Inline concentration meter and concentration detection method

An inline concentration meter, comprising:

• a light source unit for emitting mixed light having at least two wavelengths with a phase difference;
• a detecting unit including a light incident part for entering the mixed light emitted from the light source unit into a fluid passage of a detector body and at least first and second light detection parts for receiving the mixed light emitted into the fluid passage, wherein a first light path through the fluid passage is defined between the light source and the first light detection part, and a second light path through the fluid passage, longer than the first light path, is defined between the light source and the second light detection part;
• wherein the second light path includes a first portion including the first light path and a second portion defined through the fluid passage between the first light detection part and the second light detection part;
• a computing processor unit for conducting frequency analyses of detection signals of the mixed light output from the respective light detection parts and computing variations of intensities of the detection signals corresponding to absorbances in at least two frequency ranges to compute a concentration of fluid in the fluid passage based on the variations of the intensities of detection signals in the at least two frequency ranges;
• wherein, from results of the frequency analyses through FFT (fast Fourier transformation) and/or the computed results of the variations of the light intensities corresponding to the absorbances in the respective frequency ranges, respective light absorption matrixes are obtained in the computing processor unit, and then the concentration of the raw material gas is continuously computed in real time based on the light absorption matrixes by using an algorithm for computing the raw material gas concentration;
• and
• a recording/displaying unit for recording and displaying a value of the fluid concentration computed at the computing processor unit;
• wherein, the light source unit emits ultraviolet light in a wavelength region of 200 nm to 400 nm;
• and
• wherein part of the mixed light is entered into the first light detection part through a light transmitting window of the first light detection part and part of the mixed light reflected on the light transmitting window is entered into the second light detection part.