LDS 3000 basic operation.pdf - Siemens

LDS 3000
Basic Operation
CPU and
presentation
Central Unit
Laser light
Electrical signals
Return LED light
Laser
control
Hybride
cable
Measurement channel
Measurement
volume
P T
Ch 1
Signal
processing
Diode laser
P T
Measurement
Optical
volume
P 0 splitter
P R
Reference
cell
Ch 2
Measurement
volume
P T
Ch 3
Your
eye
in
hell
Basic operation
The operation of LDS 3000 is based on the fact that light propagating
through a gas mixture will be absorbed by the presence of gas molecules
at certain narrow wavelength bands. LDS 3000 uses the light
emitted from a semi-conductor laser tuned over an absorption line of the
gas to be measured. This light is split into five paths using a passive
optical splitter (a standard telecom component). The measurement is
controlled by an internal PC.
• One, two or three paths are for the measurement channels.
• Two paths are used for the internal checks of the laser: One is used to
monitor the laser power and one is used in an internal measurement
path. This path is resident inside the central unit and uses a glass
cell containing the gas the instrument is measuring.
• The internal paths compensate for any drifts in the system, thus
making recalibration of the instrument unnecessary.
Siemens Laser Analytics

Basic operation of LDS 3000
Introduction
The heart in LDS 3000 is the laser generating light which is
detected at up to five different points. The information from
these points all add up to a value of the gas concentration.
Measurement paths
LDS 3000 is almost always configured to measure in-situ.
As a result of this, the environment in the measurement
gas volume can vary within wide limits. The temperature
can range from ambient to more that 1200 °C, the gas
can be very humid (20-30% water content is common) and
the dust load, giving transmission variations, is sometimes
huge. Usually the pressure variations in our applications are
small, ~±50 mbar.
As opposed to an extractive measurement where the measurement
gas is controlled (and changed unwillingly) all
these variations must be accounted for.
LDS 3000 can, however, also be configured with an extractive
sensor. With this a long path, and consequently high
sensitivity, can be obtained. In this sensor the temperature
and the pressure are controlled. Standard gas treatment for
extractive sensors can also be applied.
Reference and Monitor paths
The optical power in the reference path is detected by the
reference detector, P R . The reference channel is built into
CU 3000 and contains a glass cell usually containing the
gas to be measured.
The reference path has two functions:
• Locking - The laser wavelength is locked to the absorption
line by tuning it so the reference absorption line is
positioned and visible in the scan window. The control
computer then locks this position by regulating the laser
current which in turn controls the laser wavelength.
• Calculating the measurement value - The measurement
value is calculated using the shape of the absorption
line in the measurement channel, the shape of the reference
absorption line and the data from the measurement
gas parameters like temperature, pressure, gas mix,
etc. These parameters can either be inputs to LDS 3000
or stored as constants in LDS 3000 or, in some cases,
derived from the absorption lines themselves.
The reference channel is an important feature in LDS 3000.
Electronically it is identical to a measurement channel and
by observing the shape of the absorption line in the reference
channel, laser degradations can be monitored and
compensated for. It also increases the sensitivity since the
presence of the reference absorption line makes it easier
to detect measurement absorption lines in a noisy background.
The optical power is also detected by the monitor detector,
P 0 , which is used to track laser variations. Laser degradation
can be spotted very early and eventually the instrument
will notify the user of this. Laser degradation is very
rare though, since very reliable telecom type lasers are used
in LDS 3000.
Calibration
In the calibration of the instrument, gas is flowed through
an oven at appropriate temperature and the calibration constant
is recorded and stored in the instrument for different
temperatures and pressures. An important aspect of the
calibration gas it that it is composed to resemble the composition
of the gas in the real measurement situation. The
dependence of the temperature, pressure and interference
from other gases, mostly water vapour, will then be compensated
for in the instrument. However, the user needs to
give the instrument a reasonable value of the temperature
and pressure when measurements are done, either manually
or by connecting appropriate sensors to LDS 3000.
Standard industrial inputs like 4-20 mA are handled.
Reference documents
• Measurement principle - Describes the theoretical fundamentals
and the measurement principle.
• Dust load compensation - Describes the method used
to compensate for the varying transmission from heavy
dust load in the measurement path.
Contact
Siemens Laser Analytics (headquarters)
Marketing Director
Jan Grimbrandt • jan.grimbrandt@siemens.com
Technical expert
Gunnar Haeffler • gunnar.haeffler@siemens.com
Marketing Assistant
Michaela Hermansson • michaela.hermansson@siemens.com
Phone: +46 (0)31 776 86 00
Recalculating to the mg/Nm 3 unit
The user can as an option choose to display the concentration data in milligrams per normal cubic meter, mg/Nm 3 . To
do this the calibrated reading in ppm is recalculated using the formula below. Note that there are two different standards
- mainly between Europe and USA.
mg/Nm 3 = ppm · (molecular weight of the gas) / (litres/mole)
at 1 atm and 0 °C -> litres/mole = 22.41 (STP) labelled mg/Nm 3 EU in LDS 3000
at 1 atm and 25 °C -> litres/mole = 24.46 labelled mg/Nm 3 US in LDS 3000
Siemens Laser Analytics
Siemens Laser Analytics AB • Box 5065 • SE-402 22 Göteborg • Sweden • www.siemens.se/sla