Emerson Rosemount CT5400, Installation, Operation And Maintenance Manual

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A.3 Process analyzer - gas concentration
measurements
In the analyzer, gas concentrations are measured using mid-infrared optical absorption
spectroscopy. The light sources are QCLs, which are operated to produce wavelength
sweeps that cover the absorption lines of the gases to be measured.
Sample gas, which may contain impurity gases that are to be detected and measured, is
conditioned and drawn into the analyzer. Inside the analyzer, the sample gas is fed into an
analysis cell, where the beams from the QCLs are passed through the gas. The analysis cell
contains a set of mirrors that bounce the light back and forth many times, which lengthens
the path of the lasers through the gas. On exiting the analysis cell, the light is detected by a
receiver unit. The variation in the intensity of light in the vicinity of absorption lines for the
gases being detected is measured, and the concentration is determined using a
comprehensive spectral fitting routine.
Theory of Operation
148 Rosemount CT5400