Measurement Principle - Siemens LDS 6 Operating Instructions Manual

Figure 2-9
2.5

Measurement principle

Mode of operation
The operation of LDS 6 is based on the fact that light propagating through a gas mixture will be
absorbed according to Beer-Lambert's law at certain narrow wavelength bands. This is where
the gases possess molecular transitions forming narrow absorption lines.
The light source in LDS 6 is a semi-conductor laser tuned to an appropriate absorption line for the
gas to be measured. The laser light is spectrally much narrower than the gas absorption line and
this, together with a proper choice of absorption line, will result in low interference from other
gases.
The light is modulated, both in frequency and in amplitude, to facilitate detection on the second
harmonic as well as elimination of contribution from spectrally broad absorption originating
from dust, smoke, etc.
LDS 6 is connected to the measuring points by fiber optics. The laser light is guided by a single-
mode fiber from the central unit to the transmitter unit of the in-situ sensor. The optics housing
consists of a transmitter and a receiver; the distance between them defines the measurement
path. In the receiver box, the light is focused onto a suitable detector. The detector signal is then
converted into an optical signal and transmitted via a second optical fiber to the central unit,
where the concentration of the gas component is determined from the detected absorption
signal.
LDS 6 usually measures a single gas component by means of the absorption capacity of a single
fully resolved molecular absorption line. The absorption results from conversion of the radiation
energy of the laser light into the internal energy of the molecule. In the working range of the LDS
6, both rotation-vibration transitions and electronic transitions – such as with O
triggered.
In some specific cases, two components can be measured simultaneously if their absorption
lines are so close to each other that they can be detected within the laser spectrum by one single
scan (for example water (H
LDS 6
Operating Instructions, 01/2022, A5E00295894-10
Measuring configuration of LDS 6 with heated flow cell
O) and ammonia (NH
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Technical information
2.5 Measurement principle
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– can be
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