MINIMIZING STRAIN-INDUCED TEMPERATURE MEASUREMENT ERRORS WHEN USING BONDABLE RTDS

Resistance Temperature Detectors (RTD), such as the Micro-Measurements®TG-series, can achieve impressive temperature measurement accuracy (better than ±0.3°C). But, achieving these results requires exercising good practices.

TG temperature sensors are installed with the same techniques and materials used for installation of wide-temperature range strain gage sensors. But unlike strain gages, TG-series temperature sensors should be installed at points of low strain magnitude on the test structure – as close to zero strain as practicable. This is to avoid strain-induced errors in the temperature measurement. At the same time, if data from the RTD is intended to correct strain gage thermal output, then the temperature sensor data must match the temperature excursion experienced by the strain gages.

Like their strain gage cousins, TG-series temperature sensors change resistance with both temperature and strain inputs, just in the opposite ratio. Strain gages are designed to maximize strain sensitivity and minimize temperature response, while TG-series temperature sensors are designed to maximize temperature sensitivity. And, although the temperature sensitivity component is much lower, TG-series RTDs do have a sensitivity to strain, up to about 0.0015 °C/µε.

Available in both high-purity Nickel and Balco® foils, Nickel produces the highest temperature measurement sensitivity, as well as the highest strain sensitivity. The strain sensitivity of pure nickel can create error signals when TG sensors are installed in areas of high mechanical strain. Fortunately, the magnitude of this effect is fairly small, as shown in Figure 1.

Fig. 1 – Typical error signal caused by strain applied to a Nickel RTD. Data applies to sensors near room temperature (+24°C)

The shape of this curve is caused by the nonlinear response of pure nickel. The strain-sensitivity coefficient has a high negative value in the central portion of the elastic region and tends toward a much smaller positive value on either side of this region. It will be observed that compressive strains result in smaller error signals, and this strain field orientation should therefore be selected for sensor placement when possible. The center of symmetry of this curve is located at approximately +750 µε, because the manufacturing process leaves the sensor with a residual compression near this value.

It is important to realize that the center of symmetry can be shifted by installing the gage on materials of different thermal expansion coefficients and/or with different adhesive cure temperatures. It is for this reason that strain gage sensor response when mounted on aluminum alloy will differ slightly from that obtained when mounted on steel.

It has been shown that repeatability of properly installed TG sensors can be better than ±0.05% of applied temperature span. To take full advantage of this repeatability, and of the other intrinsic features of TG temperature sensors, it is always advisable to conduct a calibration run of the sensor mounted on a specific material when highest measurement accuracy is required. And, to reduce the concern over strain induced errors, always install the sensors at the lowest strain area of the structure. If no readily accessible or practicable low strain area is available, then a very simple procedure for reducing strain-induced errors is to bond the RTD in the Poisson strain direction.

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