Technical Information
2.5 Measurement Principle
LDS 6
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Operating Instructions, 01/2009, A5E00295894-05
Pressure
The gas pressure can affect the line shape of the molecular absorption line. LDS 6 uses a
special algorithm to adapt the line shape. Additionally, an external pressure signal can be fed
to the instrument to provide complete compensation for the pressure influence including the
density effect.
Cross-interferences
Since LDS 6 derives its signal from a single fully resolved molecular absorption line, cross-
interferences with other gases are quite unlikely. LDS 6 is therefore able to measure the
desired gas components very selectively. In special cases, the composition of the process
gas might have an influence on the shape of the absorption line features. This influence is
compensated by analyzing the full shape of the detected signal curve applying specific
algorithms.
Optical path length
The absorption values analyzed by the LDS 6 are typically small. 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 in determining the effective optical path length in the process might
limit the overall precision of the measurement.
As the sensor openings toward the process normally need to be purged to keep them clean
over a long period of time, the thickness of the mixing zone between the purging medium
and the process gas and its concentration distribution need to be considered. In a typical in-
situ installation with some meters of path, the influence of the purging gas on the effective
path length can be neglected.
Path length and dust load are mutually influencing: the higher the dust load in the process,
the shorter the max. possible path length.
Note
Individual requirements for the measuring point can make the utilization of special sensor
equipment necessary. The possibilities for adapting the sensors are:
•
Different purging media, such as instrument air, ambient air, nitrogen or steam
•
Different purging modes on process and sensor sides
•
Special materials of purging tubes and/or sensor flanges
•
Cooling or heating of the sensors
•
EEx-proof sensor configurations