Campbell Scientific TGA100 Trace Gas Analyzer Manual - Manual (Page 50)

the long sample cell back (away from the dewar) about 5 mm from the center of its adjustment range and retighten
the axial clamping screw. Alternately adjust the horizontal and vertical adjustment screws.
6) If no detector response can be found, perform the following checks:
• If two lasers are installed, verify you are aligning to the correct laser.
• Verify the dewar cable is installed correctly. If two or more lasers are installed, verify you are using the correct
cable.
• Verify the detector cables are correctly installed.
• Verify the detector temperatures are stable at -60 deg C.
• Verify the detectors are in gain 7 and the offset is adjusted to avoid saturation.
• Verify the laser is enabled in the TGA program. The real time screen should show LASER [ON] in the
function column.
• Recheck the laser temperature, and the zero, DC, modulation, and high current settings.
7) If a detector response is still not observed, check for continuity of the laser drive circuit.
• Disable the laser. The real time screen should show LASER [ ] in the function column.
• Set the laser temperature to zero and allow the laser temperature to stabilize at its minimum temperature.
• Display the laser temperature in graph 3 on the real time screen and press to adjust the graph 3 scale.
Verify the laser temperature is stable.
• Press to enable the laser. If current is now flowing through the laser, it will generate a small amount of
heat. Watch the laser temperature displayed in graph 3 to see if it begins to rise after several seconds. If the
laser temperature does not respond to turning the laser on and off, it indicates an open circuit. Contact
Campbell Scientific for assistance.
4.2.3 Horizontal and Vertical Alignment
Once the system is aligned well enough to see a response in the sample detector, follow these steps to optimize the
horizontal and vertical alignment.
1) Change the laser DC current slightly if needed such that no absorption lines are visible in the detector signal
displays, or flow nonabsorbing gas (nitrogen) through the reference and sample cells.
2) Set the reference and sample detector gains to zero. This will disable automatic gain and offset adjustment which
can cause confusion during the alignment process.
3) Set the detector offsets and temperatures as needed to avoid saturation. These adjustments may need to be repeated
during the alignment process if the signal level increases too much. It is not important to have large signals during
alignment, so when in doubt, set the detector temperatures relatively high for relatively low signals.
4) Adjust the horizontal position (see Figure 4-4) to maximize the sample detector signal. Note that the sample and
reference signals may not reach their maxima simultaneously. If so, ignore the reference detector signal and adjust
the alignment to maximize the sample detector signal. Adjust the horizontal position past the peak in each direction
far enough to make sure there is a single response peak. If there is a single peak, leave it at the center of the peak. If
there are multiple peaks, leave the horizontal alignment at the center of the group of peaks.
5) Some older systems used a relatively coarse-pitch screw for the horizontal alignment at the dewar end and a second
horizontal adjustment screw at the detector end. This screw provides a finer adjustment of the horizontal alignment
than the one at the dewar end, allowing the signal to be more easily maximized. Newer systems have a fine-pitch
horizontal adjustment screw at the dewar end and require no adjustment at the detector end.
6) Adjust the vertical position (see Figure 4-4) to maximize the sample detector signal, in the same way as for the
horizontal alignment.
7) Iterate the horizontal and vertical alignment until the sample detector signal is maximized. If there is a single
narrow peak horizontally and vertically, the system is also in good focus. If the response peak is broad or if it has
multiple peaks, adjust the focus as outlined in the following section.
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