DETECTING METHOD OF GAS

PURPOSE:To detect gas leakage precisely by comparing photodetecting power obtained by transmitting laser light through gas to be observed or its differential value which is found from the ratio of the photodetecting power to the differential value with an actually measured value when said ratio is increased. CONSTITUTION:Light from an infrared-ray semiconductor laser 1 is transmitted through gas G and made incident to a photodetector 2. The photodetecting power and the differential photodetecting power are found out by the 1st lock-in amplifier 4a and the 2nd lock-in amplifier 4b respectively through an amplifier 3 and inputted to a microcomputer 6 through AD converters 5a, 5b. The concentration C of the gas is calculated from the ratio of both inputs. When the ratio of both inputs is suddenly increased, the concentration of the gas is calculated by the microcomputer 6 and the photodetecting power is reversely calculated and compared with the value of the actually measured photodetecting power. If both values are approximately equal, the status is judged as gas leakage, and the inconsistency of both values is judged as the disconnection of an optical path. Said judgement can be attained also by the differential photodetecting power.