Campbell Scientific TGA100 Trace Gas Analyzer Manual - Manual (Page 63)

4.4.5
Laser Modulation Current
The laser modulation current parameter controls the width of the spectral scan. In most cases, the spectral scan will
include a single absorption line. This is generally preferred, but is not required for proper operation. If a group of two or
more lines is used, make sure the line locking is enabled, and then adjust the modulation current as needed to include
the entire group of lines. The edges of the spectral scan should extend slightly past the absorption lines, to measure the
laser’s unabsorbed intensity (i.e. 100% transmittance).
If a single absorption line is used, adjust the modulation current until it occupies approximately one-third of the spectral
scan, with a nearly flat portion on either side, as illustrated in Figure 4-14. The TGA100 software includes an automatic
algorithm to set the modulation current. To use this algorithm, enable line locking and then press . Use this
algorithm only if a single absorption line is used. If more than one absorption line is present, the algorithm may give an
erroneous setting.
Modulation too low Modulation correct Modulation too high
Figure 4-14. Adjustment of Modulation Current
For dual ramp mode, the modulation current must be set individually for ramp A and ramp B. It may need to be set to a
different value for ramp A and ramp B to compensate for residual temperature perturbation that cannot be completely
removed by the high current pulse. First, set the (ramp A) modulation as described above. Then select the “Alt A & B”
detector display mode. Adjust the ramp B modulation current to match the width of the ramp B absorption line to the
ramp A absorption line. The TGA100 software includes an automatic algorithm to set the ramp B modulation current.
To use this algorithm, enable line locking (ramp A and ramp B) and then press .
4.4.6 Laser Maximum Temperature and Laser Maximum Current
The laser can be damaged by too much current or by operation at too high a temperature. The TGA100 software will
automatically disable the laser current output if the laser's temperature is above an upper limit or if the laser current
parameters are set to exceed an upper limit for any of the spectral scan points.
After the laser’s operating parameters are established, set the laser maximum temperature to one Kelvin above the
operating temperature. Set the laser maximum current to the DC current plus the high current offset plus 20 mA. This
will help to protect the laser if the laser warms up, or if the laser current parameters are inadvertently set for too much
current.
4.4.7 Laser Multimode Correction
An ideal laser would emit at only one frequency (single mode). Unfortunately, real lasers emit some of their power at
frequencies other than the desired frequency (multimode). The main mode generally contains at least 90% of the total
power, but there will likely be a few percent of the power in other modes. This multimode power is not absorbed by the
selected absorption line; therefore it gives an error in the measured concentration.
The TGA100 software corrects for the laser’s multimode power, but the user must enter a parameter that is the
percentage of total power emitted in the undesired modes. The multimode power can be estimated by temporarily
putting reference gas in the long sample cell. This will give very strong absorption in the main (desired) mode at the
center of the absorption line, essentially absorbing all of the laser’s power. The side-mode power at other frequencies
will generally not be absorbed. The measured transmittance at the center of the absorption line gives an estimate of the
laser multimode power. This transmittance measurement is affected by detector nonlinearity (see section 4.5.3). The
reference detector is more linear because it has a smaller signal. Therefore this measurement should be based on the
reference detector, not the sample detector.
It is best for this test to avoid having too much absorption, which can lead to two possible problems. First, the lines
become broader, and absorption in the tails of the absorption line can reduce the response at the edges of the spectral
scan that are assumed to be 100% transmittance. This will give an error in the estimate of multimode power, especially
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