Infrared gas analyser and humidity sensor

Abstract

An infrared gas analyser. The analyser comprises a sample cell located in a gas flow, a source of infrared radiation of selected wavelengths which is propagated through the sample cell and an infrared detector which receives radiation which has passed through the cell, The radiation incident on the detector provides a measure of the concentration of selected substances in the gas flow due to one of the wavelengths being selected to be absorbed by the gas being measured. One preferred use is as a humidity sensor in a medical respiratory circuit where at least one selected wavelength is a wavelength strongly absorbed by water. Heating means am associated with the sample cell to vaporize any liquid water which may enter it and the cell is provided with internal baffles to reduce deposition or condensation of water on internal surfaces through which infrared radiation must pass. In one embodiment two selected wavelengths are derived from a single source with the second reference wavelength being a wavelength which is not significantly absorbed by the gas being measured and the source is switched between two power supplies to ensure the output of the detector is equal for the two wavelengths when the concentration of the measured gas is zero. In a second embodiment two single wavelength sources are used and their combined outputs passed through the sample cell.