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 sensor
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
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, 01/2009, A5E00295894-05
Measuring configuration of LDS 6 with heated flow cell
– can be triggered.
2
O) and ammonia (NH
2 3
Technical Information
2.5 Measurement Principle
)).
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