Siemens LDS 6 Operating Instructions Manual page 23

Figure 2-11
Influences on the measurement
Dust load
As long as the laser beam is able to generate a suitable detector signal, the dust load of the
process gases does not influence the analytical result. By applying a dynamic background
correction, measurements can be carried out without any negative impact. Under good
conditions, particle densities up to 100 g/Nm
loads are compensated by scanning the laser over the gas absorption line and the current
background. At a scan position next to the absorption line, the instrument can "see" only
absorption caused by the dust load where at the line center the signal is composed of the
molecular absorption and the continuous, unspecific background absorption. With the
wavelength modulation technique, the actual measured transmission is always compared
with the baseline. After signal processing, phase-sensitive application delivers a signal only
from the molecular line free of background.
The influence of a high dust load is complex and depends on the path length and particle
size. The optical damping increases at longer path lengths. Smaller particles also have a
large influence on the optical damping. With a combination of high dust load, long path
length and small particle size, the technical support at Siemens should be consulted.
Temperature
The temperature influence on the absorption line strength is compensated by a correction
factor determined during calibration. A temperature signal can be fed into the instrument
from an external temperature sensor. This signal is then used to correct the influence of the
temperature on the observed line strength. If the temperature of the sample gas remains
constant, it is alternatively possible to carry out a static correction using a preset value.
At high temperatures there may be noticeable broadband IR radiation of gas and dust, or
flames may occasionally occur in the measurement path. In this case the detector is
protected by an optical bandpass filter to prevent saturation by the strong background
radiation.
LDS 6
Operating Instructions, 01/2009, A5E00295894-05
Typical spectral bandwidth of an absorption line compared to the bandwidth of the laser
light
2.5 Measurement Principle
can be handled by the LDS 6. Varying dust
3
Technical Information
23