IEC 61779-6:1999

INTERNATIONAL IEC
STANDARD
61779-6
First edition
1999-06
Electrical apparatus for the detection
and measurement of flammable gases –
Part 6:
Guide for the selection, installation, use and
maintenance of apparatus for the detection
and measurement of flammable gases
Appareils électriques de détection et de mesure
des gaz combustibles –
Partie 6:
Directives pour le choix, l'installation, l'utilisation
et l'entretien des appareils électriques de détection
et de mesure de gaz inflammables

Reference number
Numbering
As from 1 January 1997 all IEC publications are issued with a designation in the
60000 series.
Consolidated publications
Consolidated versions of some IEC publications including amendments are

available. For example, edition numbers 1.0, 1.1 and 1.2 refer, respectively, to the
base publication, the base publication incorporating amendment 1 and the base

publication incorporating amendments 1 and 2.

Validity of this publication
The technical content of IEC publications is kept under constant review by the IEC,
thus ensuring that the content reflects current technology.
Information relating to the date of the reconfirmation of the publication is available
in the IEC catalogue.
Information on the subjects under consideration and work in progress undertaken
by the technical committee which has prepared this publication, as well as the list
of publications issued, is to be found at the following IEC sources:
• IEC web site*
•
Catalogue of IEC publications
Published yearly with regular updates
(On-line catalogue)*
• IEC Bulletin
Available both at the IEC web site* and as a printed periodical
Terminology, graphical and letter symbols
For general terminology, readers are referred to IEC 60050: International
Electrotechnical Vocabulary (IEV).
For graphical symbols, and letter symbols and signs approved by the IEC for
general use, readers are referred to publications IEC 60027: Letter symbols to be
used in electrical technology, IEC 60417: Graphical symbols for use on equipment.
Index, survey and compilation of the single sheets and IEC 60617: Graphical symbols
for diagrams.
* See web site address on title page.

INTERNATIONAL IEC
STANDARD
61779-6
First edition
1999-06
Electrical apparatus for the detection
and measurement of flammable gases –
Part 6:
Guide for the selection, installation, use and
maintenance of apparatus for the detection
and measurement of flammable gases
Appareils électriques de détection et de mesure
des gaz combustibles –
Partie 6:
Directives pour le choix, l'installation, l'utilisation
et l'entretien des appareils électriques de détection
et de mesure de gaz inflammables

 IEC 1999  Copyright - all rights reserved
No part of this publication may be reproduced or utilized in any form or by any means, electronic or
mechanical, including photocopying and microfilm, without permission in writing from the publisher.
International Electrotechnical Commission 3, rue de Varembé Geneva, Switzerland
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Commission Electrotechnique Internationale
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International Electrotechnical Commission
For price, see current catalogue

– 2 – 61779-6 © IEC:1999(E)
CONTENTS
Page
FOREWORD . 4

INTRODUCTION .5

Clause
1 General.6

1.1 Scope . 6
1.2 Normative references. 6
2 Definitions.7
3 Measuring principles.11
3.1 Catalytic sensor.11
3.2 Thermal conductivity sensor. 12
3.3 Electromagnetic radiation absorption sensor – usually infrared . 13
3.4 Semi-conductor sensor.14
3.5 Electrochemical sensors.14
3.6 Flame ionization detectors (FID). 15
4 Selection of apparatus . 15
4.1 General.16
4.2 Selection criteria.16
4.3 Miscellaneous factors affecting selection of apparatus. 18
5 Behaviour of gas releases. 19
5.1 Nature of a release . 19
5.2 Outdoor sites and open structures. 21
5.3 Buildings and enclosures . 21
5.4 Environmental considerations.22
6 Installation of fixed gas detection apparatus . 22
6.1 Basic considerations for the installation of fixed systems . 22

6.2 Location of sensors. 23
6.3 Installation of sensors . 24
6.4 Timing of installation during construction operations. 25
6.5 Safety in fixed systems . 25
6.6 Environmental conditions.26
6.7 Sample lines.27
6.8 Access for calibration and maintenance . 27
6.9 Commissioning.27
6.10 Operating instructions, plans and records. 28

61779-6 © IEC:1999(E) – 3 –
Clause Page
7 Use of portable and transportable flammable gas detection apparatus. 28

7.1 General. 28

7.2 Initial and periodic check procedures for portable and transportable instrumentation. 29

7.3 Guidance on the use of portable and transportable apparatus. 29

8 Special precautions for fixed and portable apparatus. 30

9 Maintenance, routine procedures and general administrative control . 31

9.1 General.31

9.2 Operational checks.31
9.3 Maintenance.32
9.4 Sensors.34
9.5 Flow systems.34
9.6 Readout devices.34
9.7 Alarms.3 5
9.8 Workshop calibration test. 35
9.9 Routine tests and re-calibration procedures. 36
9.10 General administrative control . 36
10 Training.36
Annex A (informative) Flammability limits (LFL and UFL) of certain flammable gases
and vapours. 37
Annex B (informative) Environmental and application check-list for flammable gas
detectors (typical) . 55
Annex C (informative) Instrument maintenance record for flammable gas detectors
(typical). 57
Annex D (informative) Minimum environmental demands for IEC 61779-4
(group II 100 % LFL) and IEC 61779-5 (group II 100 % (v/v) gas) . 58

– 4 – 61779-6 © IEC:1999(E)
INTERNATIONAL ELECTROTECHNICAL COMMISSION

___________
ELECTRICAL APPARATUS FOR THE DETECTION

AND MEASUREMENT OF FLAMMABLE GASES -–

Part 6: Guide for the selection, installation, use and maintenance

of apparatus for the detection and measurement of flammable gases

FOREWORD
1) The IEC (International Electrotechnical Commission) is a worldwide organization for standardization comprising
all national electrotechnical committees (IEC National Committees). The object of the IEC is to promote
international co-operation on all questions concerning standardization in the electrical and electronic fields. To
this end and in addition to other activities, the IEC publishes International Standards. Their preparation is
entrusted to technical committees; any IEC National Committee interested in the subject dealt with may
participate in this preparatory work. International, governmental and non-governmental organizations liaising
with the IEC also participate in this preparation. The IEC collaborates closely with the International Organization
for Standardization (ISO) in accordance with conditions determined by agreement between the two
organizations.
2) The formal decisions or agreements of the IEC on technical matters express, as nearly as possible, an
international consensus of opinion on the relevant subjects since each technical committee has representation
from all interested National Committees.
3) The documents produced have the form of recommendations for international use and are published in the form
of standards, technical reports or guides and they are accepted by the National Committees in that sense.
4) In order to promote international unification, IEC National Committees undertake to apply IEC International
Standards transparently to the maximum extent possible in their national and regional standards. Any
divergence between the IEC Standard and the corresponding national or regional standard shall be clearly
indicated in the latter.
5) The IEC provides no marking procedure to indicate its approval and cannot be rendered responsible for any
equipment declared to be in conformity with one of its standards.
6) Attention is drawn to the possibility that some of the elements of this International Standard may be the subject
of patent rights. The IEC shall not be held responsible for identifying any or all such patent rights.
International Standard IEC 61779-6 has been prepared by subcommittee 31L: Electrical
apparatus for the detection of flammable gases, of IEC technical committee 31: Electrical
apparatus for explosive atmospheres.
The text of this standard is based on the following documents:
FDIS Report on voting
31L/60/FDIS 31L/61/RVD
Full information on the voting for the approval of this standard can be found in the report on
voting indicated in the above table.
This publication has been drafted in accordance with the ISO/IEC Directives, Part 3.
Annexes A, B, C and D are for information only.
A bilingual version of this standard may be issued at a later date.

61779-6 © IEC:1999(E) – 5 –
INTRODUCTION
Flammable gas detection apparatus may be used whenever there is the possibility of a hazard

to life or property caused by the accumulation of a flammable gas-air mixture. Such apparatus

can provide a means of reducing the hazard by detecting the presence of the flammable
gas and issuing suitable audible or visual warnings. Gas detectors may also be used to initiate
specific precautions (for example plant shutdown, evacuation, operation of fire extinguishing

procedures).
Apparatus may be used to monitor a gas atmosphere below the lower flammable limit in

circumstances where accumulation of gas may result in a concentration of the gas/air mixture

to potentially explosive levels.

– 6 – 61779-6 © IEC:1999(E)
ELECTRICAL APPARATUS FOR THE DETECTION

AND MEASUREMENT OF FLAMMABLE GASES –

Part 6: Guide for the selection, installation, use and maintenance

of apparatus for the detection and measurement of flammable gases

1 General
1.1 Scope
1.1.1 This part of IEC 61779 gives guidance on the selection, installation, use and
maintenance of electrically operated group II apparatus for the detection and measurement of
flammable gases complying with the requirements of IEC 61779-1 to IEC 61779-5. It is a
compilation of practical knowledge to assist the user, and applies to apparatus, instruments
and systems that indicate the presence of a flammable or potentially explosive mixture of gas
or vapour with air by using an electrical signal from a gas sensor to produce a meter reading,
to activate a visual or audible pre-set alarm or other device, or any combination of these.
NOTE – When in classified areas, the apparatus should be so installed and used that it is not capable of itself
igniting a combustible gas-air mixture. It should therefore comply with the requirements of IEC 60079-10.
For the purpose of this standard, flammable gases shall include flammable vapours.
1.1.2 This standard applies only to group II apparatus intended for use in industrial and
commercial safety applications, involving areas classified in accordance with IEC 60079-10.
For the purpose of this standard, apparatus includes
a) fixed apparatus;
b) transportable apparatus; and
c) portable apparatus.
1.1.3 This standard is not intended to cover the following:
a) apparatus intended only for the detection of non-flammable toxic gases;
b) apparatus of laboratory or scientific type intended only for analysis or measurement
purposes;
c) apparatus intended for underground mining applications;
d) apparatus intended for applications in explosives processing and manufacture;
e) apparatus intended only for process control applications;
f) apparatus intended for the detection of a potentially flammable atmosphere resulting from
dust or mist in air.
1.2 Normative references
The following normative documents contain provisions which, through reference in this text,
constitute provisions of this part of IEC 61779. For dated references, subsequent amendments
to, or revisions of, any of these publications do not apply. However, parties to agreements
based on this part of IEC 61779 are encouraged to investigate the possibility of applying the
most recent editions of the normative documents indicated below. For undated references, the
latest edition of the normative document referred to applies. Members of IEC and ISO maintain
registers of currently valid International Standards.

61779-6 © IEC:1999(E) – 7 –
IEC 60050(426):1990, International Electrotechnical Vocabulary (IEV) – Chapter 426: Electrical
apparatus for explosive atmospheres

IEC 60079 (al parts), Electrical apparatus for explosive gas atmospheres

IEC 60079-0:1998, Electrical apparatus for explosive gas atmospheres – Part 0: General
requirements
IEC 60079-10:1995, Electrical apparatus for explosive gas atmospheres – Part 10:
Classification of hazardous areas

IEC 60079-19:1993, Electrical apparatus for explosive gas atmospheres – Part 19: Repair and
overhaul for apparatus used in explosive atmospheres (other than mines or explosives)
IEC 60079-20:1996, Electrical apparatus for explosive gas atmospheres – Part 20: Data for
flammable gases and vapours, relating to the use of electrical apparatus
IEC 61779-1:1998, Electrical apparatus for the detection and measurement of flammable gases
– Part 1: General requirements and test methods
IEC 61779-2:1998, Electrical apparatus for the detection and measurement of flammable gases
– Part 2: Performance requirements for group I apparatus indicating up to 5 % methane in air
IEC 61779-3:1998, Electrical apparatus for the detection and measurement of flammable gases
– Part 3: Performance requirements for group I apparatus indicating a volume fraction up to
100 % methane in air
IEC 61779-4:1998, Electrical apparatus for the detection and measurement of flammable gases
– Part 4: Performance requirements for group II apparatus indicating a volume fraction up to
100 % lower explosive limit
IEC 61779-5:1998, Electrical apparatus for the detection and measurement of flammable gases
– Part 5: Performance requirements for group II apparatus indicating a volume fraction up to
100 % gas
2 Definitions
For the purpose of this part of IEC 61779, the following definitions apply.

2.1
aspirated apparatus
combustible gas detecting apparatus that obtains the gas by drawing it to the gas sensor – for
example by means of a hand-operated or electric pump
2.2
catalytic sensor
sensor, the operation of which depends upon the oxidation of gases on an electrically heated
catalytic element
2.3
clean air
air that is free of flammable gases and interfering or contaminating substances

– 8 – 61779-6 © IEC:1999(E)
2.4
continuous duty apparatus
combustible gas detecting apparatus that is powered for long periods of time, but may have

either continuous or intermittent sensing

2.5
continuous sensing
mode of operation in which power is applied continuously to the sensing element and readings

are taken continuously
2.6
diffusion apparatus
apparatus in which the transfer of gas from the atmosphere to the gas sensing element takes
place by diffusion, i.e. there is no aspirated flow
2.7
electrochemical sensor
sensor, the operation of which depends upon changes of the electrical parameters of
electrodes placed in an electrolyte due to redox reactions of the gas on the surface of the
electrodes
2.8
electromagnetic radiation absorption sensor
sensor, the operation of which depends upon the absorption of electromagnetic radiation by the
gas being detected
2.9
explosion protected apparatus
apparatus incorporating a type of protection covered by the IEC 60079 series of standards
2.10
explosive gas atmosphere
mixture with air, under normal atmospheric conditions, of flammable material in the form of gas
or vapour, in which, after ignition, combustion spreads throughout the unconsumed mixture
NOTE 1 – This definition specifically excludes dusts and fibres in suspension air. Mists are not covered by this
standard.
NOTE 2 – Although a mixture that has a concentration above the upper explosive limit (see 2.1.9) is not an
explosive atmosphere, in certain cases for area classification purposes, it is advisable to consider it as an explosive
gas atmosphere.
NOTE 3 – Normal atmospheric conditions include variations above and below reference levels of 101,3 kPa and
20 °C provided the variations have a negligible effect on the explosive properties of the flammable materials.

2.11
explosive range
range of gas or vapour mixtures with air between the explosive (flammable) limits
2.12
fixed apparatus
apparatus which is intended to have all its parts permanently installed
2.13
flashpoint
lowest liquid temperature at which, under certain standardized conditions, a liquid gives off
vapours in a quantity such as to be capable of forming an ignitable vapour/air mixture

61779-6 © IEC:1999(E) – 9 –
2.14
group II apparatus
electrical apparatus for places with a potentially explosive atmosphere, other than mines

susceptible to firedamp
2.15
infrared absorption sensor
sensor, the operation of which depends upon the absorption of infrared radiation by the gas

being detected
2.16
intermittent sensing
mode of operation in which the power or flow to the sensor is applied intermittently according to
a predetermined cycle and readings taken at the predetermined cycle
2.17
lower flammable limit (LFL)
volume ratio of flammable gas or vapour in air below which an explosive gas atmosphere does
not form, expressed as a percentage (see annex A)
NOTE – This is also known as lower explosive limit (LEL).
2.18
open path infrared sensor
sensor capable of detecting gas at any location along an open path traversed by an infrared
beam
2.19
portable apparatus
spot reading or continuous duty apparatus that has been designed to be readily carried from
place to place and to be used while it is being carried. Portable apparatus is battery operated
and includes, but is not limited to
a) hand-held apparatus, typically less than 1 kg, suitable for single-handed operation;
b) personal monitors, similar in size and mass to the hand-held apparatus, that are
continuously operating (but not necessarily continuously sensing) while they are attached to
the user; and
c) larger apparatus up to 5 kg that can be operated by the user while it is suspended by hand,
by a shoulder strap or by a carrying harness; it may or may not have a hand directed probe
2.20
relative density
density of gas or vapour relative to the density of air at the same pressure and at the same
temperature (air is equal to 1,0)
2.21
release rate
quantity of flammable gas or vapour emitted per unit time from the source of release which
itself could be a liquid surface
2.22
remote sensor
sensor which is not integral with the main body of the apparatus
2.23
sample line
pipeline by means of which the gas being sampled is conveyed to the sensor

– 10 – 61779-6 © IEC:1999(E)
2.24
sampling probe
separate sampling line, that may or may not be supplied with a portable apparatus that is

attached to the apparatus as required.

NOTE – The sampling probe is usually short (for example of the order of 1 m) and rigid (for example it may be

telescopic) but may be connected by a flexible tube to the apparatus.

2.25
semi-conductor sensor
sensor, the operation of which depends upon changes of the electrical conductance of a semi-

conductor due to chemisorption of the gas being detected at its surface

2.26
sensor
assembly in which the sensing element is housed that may contain associated circuit
components
2.27
sensing element
that part of a sensor that reacts in the presence of a flammable gas mixture to produce some
physical or chemical change that can be used to activate a measuring or alarm function, or
both
2.28
single point sensor
sensor capable of detecting gas at a single point location
2.29
source of release
point or location from which a flammable gas, vapour or liquid may be released into the
atmosphere such that an explosive gas atmosphere could be formed. [IEV 426-03-06,
modified]
2.30
spot reading apparatus
apparatus intended to be used for short periods of time as required (typically 5 min or less)
2.31
thermal conductivity sensor
sensor, the operation of which depends upon the change of heat lost by conduction of an
electrically heated element located in the gas to be measured, compared with that of a similar
element located in a reference gas cell
2.32
transportable apparatus
apparatus not intended to be portable, but which can be readily moved from one place to
another
2.33
upper flammable limit (UFL)
volume ratio of flammable gas or vapour in air above which an explosive gas atmosphere does
not form, expressed as a percentage (see annex A)
NOTE – This is also known as upper explosive limit (UEL).
2.34
ventilation
movements of air and replacement with fresh air due to the effects of wind, temperature
gradients or artificial means (for example fans or extractors)

61779-6 © IEC:1999(E) – 11 –
3 Measuring principles
The measuring principles of various sensors are given below as well as typical advantages and

disadvantages of each.
3.1 Catalytic sensor
The principle of operation of the catalytic sensor depends upon the oxidation of flammable gas

at the surface of an electrically heated catalytic element (filament or bead). This oxidation

causes, for example the temperature of the sensing element to change as a function of the

concentration of gas so detected. The resultant change of electrical resistance is determined.

A reasonable concentration of oxygen in the order of 10 % or greater is required for catalytic
sensors to operate.
By their very nature, catalytic sensors will directly detect flammable gases; the other types of
sensors described in 3.2 to 3.6 indirectly infer the presence of flammable gases by the
response of the sensor to other gas properties.
Since oxidation depends upon the presence of oxygen, detection apparatus should use only
this type of sensor for gas concentrations up to the lower flammable limit.
The catalytic sensor may be used in either
a) diffusion mode; or
b) aspirating mode.
3.1.1 Advantages
The sensors detect flammable gases by a process of combustion and are suitable for the
detection of a wide range of flammable gases but with possible variations in sensitivity.
3.1.2 Disadvantages
The main disadvantages with catalytic sensors are the following:
a) Range limitation
The catalytic sensor depends upon catalytic oxidation for its principle of operation and it
functions only when sufficient oxygen is present. This type of sensor should be used for the
detection of gas/air mixtures up to the lower flammable limit.
WARNING – ABOVE THE LOWER FLAMMABLE LIMIT, A CATALYTIC SENSOR
MAY RESPOND AMBIGUOUSLY AND IN SOME CASES THE INSTRUMENT
MAY ERRONEOUSLY INDICATE THAT THE FLAMMABLE GAS AND AIR MIXTURE
IS BELOW THE LFL.
b) Interfering gases and vapours
If the atmosphere to be monitored contains gas(es) that dilute(s) or displace(s) air, for
example nitrogen or carbon dioxide, the catalytic sensor may give a low or even zero
response. Similar problems may occur in steam-laden atmospheres, quite often owing to
saturation of the sintered flame arrestor due to condensation. High concentrations of inert
gas (for example argon or helium) may also change the thermal balance of the sensor
resulting in apparent reading of combustible gas.
c) Inhibition (catalyst poisoning)
Catalytic sensors are susceptible to permanent or temporary inhibition of the catalyst by
certain airborne contaminants such that the sensor may eventually produce low, or zero
response to the presence of gas.
NOTE – For this reason, it is therefore important that all catalytic gas detection apparatus is regularly tested in
accordance with 9.2.
– 12 – 61779-6 © IEC:1999(E)
This inhibition may be permanent or temporary according to the nature of the contaminant.

Permanent inhibition, usually known as "catalyst poisoning", may result from exposure to

such substances as silicones, tetraethyl lead, sulphur compounds and phosphate esters. In

some cases substances such as halogenated hydrocarbons may cause temporary

inhibition.
While many catalytic sensors are highly resistant to such contaminants and require no

additional protection in this regard, those that are not may be protected, in some cases, by

the use of activated carbon or other types of filter. However, carbon filters should be used

with great care because, while they may offer excellent protection from contaminants, they

may also prevent the detection of most hydrocarbons other than methane and may cause
considerably increased response times.

Their performance may also be affected by the level of humidity in the atmosphere.
An alternative technique sometimes used to reduce the effects of inhibition is the electrical
operation of the sensor in intermittent mode. However, there are circumstances in which
such sensors may give a false response, for example when they are suddenly exposed to a
high concentration of gas during the "power-off" part of the switching cycle. Particular
caution is necessary when using such portable apparatus in leak-detection mode or in
similar operations.
The manufacturer’s guidance should be sought where the presence of contaminants is
suspected, or where temporary or permanent inhibition is experienced.
3.2 Thermal conductivity sensor
The principle of operation of the thermal conductivity sensor depends upon the heat loss by
conduction of an electrically heated resistance element (i.e., filament, bead or thin film resistor)
located in a gas sample stream of fixed velocity or in a diffusion chamber. The resulting
change of electrical resistance is then determined.
This type of sensor is best suited for detecting individual gases of high or low conductivity
relative to air.
Thermal conductivity sensors are suitable for monitoring those gases whose thermal
conductivity differs substantially from that of air (where air is the reference environment) but
only at relatively high concentrations, usually above the lower flammable limit. Apparatus
incorporating thermal conductivity sensors should not be used for measuring gas
concentrations below the LFL except in the case of gases such as hydrogen for which such
sensors are especially sensitive.
Errors may result if
a) the apparatus is used to attempt to detect gases for which it is not calibrated;

b) a flow-sensitive type of thermal conductivity sensor is used and the gas sample flow is not
stable;
c) the gas sample is not conditioned to remove water vapour or other interfering vapours or
gases, some of which may even result in negative response;
d) there are variations in ambient temperature without compensatory ambient temperature
control on the complete sensor;
e) additional gases of different thermal conductivities compensate the resulting conductivity so
that the signal may even be zero.
3.2.1 Advantages
The sensor is capable of measuring high concentration and is independent of the oxygen level
in the gas-flow.
61779-6 © IEC:1999(E) – 13 –
3.2.2 Disadvantages
Disadvantages with the use of thermal conductivity sensors include the following:

a) these sensors are not selective for individual gases;

b) the sensitivity is often limited and the detection limit may exceed the LFL unless the thermal

conductivity of the gas is sufficiently different from that of air;

c) the thermal conductivities of flammable gases differ widely. The lighter gases, for example

methane and hydrogen, are more conductive than air, whereas the heavier gases are less

conductive. The response to a gas mixture is therefore indeterminate unless the

proportions of the constituent gases in the mixture are known. In the worst case, a mixture

of high and low conductivity gases could produce no instrument response;
d) this type of sensor is essentially dependent on thermal conduction and convection from a
heated filament and is therefore likely to give misleading results if the recommended
conditions of gas flow are not maintained as per the manufacturer's instructions;
e) some instruments specifically rely on convection and will be sensitive to orientation.
3.3 Electromagnetic radiation absorption sensor – usually infrared
The principle of operation of this type of sensor depends upon the absorption of energy of a
beam of electromagnetic radiation by the gas being detected. Most existing instruments
operate within the IR spectrum.
Detection apparatus with electromagnetic sensors may take various forms but may be
categorized as
a) specifically adapted analysers with sampling systems;
b) single-point, self-contained electromagnetic detection apparatus suitable for installation in
potentially explosive atmospheres;
c) light "pipes" – for example a fibre optic cable which directs an electromagnetic light source
from a control unit to a sensor cell at a remote location.
In cases a), b) and c) the absorption of electromagnetic radiation by the gas is detected by
spectrometric means and produces an electrical signal to provide indications of gas
concentrations and alarms.
Electromagnetic sensors may be used for the detection of most flammable gas(es) in any
specified range of concentrations up to 100 % (v/v) with the exception of hydrogen.
NOTE – Open-path, electromagnetic radiation detection apparatus differs from the other types mentioned in this
standard in that it does not measure the concentration of gas at a particular point location, but rather measures the
path integral of gas concentration along an investigative beam. It is therefore capable of detecting the presence of

gas over a wider area than other types. However, it is inherently not capable of distinguishing between a high
concentration of gas occupying a short section of the open path, and a low concentration of gas occupying a longer
section of the path, and therefore does not comply with IEC 61779-1, IEC 61779-4 and IEC 61779-5.
3.3.1 Advantages
The sensor measures the concentration of the specific gas for which it is calibrated
independent of oxygen concentration. By appropriate selection of wavelength, specific gases
can be detected. Most flammable gases of the hydrocarbon family absorb radiation in the
infrared part of the electromagnetic spectrum. By selection of the optical path length,
concentrations from 0 % to 100 % v/v can be measured. Response time is limited by the time
taken for gas to be introduced into the optical path which can be fast. However, in practice,
weather protection housings, gas filters and hydrophobic barriers, where fitted, will limit the
response times attainable.
– 14 – 61779-6 © IEC:1999(E)
Certain types of sensors using advanced signal processing have self-diagnosing and self-

calibrating capabilities, thereby reducing the need for human intervention in the absence of

malfunction alarms. Other advantages include

a) high stability capability;
b) no ambiguity at concentrations above the LFL;

c) immunity to poisoning effects;

d) reduced maintenance capability due to modern technologies, for example automatic
calibration self-check, etc.;
e) availability of warnings to detect failure of the IR source or excessive contamination of the

optical system.
3.3.2 Disadvantages
Infrared sensors are calibrated to detect a particular gas or, in some cases, a limited range of
gases. Other gases will not be detected if their infrared absorption band is outside that of the
calibration bandwidth. Apparatus incorporating such sensors should therefore be used only for
the detection of gas mixtures for which they have been calibrated. When filters are used to
keep optical components clean, they may be blocked under excessively dirty conditions.
Serious errors are likely to occur with some detectors due to the presence of water vapour or
interfering gases or both.
This sensor is not suitable for measuring hydrogen.
Some types of infrared sensors, particularly open-path designs, are sensitive to misalignment
caused by shock and vibration.
3.4 Semi-conductor sensor
The principle of operation of the semi-conductor sensor depends upon changes of electrical
conductance that occur by chemisorption when the heated semi-conductor sensing element is
exposed to gas. Gas concentrations are deduced from the measured change of conductivity.
Semi-conductor sensors may be used in either diffusion mode or in a sampling system.
This type of sensor is normally used for the detection of a specified gas in a nominated range
of concentrations.
3.4.1 Advantages
Semi-conductive sensors are sensitive to a wide range of gases and produce large signal
changes at low gas concentrations.
3.4.2 Disadvantages
Semi-conductor sensors for flammable gases are generally non-specific, vulnerable to both
humidity changes and interfering gases and may exhibit drift both of zero and span. Some
gases, for example NO , produce negative signals.
NOTE – Normally, the manufacturer will give guidance on substances that will inhibit operation of the sensor or
produce false indications.
3.5 Electrochemical sensors
The principle of operation of the electrochemical sensor depends upon the change of the
electrical parameters of electrodes when a specific gas is present. The change in the electrical

61779-6 © IEC:1999(E) – 15 –
parameters occurs due to a chemical redox reaction on the surface of electrodes placed in an

electrolyte. Electrodes and electrolyte are usually confined in semi-permeable membranes.

3.5.1 Advantages
Electrochemical cells are compact, require little power, and have a high sensitivity to certain

gases.
3.5.2 Disadvantages
Disadvantages include the following:

a) electrochemical cells are incapable of detecting the most simple hydrocarbons (for example
methane, ethane, propane, etc.);
b) the electrochemical cell can gradually be consumed;
c) electrochemical cells may detect other interfering gases;
d) periodic adjustment is necessary to correct for drifts in zero and span;
e) response and recovery times are comparatively slow (typically >30 s);
f) low temperature operations (<–15 °C) and high temperature operations (>50 °C) may be
precluded by electrolyte properties.
3.6 Flame ionization detectors (FID)
The operating principle of the flame ionization detector depends upon the ionization of the
organic compounds. The ion cloud so formed migrates under the potential gradient between
the electrodes in the ionization chamber, resulting in an electric current. This current is
proportional to the concentration of gas/vapour in the gas stream.
This type of sensor is used for the detection of gas/air mixtures up to the lower flammable limit.
3.6.1 Advantages
This type of sensor is used where high sensitivity, wide measuring range, small measuring
uncertainty, poison resistance and fast response time are of main interest.
3 –6
The sensor is suitable for the measuring range from ml/m (10 ) up to the lower flammable
limit.
3.6.2 Disadvantages
The principle of operation is not selective because generally all organic compounded carbons
will cause a signal. If different gases or vapours are expected at the place of operation, the
sensor should be calibrated for that gas or vapour to which the apparatus is least sensitive.
These sensors are not suitable for inorganic gases such as H .
The pressure of the gas sample, air and flammable gas should be kept constant, but it should
be noted that the flame arrestors used in the sampling line can be soiled and difficulty may be
experienced in keeping the sample flow constant.
4 Selection of apparatus
In this clause and clauses 5 and 6, there is a need for documentation of plant information, site
information and decisions to be made. Annex B shows a typical check-list for many of these
decisions.
– 16 – 61779-6 © IEC:1999(E)
4.1 General
In selecting flammable gas detection apparatus, account should be taken of any features of the

apparatus that may necessitate particular caution in its use and the interpretation of the output.

Each of the various types of sensor has inherent limitations, as described in clause 3.

NOTE – This standard is not intended to discourage the use of apparatus using detection principles other than

those described in 3.2 to 3.6 or to inhibit the development of new detection principles. However, it is of primary

importance that the capabilities of the detection principle should be such that the performance of the apparatus is
adequate for the intended application. Assessment of the apparatus against the performance requirements

specified in IEC 61779-1, IEC 61779-4 and IEC 61779-5 may provide a basis for judgement in appropriate circum-

stances (see annex D).
4.2 Selection criteria
4.2.1 General criteria
4.2.1.1 The following criteria are among those which should be considered when selecting
appropriate gas detection apparatus:
a) the gas(es) which the apparatus is required to detect, and the range of concentrations of
each which may be encountered;
b) the intended application of the apparatus, for example area monitoring, personnel safety,
leak detection or other purposes;
c) whether the apparatus is required to be fixed, transportable or portable;
d) the classification of the intended zone(s) of use in accordance with national regulations;
e) the environmental conditions that will be met in the area(s) of use;
f) any features of particular apparatus that require caution to be taken in the use or
interpretation of its output;
g) time dependency and interaction with safety devices;
h) calibration requirements including zero checks.
4.2.1.2 A gas detection and measurement system should be so designed that the delay time
of the whole system is less than the maximum delay time allowable for the provided
application. The following factors should be taken into account:
a) delay time of the sampling system;
b) response time of the sensor;
c) delay time of data transmission lines;
d) delay time of alarm devices and switching circuits;

e) time taken for executive action devices, for example shut-down valves, to operate;
f) potential release rate of flammable gas.
4.2.2 Gases that the apparatus is required to detect
The gas detection apparatus is required to be sensitive to each of the gases that it is required
to detect and also to be suitable for the range of gas concentrations that will be encountered.
Reference should be made to the manufacturer's information to determine the suitability of
particular detectors.
WARNING  THERMAL CONDUCTIVITY, INFRARED, ELECTROCHEMICAL,
AND SEMICONDUCTOR SENSORS MAY BE SENSITIVE TO CERTAIN NON-FLAMMABLE
GASES, IN ADDITION TO THE RANGE OF FLAMMABLE GASES WHICH THEY ARE
INTENDED TO DETECT. FOR EXAMPLE SEMICONDUCTOR SENSORS MAY BE SENSITIVE
TO WATER VAPOUR OR TO COMBUSTION PRODUCTS IN ADDITION TO FLAMMABLE
GASES. ADVICE SHOULD ALWAYS BE SOUGHT FROM THE MANUFACTURER

61779-6 © IEC:1999(E) – 17 –
CONCERNING THE EFFECT OF INTERFERING GASES ON PARTICULAR SENSORS.

THESE CAN ALSO BE SENSITIVE TO NON-FLAMMABLE GASES AND INSENSITIVE TO

FLAMMABLE GASES.
It is not normally possible to determine the concentrations of individual flammable gases in a

mixture of flammable gases using the type of apparatus covered by this standard. In general,

sensors of the type described in 3.1 to
...