Patent ID: 10337988

Abstract:
Modulation amplitude used for frequency modulation of laser light is set to a1 (<a2) (S1), and laser light is irradiated to a sample state in that state. When moisture concentration is calculated based on secondary harmonic synchronous detection signal that is obtained by synchronous detection of detection signal from transmitted light, the effects of interfering moisture in the optical chamber is ignored, and moisture concentration of the gas to be measured in the sample cell is obtained (S2). If, for example, a high vacuum is created inside the sample cell and the concentration becomes less than the detectable limit (“YES” in S3), the modulation amplitude is switched to a larger amount a2 (S4). By so doing, the detection sensitivity to interfering moisture in atmosphere at atmospheric pressure is increased, and the concentration of interfering moisture is calculated based on the secondary harmonic synchronous detection signal (S5).