A high-performance planar metal-polyimide-interdigital transducer (IDT) metal-type miniaturized capacitive humidity sensor was developed for real-time detection of moisture dissolved in transformer-insulating oil. The sensor with the sensitive film having a concave surface and a porous structure was fabricated through using a micromachined process. Additionally, the sensor interface electronics were implemented for the measurement system of the fabricated sensor and immediate use of the sensor in oil. COMSOL simulation was performed to determine the top metal IDT configuration and polyimide thickness prior to sensor fabrication. The performance of the developed sensor was tested in air and transformer oil. In the air test, the sensor showed a high sensitivity of 1.60 pF/% relative humidity (RH) and a large capacitance variation from 204 to 374 pF with a humidity change from 20% to 90% RH at a frequency of 3 kHz; a low hysteresis of 2.07% at 40% RH and a stable capacitance value with a maximum error rate of 0.15% over 450 h of continuous testing. In the transformer oil, the sensor exhibited a sensitivity of 0.90 pF/%RH and an extremely small long-term drift of the capacitance corresponding to humidity variation of ±2.0%RH. Overall, the modification of the polyimide film enhanced the performance of our sensor, such as high sensitivity, fast response, and long-term stability in the transformer–oil environment.
