A practical capacitive type CO2 sensor composed of CeO2/BaCO3/CuO mixed oxides has been developed for the IAQ (indoor air quality) monitoring system. Sensing detectors were fabricated as follows. The pressed pellets of CeO2/BaCO3/CuO mixed oxides were sintered at 800–900°C for 5 h, then RuO2 electrode was formed on both sides of the pellets.

In order to study CO2 sensing characteristics, the capacitance of detector was measured at 550°C with an LCR meter. The sensitivity of Cat2%CO2/Cin-air was measured to be −2.5 dB. They showed good stability in CO2 sensitivity at 85°C and 90% RH test for 1500 h. The sensor unit with an oscillation circuit using this detector was developed, and its monitoring characteristics were studied.

It was shown that the sensitivity change was negligible between 0% RH and 60% RH at 30°C. The measurement error of CO2 sensor was within 200 ppm in the humidity range of 40–90% RH, and within 30 ppm/°C in the temperature range of 5–50°C. A CO2 concentration profile monitored in an office by the developed sensor unit showed good agreement with that monitored by an NDIR type sensor.

These results suggest that the developed CO2 sensor unit may be applicable to indoor quality monitoring systems. The alumina substrate, which integrates the sensing detector of 1.6∗1.2∗0.5 mm3 and the printed-film heater, is small (3.6∗2.8∗0.4mm3) and need no damp proofing, so that small size and low cost of the sensor can be easily attained.