Comparison of IR LED gas sensors with thermal source products

Physical principles for infrared gas sensing using LEDs as sources are examined. A number of LEDs from different sources are compared for power output, polar diagram and temperature dependence of output. Some of these have been incorporated in a practical gas sensor based on the Edinburgh Sensors’ Gascard. Thus a direct comparison of signal levels between thermal sources and LEDs has been made using thermal detectors. Gas sensing is demonstrated for CO2. The paper describes the ranges, accuracies and stability with time and temperature presently exhibited by thermal source sensors and suggests criteria for the widespread use of LED sensors. Some advanced concepts are mentioned with respect to future sensing requirements.