Advance, Copel alloy) is the oldest and most widely used strain gage materials. - It has a high enough electric resistivity (r =  0.49 mW·m) to achieve a proper resistance for a small gage length. - It has a relatively low temperature induced strain in the temperature range of -30 to 193 ºC (-20 to 380 ºF). This material is thus considered to have self-temperature-compensation. - It has almost constant sensitivity across a wide range of strain. - Annealed Constantan can even be used in the plastic region with strain > 5%. But, - The resistance of Constantan drifts continuously when the temperature raises above 65 ºC (150 ºF), which may become troublesome for strain measurements over a long period of time or in high temperature. • Isoelastic alloy is suitable for dynamic strain measurement in vibration and impact. - It has higher sensitivity (3.6 vs. 2.1 in Constantan), which improves the signal to noise ratio. - It has higher resistance such that most Isoelastic strain gages have resistance of 350 W compared to 120 W in common Constantan strain gages, which also increases the strain sensitivity. - It has better fatigue properties among strain gage materials. But, - Isoelastic does not have self-temperature-compensation property unlike Constantan or Karma. - It is too sensitive to changes in temperature, so that it is not suitable for measurements which last a long period of time with temperature fluctuations. - Sensitivity reduces from 3.1 to 2.5 when strain exceed 7,500 µ. • Karma alloy has similar overall properties to Constantan. - It has effective self-temperature-compensation property from -73 to 260 ºC (-100 to 500 ºF). - It has higher cyclic strain resistance than Constantan. But, - It is more difficult to solder. • Nichrome V, Armour D, and the Platinum based alloys are used for high temperature environment (> 230ºC; 450°F) or for other special purposes.

 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.
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