A Film-Based Optical Humidity Sensor for Trace Moisture Measurement

Optical humidity sensors for trace moisture measurement have evolved through various techniques. However, the development of a film-free optical fiber, compact and low-cost system is still a challenge. A γ-Al2O3 based non-contact optical humidity sensor without coating the optical fiber has been proposed. The sensor system consists of an inexpensive optoelectronic emitter-detector attached with multi-mode fiber (MMF), a thermoelectric cooler (TEC), a γ-Al2O3 coated quartz placed over a mirror and temperature sensors. The entire assembly is housed in a dedicated non-hygroscopic chamber. The reduction in the intensity of incident light as per Beer Lambert law, due to moisture absorption in sensing film, brings a corresponding change in detector output voltage. The sensitivity of the sensor system at 5 °C is 0.57 mV/ppm in the range of 20-5900 ppm. The response time are 9 sec, 7 sec, and 6 sec for 5 °C, 10 °C, and 15 °C respectively. The proposed sensor system may serve as an option to conventional chilled mirror or fiber-optic based humidity sensors because of its low cost and light weight feature, even to measure trace moisture.