Siemens SITRANS SL User manual

Continuous gas analysis
In situ Laser Gas Analyzers
SITRANS SL
Operating Instructions
12/2010
A5E01132948-04
Introduction 1
Description
2
Application planning
3
Installing
4
Explosion protection
5
Commissioning
6
Operation
7
Interfaces to automation
systems
8
Service and maintenance
9
Alarm, error, and system
messages
10
Technical data
11
Dimension drawings
12
Spare parts and accessories
13
Appendix
A
List of abbreviations
B

Legal information
Warning notice system
This manual contains notices you have to observe in order to ensure your personal safety, as well as to prevent
damage to property. The notices referring to your personal safety are highlighted in the manual by a safety alert
symbol, notices referring only to property damage have no safety alert symbol. These notices shown below are
graded according to the degree of danger.
DANGER
indicates that death or severe personal injury will result if proper precautions are not taken.
WARNING
indicates that death or severe personal injury may result if proper precautions are not taken.
CAUTION
with a safety alert symbol, indicates that minor personal injury can result if proper precautions are not taken.
CAUTION
without a safety alert symbol, indicates that property damage can result if proper precautions are not taken.
NOTICE
indicates that an unintended result or situation can occur if the corresponding information is not taken into
account.
If more than one degree of danger is present, the warning notice representing the highest degree of danger will
be used. A notice warning of injury to persons with a safety alert symbol may also include a warning relating to
property damage.
Qualified Personnel
The product/system described in this documentation may be operated only by personnel qualified for the specific
task in accordance with the relevant documentation for the specific task, in particular its warning notices and
safety instructions. Qualified personnel are those who, based on their training and experience, are capable of
identifying risks and avoiding potential hazards when working with these products/systems.
Proper use of Siemens products
Note the following:
WARNING
Siemens products may only be used for the applications described in the catalog and in the relevant technical
documentation. If products and components from other manufacturers are used, these must be recommended
or approved by Siemens. Proper transport, storage, installation, assembly, commissioning, operation and
maintenance are required to ensure that the products operate safely and without any problems. The permissible
ambient conditions must be adhered to. The information in the relevant documentation must be observed.
Trademarks
All names identified by ® are registered trademarks of the Siemens AG. The remaining trademarks in this
publication may be trademarks whose use by third parties for their own purposes could violate the rights of the
owner.
Disclaimer of Liability
We have reviewed the contents of this publication to ensure consistency with the hardware and software
described. Since variance cannot be precluded entirely, we cannot guarantee full consistency. However, the
information in this publication is reviewed regularly and any necessary corrections are included in subsequent
editions.
Siemens AG
Industry Sector
Postfach 48 48
90026 NÜRNBERG
GERMANY
order number: A5E01132948
Ⓟ 12/2010
Copyright © Siemens AG 2008,
2009, 2010.
Technical data subject to change

SITRANS SL
Operating Instructions, 12/2010, A5E01132948-04 5
Table of contents
1 Introduction................................................................................................................................................ 9
1.1 Purpose of this documentation ......................................................................................................9
1.2 History............................................................................................................................................9
1.3 Product versions ..........................................................................................................................10
1.4 General information .....................................................................................................................11
1.5 Special information and warnings................................................................................................11
1.6 Warranty conditions .....................................................................................................................12
1.7 Delivery information .....................................................................................................................12
1.8 Standards and regulations...........................................................................................................12
2 Description............................................................................................................................................... 13
2.1 Overview ......................................................................................................................................13
2.2 Benefit..........................................................................................................................................14
2.3 Range of application ....................................................................................................................15
2.4 Design..........................................................................................................................................16
2.4.1 Transmitter and receiver units .....................................................................................................16
2.4.2 Display and control panel.............................................................................................................19
2.4.3 Cables for connection ..................................................................................................................20
2.4.4 Inputs/outputs...............................................................................................................................21
2.5 Functional description ..................................................................................................................22
2.5.1 Operating principle.......................................................................................................................22
2.5.2 Influences on the measurement...................................................................................................24
2.5.3 Maintenance and fault messages ................................................................................................28
2.5.4 Essential characteristics ..............................................................................................................29
2.6 Product characteristics.................................................................................................................30
3 Application planning................................................................................................................................. 31
3.1 Cable selection recommendations...............................................................................................31
3.2 Network and system integration...................................................................................................32
3.2.1 Overview ......................................................................................................................................32
3.2.2 Analog I/O and digital I/O system configuration...........................................................................33
3.2.3 PROFIBUS DP system configuration ..........................................................................................35
3.2.4 Modbus system configuration ......................................................................................................37

Table of contents
SITRANS SL
6 Operating Instructions, 12/2010, A5E01132948-04
4 Installing .................................................................................................................................................. 39
4.1 Safety information ....................................................................................................................... 39
4.2 Mounting...................................................................................................................................... 40
4.2.1 Mounting conditions .................................................................................................................... 40
4.2.2 Preparations................................................................................................................................ 41
4.2.3 Installation of process flanges..................................................................................................... 43
4.2.4 Installation of the sensors ........................................................................................................... 44
4.2.5 Installation of the purging tubes .................................................................................................. 45
4.3 Electric connections non Ex........................................................................................................ 46
4.3.1 System set-up (non Ex only)....................................................................................................... 47
4.3.2 Screw terminals........................................................................................................................... 48
4.3.3 Connecting cables....................................................................................................................... 52
4.4 Alignment of SITRANS SL .......................................................................................................... 57
4.4.1 Alignment kit................................................................................................................................ 58
4.4.2 Alignment procedure................................................................................................................... 59
4.5 Purging........................................................................................................................................ 64
4.5.1 Sensor side purging .................................................................................................................... 64
4.5.2 Process side purging................................................................................................................... 65
4.5.3 Purging set-up............................................................................................................................. 66
5 Explosion protection ................................................................................................................................ 67
5.1 ATEX........................................................................................................................................... 67
5.1.1 Safety information ....................................................................................................................... 67
5.1.2 Installation ATEX......................................................................................................................... 70
5.1.3 ATEX system set-up ................................................................................................................... 71
5.1.4 Electric connections in the ATEX system ................................................................................... 73
5.2 FM ............................................................................................................................................... 78
5.2.1 Introduction ................................................................................................................................. 78
5.2.2 Safety information ....................................................................................................................... 78
5.2.3 Installation of the FM version of SITRANS SL............................................................................ 81
5.2.4 FM system set-up........................................................................................................................ 82
5.2.5 Electric connections in the FM system........................................................................................ 84

Table of contents
SITRANS SL
Operating Instructions, 12/2010, A5E01132948-04 7
6 Commissioning ........................................................................................................................................ 87
6.1 General information for commissioning .......................................................................................87
6.2 Starting SITRANS SL for the first time.........................................................................................88
6.2.1 Defining the path length...............................................................................................................88
6.2.2 Further input parameters .............................................................................................................89
6.3 PROFIBUS configuration for SITRANS SL..................................................................................89
6.3.1 Cyclic data transfer ......................................................................................................................89
6.3.2 System integration .......................................................................................................................90
6.3.3 SIMATIC MPI cable installation ...................................................................................................90
6.3.4 PG/PC interface setting ...............................................................................................................91
6.3.5 Loading GSD file..........................................................................................................................92
6.3.6 Network configuration with SIMATIC Manager............................................................................95
6.3.7 Creation of a new project with SIMATIC Manager.......................................................................96
6.3.8 Hardware configuration..............................................................................................................101
6.4 Modbus configuration for SITRANS SL .....................................................................................106
6.4.1 Modbus settings.........................................................................................................................106
6.4.2 Byte and word ordering..............................................................................................................106
6.4.3 Supported Modbus functions .....................................................................................................107
7 Operation............................................................................................................................................... 109
7.1 General ......................................................................................................................................111
7.2 Measurement views ...................................................................................................................114
7.3 Menu system..............................................................................................................................115
7.3.1 The menu structure....................................................................................................................115
7.3.2 Configuration..............................................................................................................................116
7.3.3 Diagnostics.................................................................................................................................120
7.3.4 Service .......................................................................................................................................122
7.3.5 Communication ..........................................................................................................................125
7.3.6 Security ......................................................................................................................................127
7.3.7 Language ...................................................................................................................................128
8 Interfaces to automation systems .......................................................................................................... 129
8.1 PROFIBUS DP interface............................................................................................................129
8.1.1 PROFIBUS installation...............................................................................................................129
8.1.2 Device data base files (GSD).....................................................................................................131
8.1.3 Technical data............................................................................................................................131
8.1.4 Cyclic data transmission over PROFIBUS.................................................................................132
8.1.4.1 Cyclic data structure ..................................................................................................................132
8.1.4.2 Quality byte ................................................................................................................................133
8.2 Modbus interface .......................................................................................................................136
8.2.1 Modbus installation ....................................................................................................................136
8.2.2 SITRANS SL Modbus map for gas measurement.....................................................................137
8.2.3 Quality byte coding ....................................................................................................................138
8.2.4 Modbus adresses.......................................................................................................................141
8.2.4.1 Standard Modbus.......................................................................................................................141
8.2.4.2 Enron Modbus............................................................................................................................143
8.2.4.3 Modbus alarm list.......................................................................................................................146

Table of contents
SITRANS SL
8 Operating Instructions, 12/2010, A5E01132948-04
9 Service and maintenance ...................................................................................................................... 149
9.1 Instrument verification............................................................................................................... 149
9.1.1 Verification with the verification tube......................................................................................... 150
9.1.1.1 Verification tube ........................................................................................................................ 150
9.1.1.2 Span verification........................................................................................................................ 151
9.1.2 Verification with a verification cell ............................................................................................. 157
9.1.2.1 Span verification........................................................................................................................ 157
9.1.3 Verification of systems used in hazardous areas ..................................................................... 159
9.2 Cleaning the optical parts of the sensor.................................................................................... 160
10 Alarm, error, and system messages ...................................................................................................... 163
10.1 Alarm......................................................................................................................................... 163
10.2 Alarm list.................................................................................................................................... 170
10.3 PROFIBUS diagnosis and alarms............................................................................................. 177
10.3.1 Diagnosis and alarm handling in PROFIBUS DP ..................................................................... 177
10.3.2 Meaning of the diagnosis information ....................................................................................... 178
11 Technical data ....................................................................................................................................... 181
11.1 Technical data........................................................................................................................... 181
12 Dimension drawings .............................................................................................................................. 187
12.1 Dimensional drawings............................................................................................................... 187
13 Spare parts and accessories ................................................................................................................. 191
13.1 Spare parts................................................................................................................................ 191
13.2 Accessories............................................................................................................................... 193
A Appendix................................................................................................................................................ 195
A.1 Gas flow calculations ................................................................................................................ 195
A.2 ESD (ElectroStatic Discharge)..................................................................................................196
A.3 Verification Sheets .................................................................................................................... 198
A.4 Service and support .................................................................................................................. 201
A.5 Returned deliveries ................................................................................................................... 202
A.5.1 Returned deliveries form........................................................................................................... 203
A.5.2 Decontamination declaration..................................................................................................... 204
B List of abbreviations............................................................................................................................... 205
Glossary ................................................................................................................................................ 209

SITRANS SL
Operating Instructions, 12/2010, A5E01132948-04 9
Introduction 1
1.1 Purpose of this documentation
Before beginning work with this device, please study this manual carefully! It contains
important information and data whose observation ensures proper device function and saves
you servicing costs. The manual will help you to operate the device more easily and
efficiently, allowing you to achieve reliable results.
1.2 History
The following table shows the most important changes in the documentation compared to
each previous edition:
Edition Remark
01
03/2008
First edition
02
05/2009
Revision of contents and layout
Illustrations of the device have been changed due to changes of the device
housing.
All chapters have new enhanced contents, except the following chapters:
– NEW: " Network and system integration""
– NEW: "Process interfacing over an automation system (PLC, PC)"
– NEW: "Spare parts and accessories"
– NEW: "Service and support"
– NEW: "Interfaces to automation systems"
– NEW: "Glossary"
03
07/2010
Revision of contents and layout
Carbon monoxide (CO) introduced as measuring gas
04
12/2010
Update of contents and layout

Introduction
1.3 Product versions
SITRANS SL
10 Operating Instructions, 12/2010, A5E01132948-04
1.3 Product versions
The product version number is found under the article number on the nameplate (see
following figure).
9HUVLRQ1R
1DPHSODWHVKRZLQJWKHSRVLWLRQRIWKHYHUVLRQQR

Introduction
1.4 General information
SITRANS SL
Operating Instructions, 12/2010, A5E01132948-04 11
1.4 General information
The product described in this manual has left the factory in a high quality and tested
condition. In order to preserve this condition and to operate this product correctly and safely,
it may only be used in the manner described by the manufacturer. Furthermore, proper
transportation, storage, installation, operation and maintenance of the device is vital for
ensuring correct and safe operation.
This manual contains the information required for the intended use of the described product.
It is addressed to technically qualified personnel who are specially trained or who have the
relevant knowledge of automation technology (measuring and control systems).
Knowledge and technically correct implementation of the safety notes and warnings
contained in this manual are required for safe installation and commissioning, as well as for
safety during the operation and maintenance of the described product. Only qualified
personnel have the required professional knowledge for correctly interpreting the generally
valid safety notes and warnings in this manual in each specific case and to act accordingly.
This manual is an inherent part of the scope of delivery, despite the fact that it can be
ordered separately for logistic reasons.
Due to the variety of technical details, it is not possible to consider every single detail for all
versions of the described product and for every conceivable case in the set-up, operation,
maintenance and use in systems. For further information, or in the case of problems which
are not covered in enough detail in this document, please request the required information
from your local or responsible Siemens regional office.
Note
In particular, before using the device for new research and development applications, we
recommend that you first contact your Siemens representative or our application department
to discuss the application in question.
1.5 Special information and warnings
This manual provides you with information on using, installing, operating, and maintaining
the device.
Pay particular attention to all special information and warnings. Information of this type is set
apart from the rest of the text and is marked with the corresponding pictograms. This
information provides you with useful tips and helps to avoid faulty operation.

Introduction
1.6 Warranty conditions
SITRANS SL
12 Operating Instructions, 12/2010, A5E01132948-04
1.6 Warranty conditions
We expressly point out that the product quality is exclusively and conclusively described in
the sales contract. The content of this product documentation is neither a part of a previous
or existing agreement, promise or legal relationship, nor is it intended to modify these. All
obligations on the part of Siemens AG are contained in the respective sales contract, which
also contains the complete and solely applicable liability provisions. The provisions defined
in the sales contract for the responsibility for defects are neither extended nor limited by the
remarks in this document.
1.7 Delivery information
The respective scope of delivery is listed on the shipping documents – enclosed with the
delivery – in accordance with the valid sales contract.
When opening the packaging, please observe the corresponding information on the
packaging material. Check the delivery for completeness and undamaged condition. In
particular, you should compare the Order No. on the rating plates with the ordering data, if
available.
If possible, please retain the packaging material, since you can use it again in case of return
deliveries.
1.8 Standards and regulations
As far as possible, the harmonized European standards were the basis for the specification
and production of this device. If no harmonized European standards have been applied, the
standards and regulations for the Federal Republic of Germany are valid.
When this product is used beyond the scope of these standards and regulations, the valid
standards and regulations of the country of the operating company apply.

SITRANS SL
Operating Instructions, 12/2010, A5E01132948-04 13
Description 2
2.1 Overview
SITRANS SL is a diode laser gas analyzer with a measuring principle based on the specific
light absorption of different gas components. SITRANS SL is suitable for fast, non-contact
measurement of gas concentrations in process or flue gases. An analyzer consisting of
transmitter and receiver unit (sensors) is used for each measuring point. The hardware for
further processing of the measured signal into a concentration value, as well as the
monitoring, control and communication functions, are integrated in these two main modules.
The sensors are designed for operation under harsh environmental conditions.
Figure 2-1 SITRANS SL, typical setup

Description
2.2 Benefit
SITRANS SL
14 Operating Instructions, 12/2010, A5E01132948-04
2.2 Benefit
The in-situ SITRANS SL gas analyzer features high operational availability, unique analytical
selectivity, and a wide range of possible applications. SITRANS SL permits measurement of
a gas component directly in the process:
● With high dust load
● In hot, humid, corrosive, explosive, or toxic gases
● In applications showing strong varying gas compositions
● Under harsh environmental conditions at the measuring point
● Highly selective, i.e. mostly without cross-sensitivities
Special features of the SITRANS SL:
● Little installation effort
● Minimum maintenance requirements
● Extremely rugged design
● High long-term stability through built-in, maintenance-free reference gas cell
● Real-time measurements
Moreover, the analyzer provides warning and error messages:
● When maintenance is required
– With large variations in the reference signal
– With poor signal quality
● If the transmission violates an upper or lower limit

Description
2.3 Range of application
SITRANS SL
Operating Instructions, 12/2010, A5E01132948-04 15
2.3 Range of application
Sections
● Chemical and petrochemical plants
● Power plants
● Waste incinerators
● Iron and steel industry
Applications
● Control of combustion processes
● Process optimization
● Plant and operator safety
● Process measurements in all types of power and combustion plants
● Process control
● Explosion protection
● Measurements in corrosive and toxic gases
● Quality control
An overview of standard applications can be obtained from Technical data (Page 181),
section 'Standard applications'.

Description
2.4 Design
SITRANS SL
16 Operating Instructions, 12/2010, A5E01132948-04
2.4 Design
The SITRANS SL gas analyzer consists of a pair of cross-duct sensors - a transmitter unit
and a receiver unit - both with the same dimensions. The complete analyzer is integrated in
these two enclosures (pos. 1 and 7, fig. 2.2). The transmitter unit contains the laser source
which light is transmitted to the receiver through the measurement path. The receiver unit
contains a photodetector including electronics as well as a reference cell. The receiver unit is
connected to the transmitter unit by means of a sensor connection cable. The connection
cable of the receiver is used to connect the power supply and the communication interfaces.
The receiver enclosure contains a local user interface (LUI) with an LC display which can be
read through a window in the lid. The LUI is normally operated via a remote control panel.
2.4.1 Transmitter and receiver units
Special features of the transmitter and receiver units:
● Two in situ cross duct sensors, designed as transmitter and receiver units, connected via
sensor connection cable
● Process interface which
– consists of identical assemblies for transmitter and receiver
– is the interconnection between the transmitter/receiver units and the process
● Powder-coated aluminium; stainless steel
● Degree of protection IP65
● Adjustable flanges
● Suitable for a flange connection to DN50/PN10-40, ANSI 4"/150 lbs
● Purging facilities on the process and the sensor sides, configurable application with
purging gas connections

Description
2.4 Design
SITRANS SL
Operating Instructions, 12/2010, A5E01132948-04 17
Non Ex version and sensor connection cable kit
  












 

① Transmitter housing ⑨ Large cable gland (for cables with a diameter of
13 ... 18 mm)
② Transmitter tube unit ⑩, ⑪ Small cable gland (for cables with a diameter of
8 ... 12 mm)
③ Process interface ⑬ Reference cell inside the receiver unit
④ Customer flange (not included) ⑭ Remote control
⑤ Purging tube (accessory) Sensor connection cable kit (accessory), consisting of
⑥ Receiver tube unit ⑫ Sensor connection cable
⑦ Receiver housing ⑮ Receiver junction box
⑧ Connection cable (accessory) ⑯ Receiver junction box cable
⑰ Cable gland for Ethernet cable
Figure 2-2 Connection of a non Ex system with the sensor connection cable kit (accessory)
The sensor connection cable is used to connect the transmitter unit to the receiver unit. This
can be done either directly, or via a receiver junction box (see preceding figure). A sensor
connection cable kit is available as accessory.

Description
2.4 Design
SITRANS SL
18 Operating Instructions, 12/2010, A5E01132948-04
ATEX version
  

















 aP
aP
/
PD[
P
① Transmitter housing ⑪ ATEX cable gland
② Transmitter tube unit ⑫ Sensor connection cable
③ Process interface ⑭ Remote control
④ Customer flange (not included) Receiver junction box kit, consisting of
⑤ Purging tube (accessory) ⑮ Ex-e receiver junction box
⑥ Receiver tube unit ⑯ Receiver junction box cable
⑦ Receiver housing ⑰ Cable gland for Ethernet cable
Connection cable solution -
Analog I/O, Modbus:
Transmitter junction box kit, consisting of
⑧ Connection cable ⑱ Ex-e transmitter junction box
⑨ ATEX cable gland ⑲ Transmitter junction box cable
Connection cable solution - PROFIBUS DP  Mounting bracket for junction box
⑳ Connection cable
⑩ ATEX cable gland
Figure 2-3 Design of the ATEX version of the SITRANS SL system
See also
Electric connections in the ATEX system (Page 73)
Electric connections in the FM system (Page 84)

Description
2.4 Design
SITRANS SL
Operating Instructions, 12/2010, A5E01132948-04 19
2.4.2 Display and control panel
Special features of the receiver unit:
● Display for simultaneous output of result and device status.
● LED backlighting of display.
● Menu-driven operation for parameterization and diagnostics.
● Remote operation via infrared interface for safe use in hazardous zones.
● Remote control using membrane keypad and softkeys which are easy to clean.
Figure 2-4 Local user interface (LUI) of SITRANS SL in the receiver unit (display of measured
value)
Figure 2-5 Remote contol keypad for SITRANS SL

Description
2.4 Design
SITRANS SL
20 Operating Instructions, 12/2010, A5E01132948-04
2.4.3 Cables for connection
● The SITRANS SL is normally delivered without cabling, except for the ATEX version.
● For non-Ex versions the accessory "sensor connection cable kit" is used to connect the
transmitter and receiver unit of the analyzer, while the connection cable is needed to
connect the SITRANS SL electrically to your process control system. All materials used
are flame-retardant.
● A sensor connection cable kit is available as an accessory and provides the sensor
connection cable, i.e. the cable connecting receiver and transmitter of the SITRANS SL.
The sensor connection cable is available in lengths of 5 m, 10 m or 25 m length
according to your order.
● The sensor connection cable kit also provides the possibility of a permanent installation of
an Ethernet cable.
● A protective hose should be used as UV protection for installations in open cable ducts or
channel systems. It is available as an accessory.
● The statutory directives must be observed in the event of installation in hazardous areas.
For cable specifications refer to section Cable selection recommendations (Page 31)
Electric connections in the ATEX system
● The system is delivered with pre-mounted cables. The connections inside the unit
housings are set up in the factory before delivery and modifications can only be done by
certified technicians.
● The junction boxes mounted to the receiver and the transmitter provide connections for
the sensor connection and the Ethernet cables.
● An Ex-e classified junction box is required to install the connection cable to your process
control system. A suitable junction box is available as an accessory (not represented in
the figure above).
● The sensor connection cable (⑫, fig. 2.3) is delivered with unprepared ends. It has to be
prepared according to section Electric connections in the ATEX system (Page 73).
● The ATEX version of the SITRANS SL is delivered with a sensor connection cable of the
ordered length. This sensor connection cable must be connected between the Ex-e
junction boxes mounted on the receiver and the transmitter.
● All cable kits are part of the delivery and can be ordered separately as spare parts.
See also
Transmitter and receiver units (Page 16)

Description
2.4 Design
SITRANS SL
Operating Instructions, 12/2010, A5E01132948-04 21
2.4.4 Inputs/outputs
● 2 analog inputs (4 to 20 mA) for process gas temperature and pressure
● 2 analog outputs (4 to 20 mA) for gas concentration or for concentration and transmission
● 1 configurable binary input for quality information of external sensor
● 2 freely configurable binary outputs (display of faults, maintenance request, function
monitoring, alarms for limit violations of measured value or transmission)
● Optional: 1 PROFIBUS DP interface with:
– Output of gas concentration as cyclic data
– Alarm output, alarm classification
– Input for temperature and/or pressure data for compensation
The PROFIBUS DP protocol provides DPV0, cyclic data. Measured values are provided
with additional quality data.
Note
The PROFIBUS DP hybrid cable provides an unused pair of wires to connect one of the
analog or binary inputs or outputs mentioned above.
● Optional: 1 Modbus interface with:
– Output of gas concentration as cyclic data
– Alarm output, alarm classification
– Input for temperature and/or pressure data for compensation
● 1 Ethernet 10Base-T port, only for service and maintenance. Using Ethernet, the analyzer
supports the following software:
– LDSComm (PC software for service and maintenance engineers)
– SITRANS SL updater (PC software for service engineers)
Note
For the non-Ex version the junction box of the sensor connection cable kit is required
for permanent installation of an Ethernet cable.
WARNING
In hazardous environments, all electric cables may only be connected or
disconnected with the permission of the plant manager (hot work permit)!

Description
2.5 Functional description
SITRANS SL
22 Operating Instructions, 12/2010, A5E01132948-04
2.5 Functional description
2.5.1 Operating principle
SITRANS SL is a gas analyzer employing single line molecular absorption spectroscopy. A
diode laser emits a beam of infrared light which passes through the process gas and is
detected by a receiver unit. The wavelength of the laser diode output is tuned to a gas
specific absorption line. The laser continuously scans this single absorption line with a very
high spectral resolution (see following figure). The degree of absorption and the line shape
are used for the evaluation. The measurement is free of cross-interferences, since the quasi-
monochromatic laser light is absorbed very selectively by only one specific line in the
scanned spectral range.




① Absorption line
② Laser line
Figure 2-6 Typical spectral bandwidth of an absorption line compared to the bandwidth of the laser
light.
The field design of the SITRANS SL in-situ gas analyzer consists of a transmitter unit and a
receiver unit. The light which is not absorbed by the sample is detected in the receiver. The
concentration of the gas component is determined from the absorption.
The individual dependencies of concentration, pressure and temperature are application
specific.
An internal reference cell is used to constantly check the stability of the spectrometer, thus
assuring a continuous self-calibration of the analyzer.
The self-calibration of the analyzer is therefore valid for the time period specified in the
technical data without the necessity for external recalibration using calibration gases.