Siemens SITRANS SL, Руководство по эксплуатации, Страница: 23 / 212

Description
2.5 Functional description
SITRANS SL
Operating Instructions, 12/2010, A5E01132948-04
25
Optical path length
As a result of Beer-Lambert's law, the absorption of laser light depends on the optical path
length within the gas. Therefore the precision of the optical path length measurement has an
effect on the precision of the total measurement.
Since the sensor optics on the process side usually have to be purged to keep them clean
for a longer period, the extent of the mixed zone between the purging medium and the
process gas as well as the concentration distribution of the latter must be considered. In a
typical in-situ installation with an optical path length of several meters, the influence of the
purging gas on the effective path length can be ignored.
The path length and dust load are mutually influencing: the higher the dust load in the
process, the shorter the max. possible path length.
The transmitter and receiver units are mounted on process flanges provided by the
customer. Correct alignment of these flanges must be guaranteed, e.g. by using the optional
sensor alignment kit.
Adjustment of the sensor pair
The process flanges of the SITRANS SL must be correctly aligned so that the laser beam
generated by the transmitter hits the photodetector in the receiver unit. This is guaranteed in
that the transmitter and receiver units have a curved surface integrated in the flanges. The
adjustment is carried out by shifting the flanges on these surfaces, through which the
symmetry axis is aligned. The axis can be offset by ±1 degree, which means that the
process flanges must be welded onto the process wall with at least this accuracy - see
following figure.
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Figure 2-8 Installation adjustment requirements for the pair of cross-duct sensors