Campbell Scientific TGA100 Trace Gas Analyzer Manual - Manual (Page 49)

If the TGA is equipped with an iris in front of the focusing lens, open it fully (this is recommended for normal
operation – see section 4.5.2).
3) Start the TGA program and make sure the laser and detector parameters are set appropriately for the laser. If the
laser has not been changed this normally means just using the parameters that were loaded automatically at startup.
If switching to another laser, this normally means reading in a parameter file that was previously used with the
laser. If a new laser has been installed, see section 4.2.1 for suggested initial settings for the laser and detector
parameters.
4) Enter the real time mode and display the reference detector signal in graph 1 and display the sample detector signal
in graph 2. The goal of the alignment procedure is to maximize these signals. Set the minimum value of the Y axis
for graph 1 and 2 to zero. Set the Y axis maxima as needed to display the signals. Repeat this adjustment as needed
during the alignment process.
5) If no sample detector signal can be seen, perform the initial alignment (section 4.2.2).
6) Once a signal can be observed on the sample detector, adjust the horizontal and vertical alignment (section 4.2.3).
7) Adjust the focus (section 4.2.4).
8) When the focus and the horizontal and vertical alignment have been optimized, tighten the horizontal, vertical, and
axial clamping screws.
9) Make sure the reference detector is coaligned with the sample detector (section 4.2.5).
4.2.1 Setting Parameters to Align a New Laser
If a new laser is being tested for the first time, there will be no nominal values for the laser in the parameter file. In this
case, the following settings should allow the user to proceed with the optical alignment.
1) Set the laser temperature and DC current to values expected to give good output based on the laser vendor’s test
report. In general, a lower temperature often gives a larger output from the laser. It is not necessary to find an
absorption line before performing the optical alignment.
2) Set the Zero current below the lasing threshold. If in doubt, set it to 0 mA.
3) Set the modulation current to a nominal value such as 1 mA.
4) Set the High current to a nominal value such as 50 mA.
5) Enable automatic adjustment of the detector gains and offsets.
6) If no signal can be seen in the sample detector, set its temperatures to -60 ºC to maximize its responsivity.
7) If a signal can be seen in the sample detector, proceed to the horizontal and vertical alignment (section 4.2.3).
Otherwise perform the initial alignment (section 4.2.2).
4.2.2 Initial Alignment
If the optical system is significantly misaligned, there may be no observable detector response. This initial alignment
procedure will help to align the system well enough to see a response. As soon as a detector response is observed, the
system is ready for the horizontal and vertical alignment procedure described in section 4.2.3.
1) At the detector end, loosen the horizontal lock screw, adjust the horizontal position to near the center of its
adjustment range, and retighten the horizontal lock screw. Note that older units were supplied with a horizontal
adjustment screw at the detector end, but newer units use a fine-pitch horizontal adjustment screw at the dewar end
which makes horizontal adjustment at the detector end unnecessary.
2) Use the horizontal adjustment screw at the dewar end to align the long sample cell with the laser. Sight along the
long sample cell to point it at the laser, which can be viewed through the dewar window, hanging below the laser
mount.
3) Use the vertical adjustment screw to align the long sample cell with the laser. If no detector response is observed,
set the vertical adjustment near the center of its adjustment range.
4) Alternately adjust the horizontal and vertical alignment screws. When a response is observed in the sample
detector, proceed to the next section.
5) If a detector response is not observed, it may be helpful to defocus the optics intentionally. This will make the
laser’s image on the detector larger and easier to locate. Loosen the axial clamping screw at the dewar end. Slide
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