Campbell Scientific TGA100 Trace Gas Analyzer Manual - Manual (Page 62)

Laser
Current
CO
2
13
CO
2
12
Ramp B Offset
Ramp A Ramp B
Detector
Response
Figure 4-12. Dual Ramp Scan Sequence
To set the ramp B high current, first set the (ramp A) high current as described in section 4.4.3. Then repeat the process
for the ramp B high current. When evaluating the ramp B high current setting,, observe the ramp B line instead of the
ramp A line, and use both the ramp A line lock and the ramp B line lock to center the absorption lines. Count steps
while adjusting the DC current, as was done for the ramp A high current (not the ramp B offset current).
4.4.4 Omitted Data Count
Some additional data must be omitted from the concentration calculation to allow the laser temperature to stabilize fully
after the zero and high current phases of the scan. The omitted data count parameter specifies how many scan points to
omit It may have a value from 4 to 20 counts, where each count represents a 20
µs interval, to give a total duration of
omitted data of 80 to 400 µs. To set this parameter, look at the reference detector transmittance in Magnified mode. The
leftmost of the three vertical dotted lines on the display marks the end of the omitted data counts, i.e., the start of the
data used to calculate concentration. Increase the omitted data counts to move this line to the right; decrease it to move
it to the left. Set the omitted data counts to avoid a transient at the start of the points used in the concentration
calculations. When in doubt it is usually better to omit a few extra points. Figure 4-13 illustrates how to set this
parameter.
Too few omit counts Correct omit counts
Drops at left edge Flat at left edge
Figure 4-13. Adjustment of Omitted Data Counts
For dual ramp mode, the omitted data count parameter is applied to ramp A and ramp B, and it should generally be set
to its maximum value.
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