Infrared CO2 Sensor With Built-In Calibration Chambers

A proposed infrared CO2 sensor, operated in conjunction with suitable readout, data-processing, and control circuitry, could be calibrated repeatedly during operation to compensate for changes in sensor response induced by such phenomena as aging and changes in temperature. The sensor would include an infrared source, an infrared detector, and four chambers containing CO2 at various concentrations. Three of the chambers would be calibration chambers: they would be sealed and would contain CO2 at known low, intermediate, and high concentrations, respectively. The fourth chamber would be filled with the gas under test containing CO2 at a concentration to be determined. There would be optics for multiplexing infrared radiation from the source through the four chambers and demultiplexing the radiation from the chambers to the infrared detector. During an operation/calibration cycle, radiation would be directed through each chamber in turn, and the response of the detector recorded for each chamber. A three-point calibration for that cycle would be computed from the responses for the three calibration chambers. Then the concentration of CO2 in the fourth chamber would be computed by simply multiplying the detector response for that chamber by a factor calculated as part of the calibration.