ACOEM Met One Instruments GAS-1060, Руководство пользователя

Страница: 23 / 164

Introduction Page 23
1.4.1 Measurement Theory
The measurement of nitrogen dioxide utilizes cavity attenuated phase shift spectroscopy (CAPS).
Sample air is first drawn through a five micron filter then it is dried and filtered again by a ten
nanometer filter to remove particulates before entering the precision stainless steel 264 mm
measurement cell. After the sample is drawn through the cell and measured it is drawn through an
orifice (controlling the system flow) and then sample is exhausted out of the instrument.
Figure 1 – Simple Pneumatic Diagram
The measurement cell contains two high reflectivity mirrors located at either end to provide an
optical path length of approximately 2 km. A modulated light emitting diode (LED), with a narrow
band wavelength of 450 nm, is used as a photon source for the cavity absorption 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 of a vacuum photodiode detector.
The vacuum photodiode detector is used in conjunction with a narrow (10 nm wide) band-pass
interference filter that is optimised to the absorption wavelength of the target compound, in this
case the 450 nm absorption band of the NO
2
molecule.
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
vacuum photodiode detector.
By measuring the phase angle shift 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