Implementation of gas sampling chamber and measuring hardware for capnograph system considering thermal noise effect and time response characteristics

Most capnograph systems that can indirectly determine the partial pressure of carbon dioxide in the blood of a patient are based on NDIR (non-dispersive infrared) absorption technology. As such an NDIR gas analyzing method requires an optical absorption chamber and signal processing hardware. Accordingly, this paper designed and implemented an NDIR type CO₂ gas chamber while considering the time response characteristics and lamp chopping frequency. In addition, signal processing hardware using two infrared sources was implemented to reduce the thermal background effect. The gas chamber and signal processing hardware has been tested using temperature variation and human expiration experiment. The results showed that the system could produce a stable output signal and discerning CO₂ gas concentration curve similar to a typical capnogram.