EN IEC 62990-2:2021

SLOVENSKI STANDARD
01-december-2021
Zrak na delovnem mestu - 2. del: Plinski detektorji - Izbira, vgraditev, uporaba in
vzdrževanje detektorjev strupenih plinov in hlapov (IEC 62990-2:2021)
Workplace atmospheres - Part 2: Gas detectors - Selection, installation, use and
maintenance of detectors for toxic gases and vapours (IEC 62990-2:2021)
Arbeitsplatzatmosphäre - Teil 2: Gasmessgeräte - Auswahl, Installation, Einsatz und
Wartung von Gasmessgeräten für toxische Gase und Dämpfe (IEC 62990-2:2021)
Atmosphères des lieux de travail - Partie 2 : Détecteurs de gaz - Sélection, installation,
utilisation et maintenance des détecteurs de gaz et de vapeurs toxiques (IEC 62990-
2:2021)
Ta slovenski standard je istoveten z: EN IEC 62990-2:2021
ICS:
13.040.30 Kakovost zraka na delovnem Workplace atmospheres
mestu
13.320 Alarmni in opozorilni sistemi Alarm and warning systems
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.

EUROPEAN STANDARD EN IEC 62990-2

NORME EUROPÉENNE
EUROPÄISCHE NORM
July 2021
ICS 29.260.20
English Version
Workplace atmospheres - Part 2: Gas detectors - Selection,
installation, use and maintenance of detectors for toxic gases
and vapours
(IEC 62990-2:2021)
Atmosphères des lieux de travail - Partie 2 : Détecteurs de Arbeitsplatzatmosphäre - Teil 2: Gasmessgeräte - Auswahl,
gaz - Sélection, installation, utilisation et maintenance des Installation, Einsatz und Wartung von Gasmessgeräten für
détecteurs de gaz et de vapeurs toxiques toxische Gase und Dämpfe
(IEC 62990-2:2021) (IEC 62990-2:2021)
This European Standard was approved by CENELEC on 2021-07-09. CENELEC members are bound to comply with the CEN/CENELEC
Internal Regulations which stipulate the conditions for giving this European Standard the status of a national standard without any alteration.
Up-to-date lists and bibliographical references concerning such national standards may be obtained on application to the CEN-CENELEC
Management Centre or to any CENELEC member.
This European Standard exists in three official versions (English, French, German). A version in any other language made by translation
under the responsibility of a CENELEC member into its own language and notified to the CEN-CENELEC Management Centre has the
same status as the official versions.
CENELEC members are the national electrotechnical committees of Austria, Belgium, Bulgaria, Croatia, Cyprus, the Czech Republic,
Denmark, Estonia, Finland, France, Germany, Greece, Hungary, Iceland, Ireland, Italy, Latvia, Lithuania, Luxembourg, Malta, the
Netherlands, Norway, Poland, Portugal, Republic of North Macedonia, Romania, Serbia, Slovakia, Slovenia, Spain, Sweden, Switzerland,
Turkey and the United Kingdom.

European Committee for Electrotechnical Standardization
Comité Européen de Normalisation Electrotechnique
Europäisches Komitee für Elektrotechnische Normung
CEN-CENELEC Management Centre: Rue de la Science 23, B-1040 Brussels
© 2021 CENELEC All rights of exploitation in any form and by any means reserved worldwide for CENELEC Members.
Ref. No. EN IEC 62990-2:2021 E

European foreword
The text of document 31/1566/FDIS, future edition 1 of IEC 62990-2, prepared by IEC/TC 31
“Equipment for explosive atmospheres” was submitted to the IEC-CENELEC parallel vote and
approved by CENELEC as EN IEC 62990-2:2021.
The following dates are fixed:
• latest date by which the document has to be implemented at national (dop) 2022–04–09
level by publication of an identical national standard or by endorsement
• latest date by which the national standards conflicting with the (dow) 2024–07–09
document have to be withdrawn
Attention is drawn to the possibility that some of the elements of this document may be the subject of
patent rights. CENELEC shall not be held responsible for identifying any or all such patent rights.
Any feedback and questions on this document should be directed to the users’ national committee. A
complete listing of these bodies can be found on the CENELEC website.
Endorsement notice
The text of the International Standard IEC 62990-2:2021 was approved by CENELEC as a European
Standard without any modification.
In the official version, for Bibliography, the following notes have to be added for the standards
indicated:
IEC 60079-10-1 NOTE Harmonized as EN IEC 60079-10-1
IEC 60079-0 NOTE Harmonized as EN IEC 60079-0
Annex ZA
(normative)
Normative references to international publications
with their corresponding European publications
The following documents are referred to in the text in such a way that some or all of their content
constitutes requirements of this document. For dated references, only the edition cited applies. For
undated references, the latest edition of the referenced document (including any amendments)
applies.
NOTE 1 Where an International Publication has been modified by common modifications, indicated by (mod), the
relevant EN/HD applies.
NOTE 2 Up-to-date information on the latest versions of the European Standards listed in this annex is available
here: www.cenelec.eu.
Publication Year Title EN/HD Year
IEC 60079-29-2 - Explosive atmospheres - Part 29–2: Gas EN 60079-29-2 -
detectors - Selection, installation, use and
maintenance of detectors for flammable
gases and oxygen
IEC 62990-1 - Workplace atmospheres - Part 1: Gas - -
detectors - Performance requirements of
detectors for toxic gases
IEC 62990-2
Edition 1.0 2021-06
INTERNATIONAL
STANDARD
colour
inside
Workplace atmospheres –
Part 2: Gas detectors – Selection, installation, use and maintenance of detectors

for toxic gases and vapours
INTERNATIONAL
ELECTROTECHNICAL
COMMISSION
ICS 29.260.20 ISBN 978-2-8322-9746-9

– 2 – IEC 62990-2:2021 © IEC 2021
CONTENTS
FOREWORD . 4
INTRODUCTION . 6
1 Scope . 7
2 Normative references . 7
3 Terms and definitions . 7
4 Properties and detection of toxic gases and vapours . 13
4.1 Properties and detection . 13
4.2 The difference between detecting gases and vapours . 14
4.3 Effects of water vapour on detection . 17
4.4 Effects of temperature and pressure on detection . 17
4.5 Effects of corrosion on detection . 17
4.6 Detection by oxygen deficiency measurement . 17
5 Measurement tasks . 18
5.1 General . 18
5.2 Exposure measurement (health monitoring) . 18
5.3 General gas detection (safety monitoring) . 19
6 Selection of equipment . 20
6.1 General . 20
6.2 Performance and electrical tests . 21
6.3 Indication range, measuring range and uncertainty of measurement . 21
6.4 Selectivity requirements . 22
6.5 The influence of environmental conditions . 23
6.6 The influence of electromagnetic interference . 23
6.7 Time of response and time of recovery . 24
6.8 Time to alarm . 25
6.9 Data logging . 25
6.10 Instruction manual . 26
7 Design and installation of fixed toxic gas detection equipment . 26
7.1 General . 26
7.2 Basic considerations for the installation of fixed systems . 27
7.3 Location of detection points . 28
7.4 Access for calibration and maintenance . 33
7.5 Additional considerations for sample lines . 33
7.6 Summary of considerations for the location of sensors or sampling points . 34
7.7 Installation of sensors . 35
7.8 Integrity and safety of fixed systems . 35
7.9 Commissioning . 36
7.10 Operating instructions, plans and records . 37
8 Operation of toxic gas detection equipment . 38
8.1 Alarm setting. 38
8.2 Operation of portable equipment . 39
8.3 Operation of transportable and fixed equipment . 43
8.4 Sample lines and sampling probes . 45
8.5 Accessories . 45
9 Maintenance and calibration . 46
9.1 General . 46

IEC 62990-2:2021 © IEC 2021 – 3 –
9.2 Sensor . 46
9.3 Flow systems of aspirated equipment. 46
9.4 Readout devices . 47
9.5 Alarms . 47
9.6 Maintenance . 47
9.7 Calibration . 48
9.8 Operation test . 49
9.9 Records . 50
10 Training . 50
10.1 General . 50
10.2 Operator training . 50
10.3 Maintenance and calibration training . 51
Annex A (informative) Commonly used measurement principles . 52
A.1 General . 52
A.2 Chemiluminescence . 52
A.3 Colorimetry . 53
A.4 Electrochemical . 54
A.5 Flame-ionization . 55
A.6 Gas chromatography . 55
A.7 Infrared photometry . 56
A.8 Ion mobility spectrometry . 57
A.9 Mass spectrometry . 58
A.10 Photo-ionization . 59
A.11 Semiconductor . 60
A.12 Ultra-violet/visible photometry . 61
Bibliography . 62

Figure 1 – Relationship between indication range and measuring range (See 6.3.1) . 11
Figure 2 – Example of zero uncertainty . 11
Figure 3 – Example of warm-up time in clean air . 12
Figure 4 – Relationship between indication range and measuring range . 22
Figure 5 – Gas response curves for test gas volume fractions of 40 ppm and 100 ppm . 24
Figure 6 – Time to alarm at 25 ppm set point for test gas volume fractions of 40 ppm
and 100 ppm . 25

Table A.1 – Chemiluminescence . 52
Table A.2 – Colorimetry . 53
Table A.3 – Electrochemical . 54
Table A.4 – Flame-ionization . 55
Table A.5 – Infrared photometry . 56
Table A.6 – Ion mobility spectrometry . 57
Table A.7 – Mass spectrometry . 58
Table A.8 – Photo-ionization (PID) . 59
Table A.9 – Semiconductor . 60
Table A.10 – Ultra-violet/visible photometry . 61

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INTERNATIONAL ELECTROTECHNICAL COMMISSION
____________
WORKPLACE ATMOSPHERES –
Part 2: Gas detectors –
Selection, installation, use and maintenance
of detectors for toxic gases and vapours

FOREWORD
1) The International Electrotechnical Commission (IEC) is a worldwide organization for standardization comprising
all national electrotechnical committees (IEC National Committees). The object of 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, IEC publishes International Standards, Technical Specifications, Technical Reports,
Publicly Available Specifications (PAS) and Guides (hereafter referred to as “IEC Publication(s)”). 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. 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 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 IEC National Committees.
3) IEC Publications have the form of recommendations for international use and are accepted by IEC National
Committees in that sense. While all reasonable efforts are made to ensure that the technical content of IEC
Publications is accurate, IEC cannot be held responsible for the way in which they are used or for any
misinterpretation by any end user.
4) In order to promote international uniformity, IEC National Committees undertake to apply IEC Publications
transparently to the maximum extent possible in their national and regional publications. Any divergence between
any IEC Publication and the corresponding national or regional publication shall be clearly indicated in the latter.
5) IEC itself does not provide any attestation of conformity. Independent certification bodies provide conformity
assessment services and, in some areas, access to IEC marks of conformity. IEC is not responsible for any
services carried out by independent certification bodies.
6) All users should ensure that they have the latest edition of this publication.
7) No liability shall attach to IEC or its directors, employees, servants or agents including individual experts and
members of its technical committees and IEC National Committees for any personal injury, property damage or
other damage of any nature whatsoever, whether direct or indirect, or for costs (including legal fees) and
expenses arising out of the publication, use of, or reliance upon, this IEC Publication or any other IEC
Publications.
8) Attention is drawn to the Normative references cited in this publication. Use of the referenced publications is
indispensable for the correct application of this publication.
9) Attention is drawn to the possibility that some of the elements of this IEC Publication may be the subject of patent
rights. IEC shall not be held responsible for identifying any or all such patent rights.
International Standard IEC 62990-2 has been prepared IEC technical committee 31: Equipment
for explosive atmospheres and ISO technical committee 146: Air quality, sub-committee 2:
Workplace atmospheres.
It is published as a double logo standard.
The text of this International Standard is based on the following documents:
FDIS Report on voting
31/1566/FDIS 31/1568/RVD
Full information on the voting for the approval of this International Standard can be found in the
report on voting indicated in the above table.
This document has been drafted in accordance with the ISO/IEC Directives, Part 2.

IEC 62990-2:2021 © IEC 2021 – 5 –
A list of all parts in the IEC 62990, published under the general title Workplace atmospheres,
can be found on the IEC website.
The committee has decided that the contents of this document will remain unchanged until the
stability date indicated on the IEC website under "http://webstore.iec.ch" in the data related to
the specific document. At this date, the document will be
• reconfirmed,
• withdrawn,
• replaced by a revised edition, or
• amended.
IMPORTANT – The 'colour inside' logo on the cover page of this publication indicates that it
contains colours which are considered to be useful for the correct understanding of its
contents. Users should therefore print this document using a colour printer.

– 6 – IEC 62990-2:2021 © IEC 2021
INTRODUCTION
Toxic gas detection equipment can be used whenever there is the possibility of a hazard to life
or adverse health effects caused by the accumulation of a toxic gas or vapour. Such equipment
can provide a means of reducing the exposure to the hazard by detecting the presence of a
toxic gas or vapour and issuing suitable audible or visual warnings. Gas detectors can also be
used to initiate precautionary steps (for example, plant shutdown and evacuation).
Performance requirements for gas detection equipment for workplace atmospheres are set out
in IEC 62990 series standards.
However performance capability alone cannot ensure that the use of such equipment will
properly safeguard life and health where toxic gases and vapours might be present. The level
of safety obtained depends heavily upon correct selection, installation, calibration and periodic
maintenance of the equipment, combined with knowledge of the limitations of the detection
technique required. This cannot be achieved without responsible informed management.
This document has been specifically written to cover all the functions necessary from selection
to ongoing maintenance for a successful gas detection operation.

IEC 62990-2:2021 © IEC 2021 – 7 –
WORKPLACE ATMOSPHERES –
Part 2: Gas detectors –
Selection, installation, use and maintenance
of detectors for toxic gases and vapours

1 Scope
This document gives guidance on the selection, installation, use and maintenance of electrical
equipment used for the measurement of toxic gases and vapours in workplace atmospheres.
The primary purpose of such equipment is to ensure safety of personnel and property by
providing an indication of the concentration of a toxic gas or vapour and warning of its presence.
This document is applicable to equipment whose purpose is to provide an indication, alarm or
other output function to give a warning of the presence of a toxic gas or vapour in the
atmosphere and in some cases to initiate automatic or manual protective actions. It is applicable
to equipment in which the sensor automatically generates an electrical signal when gas is
present.
For the purposes of this document, equipment includes:
a) fixed equipment;
b) transportable equipment, and
c) portable equipment.
This document is intended to cover equipment defined within IEC 62990-1, but can provide
useful information for equipment not covered by that document.
2 Normative references
The following documents are referred to in the text in such a way that some or all of their content
constitutes requirements of this document. For dated references, only the edition cited applies.
For undated references, the latest edition of the referenced document (including any
amendments) applies.
IEC 60079-29-2, Explosive atmospheres – Part 29-2: Gas detectors – Selection, installation,
use and maintenance of detectors for flammable gases and oxygen
IEC 62990-1, Workplace atmospheres – Part 1: Gas detectors – Performance requirements of
detectors for toxic gases
3 Terms and definitions
For the purposes of this document, the terms and definitions given in IEC 62990-1 and the
following apply.
NOTE 1 Certain definitions within IEC 62990-1 are repeated below for the convenience of the reader.

– 8 – IEC 62990-2:2021 © IEC 2021
ISO and IEC maintain terminological databases for use in standardization at the following
addresses:
• IEC Electropedia: available at http://www.electropedia.org/
• ISO Online browsing platform: available at http://www.iso.org/obp
NOTE 2 Additional definitions applicable to explosive atmospheres can be found in Chapter 426 of the International
Electrotechnical Vocabulary (IEC 60050-426).
3.1
toxic gas
gas or vapour that can be harmful to human health and/or the performance of persons due to
its physical or physico-chemical properties
Note 1 to entry: For the purpose of this document, the term “toxic gas” includes “toxic vapours”.
3.2
interfering gas
any gas other than the gas to be detected, including water vapour, which affects the indication
3.3
clean air
air that is free of gases or vapours to which the sensor is sensitive or which influence the
performance of the sensor
3.4
zero gas
gas recommended by the manufacturer, which is free of toxic gases and interfering and
contaminating substances, the purpose of which is calibration or adjustment of the equipment
zero
3.5
volume fraction
quotient of the volume of a specified component and the sum of the volumes of all components
of a gas mixture before mixing, all volumes referring to the pressure and the temperature of the
gas mixture
Note 1 to entry: The volume fraction and volume concentration take the same value if, at the same state conditions,
the sum of the component volumes before mixing and the volume of the mixture are equal. However, because the
mixing of two or more gases at the same state conditions is usually accompanied by a slight contraction or, less
frequently, a slight expansion, this is not generally the case.
3.6
occupational exposure limit value
OELV
limit of the time-weighted average of the concentration of a chemical agent in the air within the
breathing zone of a worker in relation to a specified reference period
Note 1 to entry: The term “limit value” is often used as a synonym for “occupational exposure limit value”, but the
term “occupational exposure limit value” is preferred because there is more than one limit value (e.g., biological limit
value and occupational exposure limit value).
Note 2 to entry: Occupational exposure limit values (OELVs) are often set for reference periods of 8 h, but can also
be set for shorter periods or concentration excursions.
[SOURCE: ISO 18158:2016, 2.1.5.4, modified (Note 2 to entry is shortened)]
3.7
exposure (by inhalation)
situation in which a chemical agent is present in air that is inhaled by a person

IEC 62990-2:2021 © IEC 2021 – 9 –
3.8
time weighted average concentration
TWA concentration
concentration of gas in air averaged over a reference period
3.9
fixed equipment
equipment fastened to a support, or otherwise secured in a specific location, when energized
3.10
transportable equipment
equipment not intended to be carried by a person during operation, nor intended for fixed
installation
3.11
portable equipment
equipment intended to be carried by a person during its operation
Note 1 to entry: Portable equipment is battery powered and includes, but is not limited to;
a) hand-held equipment, typically less than 1 kg, which requires use of only one hand to operate,
b) personal monitors, similar in size and mass to the hand-held equipment, that are continuously operating while
they are attached to the user, and,
c) larger equipment that can be operated by the user while it is carried either by hand, by a shoulder strap or
carrying harness and which might or might not have a hand directed probe.
3.12
aspirated equipment
equipment that samples the atmosphere by drawing it to the sensor
Note 1 to entry: A hand operated or electric pump is often used to draw gas to the sensor.
3.13
alarm-only equipment
equipment with an alarm but not having an indication of measured value
3.14
sensing element
part of the sensor which is sensitive to the gas or vapour to be measured
3.15
sensor
assembly in which the sensing element is housed and that can also contain associated circuit
components
3.16
remote sensor
sensor which is installed separately, but is connected to a gas detection control unit, gas
detection transmitter, or transportable or portable equipment
3.17
gas detection transmitter
fixed gas detection equipment that provides a conditioned electronic signal or output indication
to a generally accepted industry standard (such as 4 to 20 mA), intended to be utilized with
separate gas detection control units or signal processing data acquisition, central monitoring
and similar systems, which typically process information from various locations and sources
including, but not limited to gas detection equipment

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3.18
separate gas detection control unit
equipment intended to provide display indication, alarm functions, output contacts or alarm
signal outputs or any combination when operated with gas detection transmitters(s)
3.19
alarm set point
setting of the equipment at which the measured concentration will cause the equipment to
initiate an indication, alarm or other output function
3.20
fault signal
audible, visible or other type of output, different from the alarm signal, permitting, directly or
indirectly, a warning or indication that the equipment is not working satisfactorily
3.21
sample line
means by which the gas being sampled is conveyed to the sensor
Note 1 to entry: Accessories such as filter or water trap are often included in the sample line.
3.22
sampling probe
separate accessory sample line which is optionally attached to the equipment
Note 1 to entry: It is usually short (for example in the order of 1 m) and rigid, although it can be telescopic. In some
cases it is connected by a flexible tube to the equipment.
3.23
field calibration kit
means of presenting test gas to the equipment for the purpose of calibrating, adjusting or
verifying the operation of the equipment
Note 1 to entry: The field calibration kit can be used for verifying the operation of the alarms if the concentration of
the test gas is above the alarm set-point.
Note 2 to entry: A mask for calibration and test is an example of a field calibration kit.
3.24
zero indication
indication given by an equipment when exposed to zero gas in normal operating conditions
3.25
indication range
range of measured values of gas concentration over which the equipment is capable of
indicating (see Figure 1)
3.26
lower limit of indication
smallest measured value within the indication range (see Figure 1)
3.27
upper limit of indication
largest measured value within the indication range (see Figure 1)
3.28
measuring range
range of measured values of gas concentration over which the accuracy of the equipment lies
within specified limits (see Figure 1)

IEC 62990-2:2021 © IEC 2021 – 11 –
3.29
lower limit of measurement
smallest measured value within the measuring range (see Figure 1)
3.30
upper limit of measurement
largest measured value within the measuring range (see Figure 1)

Figure 1 – Relationship between indication range and measuring range
3.31
expanded uncertainty
U
quantity defining an interval about a result of a measurement, expected to encompass a large
fraction of the distribution of values that could reasonably be attributed to the measurand
[SOURCE: ISO 18158:2016, 2.4.2.5]
3.32
zero uncertainty
quantity defining an interval about zero expected to encompass a large fraction of the
distribution of values that could reasonably be attributed to the measurement in clean air
Note 1 to entry: In Figure 2 the mean value of the measured values in clean air is not equal to zero to illustrate that
there can be an offset due to drift. The mean value can be above or below zero.

Figure 2 – Example of zero uncertainty

– 12 – IEC 62990-2:2021 © IEC 2021
3.33
selectivity
degree of independence from interfering gases
3.34
averaging time
period of time for which the measuring procedure yields an averaged value
3.35
drift
variation in the equipment indication over time at any fixed gas volume fraction (including clean
air) under constant ambient conditions
3.36
time of recovery
t(x)
time interval, with the equipment in a warmed-up condition, between the time when an
instantaneous change from standard test gas to clean air is produced at the equipment inlet
and the time when the indication reaches a stated percentage (x) of the initial indication
Note 1 to entry: For alarm only equipment the stated indication can be represented by the de-activation of the alarm
set at a stated value.
3.37
time of response
t(x)
time interval, with the equipment in a warmed-up condition, between the time when an
instantaneous change between clean air and the standard test gas is produced at the equipment
inlet, and the time when the indication reaches a stated percentage (x) of the final indication
Note 1 to entry: For alarm only equipment the stated indication can be represented by the activation of the alarm
set at a stated value
3.38
warm-up time
time interval, with the equipment in a stated atmosphere, between the time when the equipment
is switched on and the time when the indication reaches and remains within the stated
tolerances
Note 1 to entry: See Figure 3.

Figure 3 – Example of warm-up time in clean air

IEC 62990-2:2021 © IEC 2021 – 13 –
3.39
calibration
procedure which establishes the relationship between a measured value and the concentration
of a test gas
Note 1 to entry: If the deviation at calibration is too high, usually an adjustment will be carried out subsequently.
3.40
adjustment
procedure carried out to minimize the deviation of the measured value from the test gas
concentration
Note 1 to entry: If the equipment is adjusted to give an indication of zero in clean air, the procedure is called ’zero
adjustment’.
3.41
special state
state of the equipment other than those in which monitoring of gas concentration or alarming is
the intent
Note 1 to entry: Special state includes warm-up, calibration mode or fault condition.
3.42
ventilation
movement of air and its replacement with fresh air due to the effects of wind, temperature
gradients, or artificial means (for example, fans or extractors)
4 Properties and detection of toxic gases and vapours
4.1 Properties and detection
A distinction is drawn between gases, which remain gaseous at typical ambient pressures and
temperatures, and vapours where liquid can also exist at any relevant pressure or temperature.
The following properties and behaviours of gases should be taken into account, in particular
when locating detectors or deciding on a sampling strategy, in order to obtain representative
indications. Failure to take proper consideration of these gas properties and behaviours can
lead to failure to alarm and failure to take appropriate action or false alarms and incorrect action.
It can also lead to false estimates of exposure.
Toxic gases typically become harmful at low concentrations (occupational exposure limit values
typically range from parts per billion (ppb) to 1 % v/v levels). At distances far from the source
of toxic gas release, the relative density of such a gas mixture is not significantly different from
that of air. However, close to the source, the relative density can be significantly different,
although consideration should be given to influences by the thermal effect of pressurised gas.
Gases and mixtures with relative densities between 0,8 and 1,2 should generally be considered
to behave like air at ambient temperatures and are therefore capable of propagating in all
directions.
High pressure leaks can generate gas clouds that propagate over significant distances from the
source before mixing. This can occur for sources where the gas can be of any density.
In stagnant environments low pressure leaks can build up local high concentration pockets due
to insufficient passive air movement.
Spillage of liquids can result in toxic vapour clouds that can disperse over long distances and
duration and can accumulate in trenches, drains, tunnels etc. This is a result of liquid and
vapour flow under gravity, cooling due to evaporation, and densities greater than air. The vapour

– 14 – IEC 62990-2:2021 © IEC 2021
cloud tends to stay close to the ground until well mixed with air. Nevertheless, concentrations
in the breathing zone can approach harmful levels.
Gases and vapours fully mix with each other by diffusion over time or if stirred (for example, by
convection or mechanical ventilation). Once they have been mixed, they will remain mixed,
unless a component is removed chemically or is absorbed, for instance on a charcoal filter.
Additionally, in the case of vapours, the concentration can be lowered by condensation due to
increased pressure or reduced temperature. Some gases can react chemically with each other
on mixing, for example, nitric oxide and oxygen.
The toxic component within a gas mixture follows the characteristics of the mixture, irrespective
of the physical characteristics of the toxic component in pure form. The detection of H S for
sour gas applications should be based on consideration of the characteristics of the sour gas
mixture as a whole – typically dominated by methane, i.e. a “lighter than air” mixture,
irrespective of the properties of pure H S.
Air movement by convection, mechanical ventilation or wind can have a marked effect on gas
distribution. A heat source in an enclosed space, for example, can create a circular flow where
the heated gas rises, runs along the ceiling which is at a lower temperature and falls as it cools,
then runs along the floor back to the heat source.
Flow patterns can become very complicated and voids might well exist in which the gas can
accumulate. Consequently, each workplace scenario could be different. The use of smoke
tubes, mathematical modelling or scale models placed in wind tunnels can help to optimize the
location of fixed detectors.
Some gases tend to stick (adsorb) on surfaces, which leads to a decrease of their concentration
in air. This behaviour can be significant, especially with low gas concentrations and for reactive
gases. Adsorbed gases can desorb and produce a response even when there is no gas present
in the monitored air. The adsorption/desorption properties of each gas should be considered
before the measurement task is undertaken. This is particularly important where sampling
probes or sample lines are used to convey the gas to the equipment. The gas flow rate,
temperature, length, diameter and material from which the probe or line is made are important
factors.
Hygroscopic gases can form aerosols, which could be hazardous. A detector, which is only
capable of measuring gas phase concentrations, will underestimate the true hazard.
4.2 The difference between detecting gases and vapours
4.2.1 Gases
4.2.1.1 Characteristics of gases
Substances that remain gaseous under the range of temperatures and pressures relevant to
the gas detection application will closely follow the Gas Laws and behave predictably.
Gases can be pure, or any mixture of gases can be made, unless they react chemically. The
composition of non-reacting gas mixtures does not change with temperature or pressure.
4.2.1.2 Calibration considerations
It is possible to make and store under high pressure, calibration and other test gas mixtures
fully representative of the intended gas detection application. Many can be made with a dry or
synthetic air background. However, the more-reactive gases tend to have longer storage life if
the background is specially dried nitrogen, and this is normally chosen unless it is incompatible
with the sensor.
IEC 62990-2:2021 © IEC 2021 – 15 –
Where a sensor is intended for use with more than one toxic gas (or vapour), the calibration
gas should be the determination of worst case sensitivity combined with selected alarm
threshold level. If more than one sensor is necessary to monitor multiple gases (or vapours),
each sensor needs to be individually calibrated with the intended gas (or vapour) to be detected.
Cross-sensitivity needs to be considered and fully understood.
4.2.1.3 Propagation and sampling considerations
Even when a pure gas is lighter or heavier by density than air, this is not a reliable means of
determining propagation of a gas cloud.
The density of the gas to be detected should be taken into consideration when using sampling
and diffusion equipment and when installing fixed detection equipment.
4.2.2 Vapours
4.2.2.1 Characteristics of vapours
Substances, where the liquid or solid can coexist with their gaseous state at normal or slightly
abnormal temperatures and pressures are considered to be vapours. Vapours behave
differently than gases and can be more difficult to detect accurately.
Where a liquid is present, the rate of evaporation will increase with temperature. Similarly, the
maximum volume fraction of the vapour that can be achieved in a closed system (saturated
vapour) will increase with temperature. This is dependent on the temperature and pressure and
is independent of the quantity of liquid, provided there is some liquid remaining. The maximum
volume fraction of the vapour is also independent of the background gas provided it is at the
same temperature and pressure and is not soluble in the liquid.
The volume fraction that can be reached by the vapour at any temperature is inversely
proportional to the absolute pressure. So an increase in pressure can cause condensation.
As a rule of thumb, at constant pressure, the maximum (saturated) volume fraction of any
vapour will increase by a factor between 1,5 and 2,0 for each 10 K rise in liquid temperature,
and will fall by a factor of 1,5 to 2,0 for every 10 K reduction.
The effect of doubling the absolute pressure has an equi
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