A Novel Linear Capacitive Temperature Sensor Using Polydimethylsiloxane

Abstract

A linear, simple, and reliable capacitive temperature sensor is fabricated with inexpensive materials. The sensor is a parallel plate capacitor, in which the dielectric material polydimethylsiloxne (PDMS) is sensitive to temperature. The sensor works on the principle that the density of sensing layer decreases with an increase in temperature following the Clausius–Mosotti equation, because of which the permittivity also decreases. Moreover, the thickness of the sensing layer also increases with an increase in temperature due to a positive coefficient of linear expansion. The sensor is tested in the range of 20 °C–200 °C and shows a mean sensitivity ( $\mu$ ) of 156 fF/°C with standard deviation ( $\sigma$ )± 0.037 pF. It shows a linear characteristic with the coefficient of correlation ( $R^{2})~0.99$ , and maximum nonlinearity is 1.11% at 10 kHz. The measured resolution of the sensor is 0.08 °C, and the time constant for a unit step input is 0.6 s/°C. The average repeatability index ( $\Re$ ) of the sensor is 0.51% over a month. Finally, an interfacing circuit based on the integrator and instrumentation amplifier is designed, which gives a DC output with a sensitivity of 67.9 mV/°C.