A simple, low-cost non-dispersive infrared CO2 monitor

A non-dispersive infrared (NDIR) CO₂ monitor is developed for air quality assessment, The NDIR CO₂ sensor consists of one IR source, a pipe in which IR beam interacts with CO₂ 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 CO₂ 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 CO₂ concentration measurement insensitive to the aging of the IR source and temperature. The ratio is a nonlinear function of CO₂ concentration and the cubic spline function is used for the linearization and the calibration. A prototype monitor demonstrates performances quite satisfactory for practical applications.