Campbell Scientific TGA100 Trace Gas Analyzer Manual - Manual (Page 54)

Return to the Main Menu, select the Laser Mapping Menu, and then select the File Format screen.
• Set the File Format to ASCII or binary, as desired.
• Set the Concentration File Decimation Factor to 1
• Set the Interval to start new data files (min) to 0
3) Return to the Laser Mapping Menu and select the File Output Selection screen. Select the following data to be
saved to the 10 Hz file (additional data may be selected if desired).
• Laser Temp
• Laser DC Current
• Ref Det Signal
4) Return to the Laser Mapping Menu and select the Laser Map Temperature and Current screen. Enter the map
limits:
• Laser Map Temperature Low (K): Choose the lower temperature limit to include the range of interest. There is
no danger to the laser at too low temperature. However, if the lower temperature limit is lower than can be
maintained, the mapping sequence will be suspended waiting for the temperature to stabilize. Make sure the
system can achieve the desired lower map limit. This should be checked at the maximum current, because of
the power dissipated in the laser by its current.
• Laser Map Temperature High (K): Choose the upper temperature limit to include the range of interest, but do
not exceed the operating range of the laser. Lasers can be damaged by operation at temperatures (or currents)
that are too high. Generally the temperature can be increased until either the threshold current begins to
increase more rapidly with increased temperature, or the laser power begins to decrease at higher current. This
parameter must also be set lower than the Maximum laser temperature rating (K) parameter, or the laser
mapping function will not be completed.
• Laser Map Temperature Step Size (K): The step size should be small enough that the laser's mode structure
does not change too much between temperatures. This depends on the characteristics of the individual laser,
but generally a 1or 2 K increment works well.
• Laser Map Current Low (mA): Choose the lower current limit to include the range of interest. It is often
helpful to set this parameter just below the laser’s threshold current (the current at which it starts to emit). The
threshold current will increase with increasing temperature, so the lower current limit should be chosen to
include the laser’s threshold current at the lowest temperature to be mapped.
• Laser Map Current High (mA): Choose the upper current limit to include the range of interest. However,
lasers can be damaged by excessive current. Do not exceed the operational range of the laser, as noted on the
laser vendor’s specification sheet. This parameter must also be set lower than the Maximum laser current
rating (mA) parameter, or the laser mapping function will not be completed.
• Laser Map Current Step Size (mA): The current increment must be small enough to provide sufficient detail in
the laser maps. Generally 0.03 to 0.05 mA works well. Maps collected at very low pressure (which reduces
line width) may require a smaller current increment. Conversely, maps collected at higher pressure may be
collected with a larger current increment.
5) Start the laser mapping function. Return to the Laser Mapping Menu, and then select Begin Laser Map. The real
time screen will appear, with a run mode of LaserMap. This mode allows the user to change the graphical displays,
but it does not allow the run mode, functions or dynamic parameters to be changed. Graphs 1, 2, and 3 will be set
to display the reference detector signal, the laser temperature, and the DC current. The laser temperature will be set
to the starting (lowest) temperature. As soon as the temperature stabilizes at this temperature, the data collection
will start and the laser DC current will increment from the lower limit to the upper limit. The current will increment
every sample (0.1 s), so the time for each current scan will be approximately:
step
low high
t
) (
1 . 0
−
=
For example, if the current high, low, and step size are 500, 300, and 0.05 mA, the time for each temperature will be
approximately 400 s.
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