Yokogawa In-Situ Gas Analyzer TDLS200 - Manual (Page 82)

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IM 11Y01B01-01E-A 6th Edition :Feb 13, 2013-00
<6. BASIC OPERATION>
6.5 Large Aperture Optics
For in-situ application (typically large scale combustion systems) the optical path lengths are generally very long
(7-30 m for large scale combustion and ethylene furnaces). Standard TDLS optics and laser beam configurations
are unsuitable because of the mechanical stability of these large scale combustion systems. Yokogawa Laser
Analysis Division therefore designed and developed the concept of a diverging beam (i.e. a laser beam that
expands over distance) and a large aperture optics detector scheme (i.e. a large target for the laser to hit). The
general concept of diverging beam and large aperture optics is shown below:
In a standard TruePeak TDLS200 analyzer, the laser beam exiting the launch unit is normally collimated parallel
before hitting the opposing detect unit. The collimated beam size is typically less than 1” diameter. However,
this optical layout is not appropriate for long-path applications (the dimension of process is longer than 30 feet).
During initial installation, it is also difficult to align the laser beam so that it can hit the targeted detect unit over
a long distance (small changes in the launch unit angle are magnified over long distances). Also it is almost
impossible to keep good alignment with varying ambient and process conditions – especially during cold starts
and shut-downs (the most extreme thermal changes on the mechanical structures).
To resolve the above issues, Yokogawa Laser Analysis Division has developed a diverging beam and large
aperture optics strategy. At the launch side, the output laser beam has a small diverging angle. For example, the
beam size is about 20” (~50 cm) diameter at 60’ (~18 m) optical distance. The optical aperture at the receive unit
is enlarged from the original 1¾” diameter. With these two changes, it is much easier to do initial alignment and
keep good transmission during a wide range of operating conditions.
Please note that when an analyzer is mounted on the standard 0.726m (28.6”) OPL off-line calibration cell, there
is a large amount of laser light on the detector. Once moved onto a long path installation, the diverging beam
power is spread out over a larger area and hence weaker, this means that once installed on the long path, the
detector gain may require some manual adjustment (increase typically) – please use the following procedure.
TDLS200 TDL Analyzer Instruction Manual V2.1
Page 76 of 131 - 76 -
4.6 Large Aperture Optics
For in-situ application (typically large scale combustion systems) the optical path lengths are
generally very long (7-30m for large scale combustion and ethylene furnaces). Standard TDL
optics and laser beam configurations are unsuitable because of the mechanical stability of these
large scale combustion systems. Yokogawa Laser Analysis Division therefore designed and
developed the concept of a diverging beam (i.e. a laser beam that expands over distance) and a
large aperture optics detector scheme (i.e. a large target for the laser to hit).
The general concept of diverging beam and large aperture optics is shown below:
The general concept of diverging beam and a standard Detect Unit is shown below:
In a standard TruePeak TDL analyzer, the laser beam exiting the launch unit is normally collimated
parallel before hitting the opposing detect unit. The collimated beam size is typically less than 1”
diameter. However, this optical layout is not appropriate for long-path applications where the
distance between the laser and detector is > 30 feet. During initial installation, it is also difficult to
align the laser beam so that it can hit the targeted detect unit over a long distance (small changes
in the launch unit angle are magnified over long distances). During conditions that will cause flex
of the furnace walls, it is difficult to maintain alignment. This is common during process cold starts
and shut-downs (the most extreme thermal changes on the mechanical structure of the process).
To resolve the above issues, Yokogawa Laser Analysis Division has developed a diverging beam
PLUS large aperture optics strategy. At the launch side, the output laser beam has a small
diverging angle. For example, the beam size is about 20” (~50cm) diameter at 60’ (~18m) optical
distance. The optical aperture at the receive unit is enlarged from the original 1¾” diameter. With
these two changes, it is much easier to do initial alignment and keep good transmission during a
wide range of operating conditions.
Figure 52