Experimental Evaluation of a Thermopile Detector With SU-8 Membrane in a Carbon Dioxide Meter Setup

Continuous control of the carbon dioxide levels in the ventilation systems in office buildings and public schools has been shown to increase productivity and save money. However, these measurement systems require further developments in order to be more cost effective. This paper presents an evaluation of an Al/Bi thermopile detector with a 4 mum thin SiO₂/SU-8 membrane in a CO₂ meter application using the nondispersive infrared technology (NDIR). The system consists of an 11 cm aluminum tube, used as the sample chamber and in which a light source and a thermopile detector with a 4.26 mum optical bandpass filter are positioned on its opposite sides. The light source is pulsed with a frequency of 0.5 Hz. The voltage response of the Al/Bi thermopile is measured for different CO₂ concentrations, and, as expected according to the Lambert-Beer law, there is an exponential decrease in the measured intensity. The absolute response is about 50% lower than for a commercial HMS J21 thermopile from Heimann Sensor GmbH. In relative terms, on the other hand, the Al/Bi thermopile is more sensitive for changes in the CO₂ concentration. At 7500 ppm, the voltage response has decreased to 40% of the reference response measured in the nitrogen atmosphere.