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Optical gas detection system for monitoring industrial sites with widely varying ground temperatures, where temperature effects are eliminated by calculating flow difference ratios for two filters
Optical gas detection system for monitoring industrial sites with widely varying ground temperatures, where temperature effects are eliminated by calculating flow difference ratios for two filters
A measurement scale independent of temperature is determined in the calculating window from the ratio of differences in flow through the two filters. The gas concentration can be calculated from this scale by weighting the ratio using the mean flow in the calculating window or as a function of the difference between a preset temperature and the ground temperatures during the calculating window Optical gas detection system by observation at a distance of an area using a measurement filter with a transmission band comprising a specific absorption ray for the gas being sought and a reference filter with a transmission band corresponding to that of the measurement filter but not including the specific absorption ray for the gas. The process involves detecting the presence of the gas by determining the difference in flow through the gas (22) and coming from points at different temperatures in a calculation window, and of the ratio of the flow differences seen through the measurement filter and through the reference filter. The scale is equal to ( eta . phi p)/K, where eta is the ratio, phi the mean flow in the calculating window, p a parameter depending on the measuring and reference filters and K a normalization parameter equal to phi p for a preset temperature in the calculating window, e.g. 20 deg C. The values of the derivative with respect to the temperature of the spectral luminescence of a black body at a given mean temperature are calculated for difference ground temperatures to deduce the derivative with respect to the temperature of a ratio for the same mean temperature in the absence of gas. The values are recorded and the measured ratios weighted as a function of the difference between the ground temperatures and the preset mean temperature. For the detection of several gases, a filter assembly (F1, F2, F3) is used, with the transmission bands determined relative to each other as a function of the absorption rays of the gases (G1, G2, G3). Thus, a filter (F2) can be used as a reference filter for a gas (G1) and as a measuring filter for another gas (G2,G3), or vice versa, connecting the filters in pairs, each pair detecting one or more gases. An identification matrix is formed where the rows correspond to the pairs of filters and the columns to the gases to be detected. The matrix is formed by calibrating the various filters and is used to detect the presence and concentration of the gases. A series of filters with stepped transmission bands are used.
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