Campbell Scientific TGA100 Trace Gas Analyzer Manual - Manual (Page 44)

4 DETAILED SETUP INSTRUCTIONS
When the TGA100 is first installed, or if it is reconfigured (with a new laser, for example) the operational parameters
must be set for optimal performance. This section gives detailed instructions to set up the TGA100. If the TGA100 has
already been configured, see section 2.3.1 for routine startup instructions.
4.1 Configuring the System for a Specific Gas Species
The TGA100 can measure gases with appropriate absorption lines in the 3 to 10 µm range, by selecting appropriate
lasers, reference gas, and detectors. Some applications, such as isotope ratio or ammonia measurements, require options
such as an air gap purge or polyethylene sample cell liner. This section describes how to configure the system for the
specific gas of interest.
4.1.1 Laser Selection
Each gas species has a unique set of absorption lines, and tunable diode lasers have limited tuning ranges. Therefore, in
most cases a different laser is required for each gas species to be measured. The laser dewar can accommodate up to
four lasers, allowing the user to select a different gas without opening the dewar to install a different laser. Note that
some dewars require an optional second laser mount assembly to allow more than 2 lasers to be installed, and each laser
position requires a corresponding dewar cable. The following steps outline the laser selection procedure.
1) Turn the analyzer electronics off.
2) Disconnect the dewar cable from the dewar and electronics.
3) If the dewar must be rotated to select the other laser port, remove the four dewar mounting bolts, rotate the dewar
and reinstall and tighten the dewar mounting bolts.
4) Connect the dewar cable corresponding to the new laser position.
5) Read an appropriate parameter file. To avoid damaging the laser, ensure that the laser maximum temperature and
laser maximum current parameters are valid for the new laser.
6) Turn the analyzer electronics on.
7) Adjust the optical alignment (see section 4.2).
8) Resume real time operation and verify the parameter settings (see sections 4.4 and 4.5).
4.1.2 Reference Gas
A prepared reference gas having a known concentration of the target gas must flow through the reference cell. The
beam splitter directs a small fraction of the laser power through the reference cell to the reference detector. This gives a
reference signal proportional to the laser power, with the spectral absorption signature of the reference gas. The
reference signal provides a template for the spectral shape of the absorption feature, allowing the concentration to be
derived without measuring the temperature or pressure of the sample gas, or the spectral positions of the scan samples.
It provides feedback for a digital control algorithm to maintain the center of the spectral scan at the center of the
absorption line. The reference signal also allows the user to identify the wavenumber of an absorption line by
comparing it to the theoretical absorption spectrum of the gas.
The reference cell is kept at the same pressure as the sample cell by connecting the outlets of both cells to a common
vacuum manifold. A continuous flow of reference gas must be maintained to avoid dilution of the reference gas with
the sample gas. A flow of 10 ml/min is recommended.
The reference gas and sample gas are brought to the same temperature by flowing each of them through sufficient
length of tubing inside the analyzer enclosure to bring them both to the temperature of the inside of the enclosure.
The absorbance of the reference gas depends primarily on the line strength of the selected absorption line, the
concentration of the reference gas, and the path length. Pressure and temperature also affect the reference absorbance.
The reference gas concentration should be chosen to give an absorbance (in the center of the absorption line) of 0.3 to
0.9 (transmittance of 75% to 40%). If the absorbance is significantly more or less than this, the concentration noise may
increase. Suggested reference gas concentrations for the most commonly measured gases are listed in Table 9.
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