A simple, low-cost non-dispersive infrared CO/sub 2/ monitor

A non-dispersive infrared (NDIR) CO/sub 2/ monitor is developed for air quality assessment, The NDIR CO/sub 2/ sensor consists of one IR source, a pipe in which IR beam interacts with CO/sub 2/ molecules in the air, and a beam splitter terminated by two thermopiles with optical band-pass windows. One thermopile measures the spectrum absorption due to CO/sub 2/ molecules, while the other monitors the light emission level of the IR source. The signal processing electronics take the ratio between the outputs of the two detectors to make the CO/sub 2/ concentration measurement insensitive to the aging of the IR source and temperature. The ratio is a nonlinear function of CO/sub 2/ concentration and the cubic spline function is used for the linearization and the calibration. A prototype monitor demonstrates performances quite satisfactory for practical applications.