Campbell Scientific TGA100 Trace Gas Analyzer Manual - Manual (Page 64)

if there is another absorption line near by. Second, too much absorption will increase the chances of absorbing the
multimode power in some other absorption lines of the gas.
To achieve the optimum amount of absorption, first note the reference transmittance with reference gas in the reference
cell only (the normal configuration.) Normally the reference gas concentration is chosen to give approximately 50%
absorption. However, for this test, it is best to have between 75% and 85% absorption. Usually this can be
accomplished by reducing the pressure in the analyzer. Then put the reference gas in the long sample cell (in addition to
the reference cell) at this same pressure. The long sample cell is much longer than the reference cell, giving
proportionally higher absorbance. Starting with 85% or lower transmittance will result in transmittance well below 1%,
as shown in
. This table also shows that there is no real advantage to starting with a transmittance below 75%. Table 11
Table 11. Reference Transmittance for Laser Multimode Power Test
Reference Transmittance (%) (reference
gas in reference cell only)
Reference Transmittance (%) (reference gas in
reference and sample cells)
95 16.7
90 2.5
85 0.3
80 0.04
75 0.004
Once the proper absorption is achieved, go to the Laser parameter screen and make sure the Laser multimode power
(%) parameter is set to zero. Return to the real time screen and read the value of the reference transmittance at the center
of the ramp (displayed in the graph in the lower right corner of the real time screen). This is the estimate of the laser's
multimode power. Enter this value into the Laser multimode power (%) parameter on the Laser parameter screen.
Return to the real time screen and verify the reference transmittance now reads zero.
For dual ramp operation the laser multimode power may be different for each absorption line. Repeat the process
described above using ramp B to set the value of the Ramp B laser multimode power (%) parameter on the Ramp B
parameter screen.
4.5 Optimizing Detector Parameters
The detector parameters should be set after the laser parameters are adjusted.
4.5.1 Detector Gain and Offset
The detector signals are processed in the TGA electronics, which include an amplifier with programmable gain and
offset in the input module and a second programmable-gain amplifier in the analog module. The detector gains and
offsets are normally controlled automatically by the TGA software using the “OffsetGn” function. This function is
turned on when the TGA software starts up, and it can be toggled on or off by pressing <O> at the real time screen (see
section 3.4.6).
The automatic gain algorithm adjusts the sample detector gain to maximize the sample detector signal while staying
within the analog input range. If the offset corrected detector signal (maximum of the points used in the concentration
calculation) is less than 50% of the analog input range, it will increase the sample gain. If any of the points used in the
calculations are within 2% of the upper or lower limit, it will reduce the gain.
The automatic gain algorithm adjusts the reference detector gain to use between 5% and 10% of the input range.
Keeping the reference detector signal below 10% of the analog input range minimizes electronic crosstalk without
increasing concentration noise.
The detector gains and offsets can be viewed or changed using the dynamic parameter feature of the real time screen
(see section 3.4.4.) The sample detector gain parameter is an integer from 0 to 55, organized into seven groups of eight
(0 to 7, 8 to 15, 16 to 23, 24 to 31, 32 to 29, and 40 to 55). The total gain for each setting is the product of the gain in
the input module and the gain in the analog module. In each group of eight, the input module gain increases from 1.00
to 25.71, with each step increasing the gain by approximately 60%. Moving to the next group of eight increases the
gain in the analog module.
The reference and sample detector signals each have their own amplifier channels on the input module, but they are
multiplexed to share the same amplifier channel in the analog module. The sample detector gain parameter determines
the gain in the analog module for both detectors. Because the reference detector gain is automatically placed in the
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