Campbell Scientific TGA100 Trace Gas Analyzer Manual - Manual (Page 94)

8 TGA100 OPTIONS
8.1 Laser Cooling
The laser must be cooled to as low as 80 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. An optional second laser mount is available if more than two lasers are
to be mounted in the same dewar. Contact Campbell Scientific for details on the second laser mount
8.1.1 LN2DEWAR TGA100 LN2 Laser Dewar
The TGA100 LN2 Laser Dewar mounts inside the analyzer enclosure. It holds 10.4 liters of liquid nitrogen, and must
be refilled twice per week. The top of the analyzer enclosure has a screw-out deck plate to allow access to fill the
dewar.
The dewar must be evacuated periodically to maintain its insulating vacuum. Typically this must be done once per year.
There are two indications that the dewar vacuum has degraded. First, as the vacuum degrades more heat will be
transferred to the LN2 tank, which reduces the hold time. The hold time is typically 10 to 12 days if the laser is off, but
the operational hold time depends on the operating temperature of the laser and on the thermal conductance between the
laser mount and the LN2 tank. This thermal conductance is adjusted at the factory to give at least 4 days of operational
hold time. As the vacuum degrades the hold time may be reduced. The other indication of degraded vacuum is a
reduction in the laser heater voltage. As the vacuum degrades inside the dewar, more heat is transferred to the laser by
convection, and less heat must be applied to the laser heater to maintain its temperature. The laser heater voltage will
tend to decrease slowly as the vacuum degrades. Some lasers must operate at very low temperature (near 80 K), which
may require more frequent evacuation of the laser dewar.
8.1.2 CRYODEWAR TGA100 Laser Cryocooler System
The TGA100 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, a compressor mounted outside the
enclosure, and 3.1 m (10 ft) flexible gas transfer lines. The compressor and gas lines must be sheltered from the
environment and maintained between 10 and 35 °C.
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