Campbell Scientific TGA100 Trace Gas Analyzer Manual - Manual (Page 61)

8) Press “I” to start line lock again. Watch the reference detector display as the lines to come to the center of the
spectral scan.
9) Evaluate the results and iterate as needed (refer to Fig ): ure 4-11
• If the High current is too low, it will take more “+” steps to move the line to the left edge than “-” steps to
move the line to the right edge, the absorption line may become noticeably narrower as it approaches the left
edge, and when line lock is started with the absorption line at the left or right edge, the absorption line may
move relatively slowly to the center. Increase the High current and repeat the steps above.
• If the High current is set correctly, it will take the same change in DC current to move the absorption line to
the left or right edge (and this will be approximately equal to the modulation current), the width of the
absorption line will not change noticeably as it is moved from left edge to right edge, and when line lock is
started with the absorption line at the left or right edge, the absorption line will jump quickly to the center.
• If the High current too high, the laser’s frequency will overshoot the absorption line at the beginning of the
spectral scan, quickly scan backwards through the absorption line, and then scan forward through the
absorption line. This is visible in the reference detector display as a second narrow absorption line at the left
edge of the spectral scan data. When incrementing the DC current, it may not be possible to move the
absorption line to the left edge because the two absorption lines may merge just inside the left vertical dotted
line. When line lock is started with the absorption line at the left or right edge, the absorption line may come
quickly to the center, it may overshoot the center and then approach from the other side, or it may oscillate
about the center. Decrease the High current and repeat the steps above.
4.4.3.1 High Current Counts
The high current counts parameter must be set in conjunction with the high current offset. The high current counts
parameter sets the duration of the high current pulse, from 2 to 8 counts, where each count represents a 20
µs interval,
for a total duration of 40 to 160 µs. Generally, it is best to start with the high current counts at its maximum value, for a
low-amplitude, long duration pulse. However, if the high current is set to a small value (less than 20 mA) it may be
useful to reduce the high current counts and increase the high current offset. This will allow more of the samples to be
used in the concentration calculation.
4.4.3.2 Ramp B High Current
For dual ramp mode, the high current pulse must also compensate for a temperature perturbation caused by the ramp B
offset. The high current count parameter applies to both ramp A and ramp B, and it should generally be set to its
maximum value. The high current offset must be set individually for ramp A and ramp B. If the ramp B offset is
negative (as illustrated in Figure 4-12), the laser will be warmer than “normal” at the start of the ramp B, because its DC
current was higher in the previous ramp (ramp A). The ramp B high current offset must be set lower than “normal” to
compensate for this additional heating. In this case the (ramp A) high current must be set to a higher value than
“normal”, because the previous scan (ramp B) was at a lower DC current. It is possible that the ramp B high current
must be set to a negative value in some cases. If the ramp B offset is positive, the situation is reversed, and the ramp B
high current must be set to a higher value than the (ramp A) high current.
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