A novel capacitive sensor is developed which can detect 2-Furfuraldehyde (2-FAL, C5H4O2, in ppm order) in transformer oil. No such sensor is reported earlier in the literature. The 2-FAL is released in transformer oil with the ageing of the transformer and is a promising indicator for remaining useful life of the solid insulation. The sensor is fabricated by coating a Molecularly Imprinted Polymer (MIP) made from Polydimethylsiloxane (PDMS) on a copper electrode. The MIP coating has the template of 2-FAL molecules. On testing, the 2-FAL molecules in transformer oil are adsorbed on the templates which result in a change of capacitance of the sensor. The sensor is tested in the range of 1-20 ppm for measuring 2-FAL concentration and shows an average sensitivity of 0.084 pF/ppm. The sensors are fabricated in different months (batches), and average capacitance values for different oil sample are presented. The sensor reliability is analysed using statistical analysis and repeatability study which shows repeatable results consistently.

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 (μ) of 156 fF/°C with standard deviation (σ)±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 (R) 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.

A molecularly imprinted polymer (MIP) based 2-Furfuraldehyde (2-FAL, C5H4O2) capacitive sensor with improved sensitivity has been presented. The electrode is shaped into a novel comb structure to increase the fringe field capacitance which is responsible for sensing 2-FAL. The MIP solution sensitive to 2-FAL molecules is prepared using Polydimethylsiloxane (PDMS) and is coated on the printed circuit board (PCB) comb electrode. When the sensor is dipped in the transformer oil, the 2-FAL molecules are adsorbed on the template cavity of the MIP. The adsorbed layer of 2-FAL is responsible for the change in capacitance of the sensor. The sensor is tested for six oil samples containing the 2-FAL concentration (0-20 ppm range) and shows a mean sensitivity (μ) of 0.184 pF/ppm with standard deviation (σ) 0.022 pF at 1 kHz. The sensor shows a linear result with the coefficient of correlation (R2) 0.998 and maximum nonlinearity is 0.37% at 1 kHz in the 0-20 ppm range. The average repeatability of the sensor is ±0.12% over two weeks. Further, the sensor is connected to a suitably designed electronic circuit based on the instrumentation amplifier for obtaining an electrical signal. The circuit gives an output voltage of 0.35 V/ppm at 1 kHz.

A novel dual-slope resistance to digital converter (RDC) for compensating lead resistance is presented. The RDC uses a dual-slope ADC structure along with three switches and a diode. It completes measurement in one charging-discharging cycle only, hence provides an improvement to existing RDCs based on direct microcontroller interfaces (DMI) where 2 or 3 charging-discharging cycles are required. Moreover, it incorporate the feature of lead resistance compensation in dual-slope RDCs. The scheme works by connecting the resistive sensor to the input terminal of the integrator of the dual-slope ADC. In the charging path (or mode), sensor resistance is present along with lead resistance; and in discharging path, sensor resistance is bypassed, and only lead resistance is there. The difference between the RC time constant during these two modes is used to find the sensor resistance. The diode is used to turn on a switch for bypassing the resistive sensor. The diode voltage drop is not there in the charging and discharging path, which is another novelty. A detailed error analysis is also carried to evaluate the performance of the scheme considering switch resistance, diode voltage drop, voltage mismatch and opamp noises. The scheme works with low-value resistive sensors (≈ 100 Ω), a feature not found in existing dual-slope RDCs. The hardware prototype of the proposed scheme on the breadboard shows a measurement time of 2.09 ms with an average error of ±0.04 % at 100 Ω when lead resistance varied from 20 Ω to 100 Ω. The scheme takes 23.44 % less time for measurement.