Campbell Scientific TGA200 Trace Gas Analyzer Overview - Manual (Page 9)

TGA200 Trace Gas Analyzer Overview
2. Theory of Operation
2.1 Optical System
The TGA200 optical system is shown schematically in Figure 2-1. The optical
source is a lead-salt tunable diode laser that operates between 80 and 140 K,
depending on the individual laser. The TGA200 LN2 Laser Dewar includes a
laser mount that can accommodate one or two lasers. The LN2 Laser Dewar
mounts inside the analyzer enclosure. It holds 14.5 liters of liquid nitrogen and
must be refilled once per week.
FIGURE 2-1. Schematic diagram of TGA200 optical system
The laser is simultaneously temperature and current controlled to produce a
linear wavelength scan centered on a selected absorption line of the trace gas.
A beam splitter allows most of the energy from the laser to pass through a
1.5 m sample cell, where it is absorbed proportional to the concentration of the
target gas. A lens at the outlet of the sample cell focuses the beam onto the
sample detector. The portion of the beam that is reflected by the beamsplitter
passes through a reference cell similar to the sample cell and is focused on the
reference detector. A prepared reference gas having a known concentration of
the target gas flows through the reference cell. The reference signal provides a
template for the spectral shape of the absorption line, allowing the
concentration to be derived independent of the temperature or pressure of the
sample gas or the spectral positions of the scan samples. The reference signal
also provides feedback for a digital control algorithm to maintain the center of
the spectral scan at the center of the absorption line. The simple optical design
avoids the alignment problems associated with multiple-path absorption cells.
The number of reflective surfaces is minimized to reduce errors caused by
Fabry-Perot interference.
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