temperature Eflects in Strain Gauges The strain gauge measuring environment is often influenced by temperature changes. The electrical resistivity of most alloys changes with temperature, increasing as temperature rises and decreasing as it falls. As shown in Table 3-3, metals used in strain gauges have a temperature coefficient (cq, ) on the order of 0.00QPC. The resistance at temperature T is given as RT = RTo (1 + a, AT) Eq. 3-63 Resistance change due to change in temperature AT is ART = RToa,AT Eq. 3-64 For example, if the temperature changes by one degree, the change in resistance is calculated as AT = lo, a, = 0.004PC, RTo = 120 R, and ART = 0.48 a. When a strain gauge is bonded to the member under test, its resistance will be affected by a change in temperature. This effect is independent of any strain applied to the gauge. The recording instrument cannot differentiate between the changes in the resistance due to temperature and those due to strain. In addition, unless the coefficient of the linear expansion of the gauge is the same as that of the material to which it is bonded, the temperature change during measurement will also be a source of false strain due to differential expansion. Temperature compensation is achieved in two ways: 1. Using a dummy gauge 2. Using more than one active gauge, with proper arrangement of gauges If active and dummy gauges are mounted on the adjacent arms of a bridge, variation in temperature will not affect the bridge. The active gauge is subjected to strain as well as temperature change, whereas the dummy gauge is subjected to temperature change only. Because active and dummy gauges form adjacent arms of the bridge, the output due to temperature change is zero; both active and dummy gauges change identically due to temperature. Further, it is desirable to choose a gauge material with a coefficient of thermal expansion very close to that of the material under test. Because it is inconvenient to calculate and apply temperature correction after the measurement is made, the temperature compensation can be made in the experimental setup itself. The gauges are suitably arranged so that adjacent arms have strains of opposite nature. This procedure ensures signal enhancement as well as temperature compensation.