Campbell Scientific TGA100 Trace Gas Analyzer Manual - Manual (Page 13)

1.2 Theory of Operation
1.2.1 Optical System
The TGA100 optical system is shown schematically in . The optical source is a lead-salt tunable diode laser
that operates between 80 and 140 K, depending on the individual laser. Two options are available to mount and cool the
laser: the TGA100 LN2 Laser Dewar and the TGA100 Laser Cryocooler System. Both options include a laser mount
that can accommodate one or two lasers. The LN2 Laser Dewar mounts inside the analyzer enclosure. It holds 10.4
liters of liquid nitrogen, and must be refilled twice per week. The Laser Cryocooler System uses a closed-cycle
refrigeration system to cool the laser without liquid nitrogen. It includes a vacuum housing mounted inside the analyzer
enclosure, an AC-powered compressor mounted outside the enclosure, and 3.1 m (10 ft) flexible gas transfer lines.
Figure 1-2
Figure 1-2. Schematic Diagram of TGA100 Optical System
Reference
detector
The laser is simultaneously temperature and current controlled to produce a linear wavelength scan centered on a
selected absorption line of the trace gas. The IR radiation from the laser is collimated and passed through a 1.5 m
sample cell, where it is absorbed proportional to the concentration of the target gas. A beam splitter directs most of the
energy through a focusing lens to the sample detector, and reflects a portion of the beam through a second focusing lens
and a short reference cell to 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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