Serinus ® 60 NO 2 ANALYSER THEORY OF OPERATION The measurement of nitrogen dioxide (NO 2 ) utilises Cavity Attenuated Phase Shift (CAPS) spectroscopy technology. Sample air is filtered to remove particulates and dried before entering the precision stainless steel 264 mm measurement cell. The measurement cell contains two high reflectivity mirrors located at either end to provide an average optical path length of several kilometres. A modulated blue light source emits a narrow band wavelength of 450 nm which is used as a photon source for the cavity attenuation process. The LED is pulsed into the optical cavity and the photon ‘leakage’, as a result of the optical cavity, is determined through the output voltage measured by the vacuum photodiode detector. The detector is used in conjunction with a narrow band-pass interference filter (450 nm). During LED modulation the photon leakage of the cavity combined with photon absorption from gas molecules within the cavity produces a shift in the phase of the response signal measured by the detector. By measuring the shift in phase angle without NO 2 in the sample (background) a baseline phase shift can be measured. During operation, NO 2 gas molecules within the cavity will increase photon decay and this will be reflected as a change in phase of the detector signal. This change in phase is proportional to the NO 2 concentration. Using this principle it is possible to reliably measure NO 2 concentrations of well under 1 ppb up to 1000 ppb. Due to the continuous measurement of the sample, the analyser has a fast response time of less than 30 seconds. Sample IN Sample Sample IN IN 5 Micron Sample Filter 5 Micron 5 Micron Sample Sample Filter Filter Calibration Valve Manifold Background Scrubber Disposable Absorption Unit Calibration Calibration Valve Valve Manifold Manifold Background Background Scrubber Scrubber Disposable Disposable Absorption Absorption Unit Unit To Vacuum Source To To To Vacuum Source Dryer 0.01 Micron Sample Disposable Filtration Unit Orifice Dryer Dryer 0.01 0.01 Micron Micron Sample Sample Disposable Filtration Unit Unit Orifice Orifice Attenuated Modulated Wave Attenuated Form Attenuated Modulated Square Modulated Square Wave Modulated Measured Attenuated Wave Wave Form Form Wave Wave Form Measured Form Square Emitted Square Wave Wave Lens Measured Measured Lens Form Form Emitted Emitted Lens Lens Lens Lens Detector Detector Detector Blue LED Light Source Mirror Measurement Cell Mirror Bandpass Filter Blue LED Light Blue 450 Blue Source LED nm LED Light Light 450 Source nm Source Mirror Mirror Measurement Measurement Cell Cell Mirror Mirror Bandpass Filter Filter 450 450 nm nm US EPA Designated Equivalent Method
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