IEC 62990-2:2021
(Main)Workplace atmospheres — Part 2: Gas detectors — Selection, installation, use and maintenance of detectors for toxic gases and vapours
Workplace atmospheres — Part 2: Gas detectors — Selection, installation, use and maintenance of detectors for toxic gases and vapours
IEC 62990-2:2021 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.
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 donne des recommandations pour la sélection, l’installation, l’utilisation et la maintenance du matériel électrique utilisé pour le mesurage des gaz et des vapeurs toxiques dans les atmosphères des lieux de travail. Le principal objectif de ce type de matériel est d’assurer la sécurité des personnes et des biens en fournissant une indication de la concentration d’un gaz ou d’une vapeur toxique et en avertissant de sa présence. Le présent document s’applique aux matériels dont l’objectif principal est de donner une indication, une alarme ou une autre fonction de sortie pour avertir de la présence d’un gaz ou d’une vapeur toxique dans l’atmosphère et, dans certains cas, de déclencher des actions protectrices automatiques ou manuelles. Il s’applique aux matériels dont le capteur génère automatiquement un signal électrique en présence de gaz. Pour les besoins du présent document, les matériels comprennent: a) le matériel fixe; b) le matériel transportable; et c) le matériel portable. Le présent document est destiné à couvrir les matériels définis dans l’IEC 62990-1, mais peut fournir des informations utiles pour des matériels non couverts par ce document.
General Information
Standards Content (Sample)
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
All rights reserved. Unless otherwise specified, 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
either IEC or IEC's member National Committee in the country of the requester. If you have any questions about IEC
copyright or have an enquiry about obtaining additional rights to this publication, please contact the address below or
your local IEC member National Committee for further information.
IEC Central Office Tel.: +41 22 919 02 11
3, rue de Varembé info@iec.ch
CH-1211 Geneva 20 www.iec.ch
Switzerland
About the IEC
The International Electrotechnical Commission (IEC) is the leading global organization that prepares and publishes
International Standards for all electrical, electronic and related technologies.
About IEC publications
The technical content of IEC publications is kept under constant review by the IEC. Please make sure that you have the
latest edition, a corrigendum or an amendment might have been published.
IEC publications search - webstore.iec.ch/advsearchform IEC online collection - oc.iec.ch
The advanced search enables to find IEC publications by a Discover our powerful search engine and read freely all the
variety of criteria (reference number, text, technical publications previews. With a subscription you will always
committee, …). It also gives information on projects, replaced have access to up to date content tailored to your needs.
and withdrawn publications.
Electropedia - www.electropedia.org
IEC Just Published - webstore.iec.ch/justpublished
The world's leading online dictionary on electrotechnology,
Stay up to date on all new IEC publications. Just Published
containing more than 22 000 terminological entries in English
details all new publications released. Available online and
and French, with equivalent terms in 18 additional languages.
once a month by email.
Also known as the International Electrotechnical Vocabulary
(IEV) online.
IEC Customer Service Centre - webstore.iec.ch/csc
If you wish to give us your feedback on this publication or
need further assistance, please contact the Customer Service
Centre: sales@iec.ch.
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
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
– 4 – IEC 62990-2:2021 © IEC 2021
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.
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.
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
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
– 10 – IEC 62990-2:2021 © IEC 2021
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)
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
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.
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 equivalent effect to a decrease in
temperature of 10 K to 17 K at constant pressure. Halving the pressure has an effect equivalent
to a similar rise in temperature.
The temperature at which the saturated volume fraction can reach 100 % at the prevailing
pressure is the boiling point.
It is only possible to have 100 % volume fraction of a vapour at or above the boiling point at
that atmospheric pressure. Below the boiling point of the liquid, the maximum possible
concentration of vapour in air or other gases will be less than 100 % volume fraction.
The actual amount of vapour will be less than the amounts predicted above if fresh atmosphere
is being continually passed over the liquid surface, or if there has not been enough time for
equilibration to be established. However, this maximum amount can be achieved in an enclosed
space, particularly if it has been closed for some time and the air space is slowly stirred by
convection or mechanical means.
Toxic vapours can be present when the vapour is well below a flammable level.
All of these points impose limits on mixtures of vapours with gases. For any particular volume
fraction of a vapour in a mixture with gas, a fall in temperature or increase in pressure will, at
a certain stage, reach the saturation point, below which vapour will start to condense as a fog
– 16 – IEC 62990-2:2021 © IEC 2021
or liquid. When applied to water vapour this is known as the “dew point”. This term is frequently
applied to other vapours. Below the dew point the composition of any vapour-gas mixture will
therefore change.
4.2.2.2 Calibration considerations
Real restrictions on what calibration or test vapours are practical for field use are imposed by
the minimum temperature at which they will be used, and the required pressure in a calibration
gas or test gas cylinder.
Whenever test or calibration gases other than the gas or vapour of interest are used, it is
recommended that additional safety margins, lower alarm settings etc. are employed in order
to compensate for the added uncertainty of relative response data due to the factors mentioned
above.
4.2.2.3 Propagation and sampling considerations
There are toxic vapours that are lighter than air.
All other vapours are denser than air. Most are considerably denser than air. At a point of
release, unless heated, they will initially flow downwards or across level surfaces, and until they
are well mixed with air they will stay close to the ground, possibly only a few centimetres deep,
which is where they will present the greatest flammability problems. A severe instance of this
is in the case of pits, trenches and tunnels which will tend to fill from the bottom upwards and
can permit extensive propagation over hundreds of metres. This can also cause the most
serious toxicity problems for personnel entering pits and tunnels.
When sampling for toxic vapours in relatively undisturbed atmospheres, some tests need to be
performed at very low levels, ideally within a centimetre of the floor or ground.
Once mixed with an excess of air, all vapours will be found at all levels, possibly at volume
fractions well below their Lower Flammable Limit (LFL) but still presenting major toxicity
problems.
Conversely, if the temperature rises, and particularly in an enclosed area, the concentration
can change dramatically. Again, using the rule of thumb, the vapour concentration in an
enclosed space could increase by a factor up to eight for a 30 K rise in temperature, caused,
for instance, by the sun on an external tank. A previously undetected amount of vapour while
the tank was cold could become very significant when it is heated up. When
...
IEC 62990-2
Edition 1.0 2021-06
INTERNATIONAL
STANDARD
NORME
INTERNATIONALE
colour
inside
Workplace atmospheres –
Part 2: Gas detectors – Selection, installation, use and maintenance of detectors
for toxic gases and vapours
Atmosphères des lieux de travail –
Partie 2: Détecteurs de gaz – Sélection, installation, utilisation et maintenance
des détecteurs de gaz et de vapeurs toxiques
All rights reserved. Unless otherwise specified, 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
either IEC or IEC's member National Committee in the country of the requester. If you have any questions about IEC
copyright or have an enquiry about obtaining additional rights to this publication, please contact the address below or
your local IEC member National Committee for further information.
Droits de reproduction réservés. Sauf indication contraire, aucune partie de cette publication ne peut être reproduite ni
utilisée sous quelque forme que ce soit et par aucun procédé, électronique ou mécanique, y compris la photocopie et
les microfilms, sans l'accord écrit de l'IEC ou du Comité national de l'IEC du pays du demandeur. Si vous avez des
questions sur le copyright de l'IEC ou si vous désirez obtenir des droits supplémentaires sur cette publication, utilisez
les coordonnées ci-après ou contactez le Comité national de l'IEC de votre pays de résidence.
IEC Central Office Tel.: +41 22 919 02 11
3, rue de Varembé info@iec.ch
CH-1211 Geneva 20 www.iec.ch
Switzerland
About the IEC
The International Electrotechnical Commission (IEC) is the leading global organization that prepares and publishes
International Standards for all electrical, electronic and related technologies.
About IEC publications
The technical content of IEC publications is kept under constant review by the IEC. Please make sure that you have the
latest edition, a corrigendum or an amendment might have been published.
IEC publications search - webstore.iec.ch/advsearchform IEC online collection - oc.iec.ch
The advanced search enables to find IEC publications by a Discover our powerful search engine and read freely all the
variety of criteria (reference number, text, technical publications previews. With a subscription you will always have
committee, …). It also gives information on projects, replaced access to up to date content tailored to your needs.
and withdrawn publications.
Electropedia - www.electropedia.org
IEC Just Published - webstore.iec.ch/justpublished
The world's leading online dictionary on electrotechnology,
Stay up to date on all new IEC publications. Just Published
containing more than 22 000 terminological entries in English
details all new publications released. Available online and once
and French, with equivalent terms in 18 additional languages.
a month by email.
Also known as the International Electrotechnical Vocabulary
(IEV) online.
IEC Customer Service Centre - webstore.iec.ch/csc
If you wish to give us your feedback on this publication or need
further assistance, please contact the Customer Service
Centre: sales@iec.ch.
A propos de l'IEC
La Commission Electrotechnique Internationale (IEC) est la première organisation mondiale qui élabore et publie des
Normes internationales pour tout ce qui a trait à l'électricité, à l'électronique et aux technologies apparentées.
A propos des publications IEC
Le contenu technique des publications IEC est constamment revu. Veuillez vous assurer que vous possédez l’édition la
plus récente, un corrigendum ou amendement peut avoir été publié.
Recherche de publications IEC - Découvrez notre puissant moteur de recherche et consultez
webstore.iec.ch/advsearchform gratuitement tous les aperçus des publications. Avec un
La recherche avancée permet de trouver des publications IEC abonnement, vous aurez toujours accès à un contenu à jour
en utilisant différents critères (numéro de référence, texte, adapté à vos besoins.
comité d’études, …). Elle donne aussi des informations sur les
projets et les publications remplacées ou retirées. Electropedia - www.electropedia.org
Le premier dictionnaire d'électrotechnologie en ligne au monde,
IEC Just Published - webstore.iec.ch/justpublished
avec plus de 22 000 articles terminologiques en anglais et en
Restez informé sur les nouvelles publications IEC. Just
français, ainsi que les termes équivalents dans 16 langues
Published détaille les nouvelles publications parues.
additionnelles. Egalement appelé Vocabulaire
Disponible en ligne et une fois par mois par email.
Electrotechnique International (IEV) en ligne.
Service Clients - webstore.iec.ch/csc
Si vous désirez nous donner des commentaires sur cette
publication ou si vous avez des questions contactez-nous:
sales@iec.ch.
IEC online collection - oc.iec.ch
IEC 62990-2
Edition 1.0 2021-06
INTERNATIONAL
STANDARD
NORME
INTERNATIONALE
colour
inside
Workplace atmospheres –
Part 2: Gas detectors – Selection, installation, use and maintenance of detectors
for toxic gases and vapours
Atmosphères des lieux de travail –
Partie 2: Détecteurs de gaz – Sélection, installation, utilisation et maintenance
des détecteurs de gaz et de vapeurs toxiques
INTERNATIONAL
ELECTROTECHNICAL
COMMISSION
COMMISSION
ELECTROTECHNIQUE
INTERNATIONALE
ICS 29.260.20 ISBN 978-2-8322-1048-5
– 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
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
– 4 – IEC 62990-2:2021 © IEC 2021
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.
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.
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
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
– 10 – IEC 62990-2:2021 © IEC 2021
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)
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
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.
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 equivalent effect to a decrease in
temperature of 10 K to 17 K at constant pressure. Halving the pressure has an effect equivalent
to a similar rise in temperature.
The temperature at which the saturated volume fraction can reach 100 % at the prevailing
pressure is the boiling point.
It is only possible to have 100 % volume fraction of a vapour at or above the boiling point at
that atmospheric pressure. Below the boiling point of the liquid, the maximum possible
concentration of vapour in air or other gases will be less than 100 % volume fraction.
The actual amount of vapour will be less than the amounts predicted above if fresh atmosphere
is being continually passed over the liquid surface, or if there has not been enough time for
equilibration to be established. However, this maximum amount can be achieved in an enclosed
space, particularly if it has been closed for some time and the air space is slowly stirred by
convecti
...
IEC 62990-2
Edition 1.0 2021-06
INTERNATIONAL
STANDARD
NORME
INTERNATIONALE
colour
inside
Workplace atmospheres –
Part 2: Gas detectors – Selection, installation, use and maintenance of detectors
for toxic gases and vapours
Atmosphères des lieux de travail –
Partie 2: Détecteurs de gaz – Sélection, installation, utilisation et maintenance
des détecteurs de gaz et de vapeurs toxiques
All rights reserved. Unless otherwise specified, 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
either IEC or IEC's member National Committee in the country of the requester. If you have any questions about IEC
copyright or have an enquiry about obtaining additional rights to this publication, please contact the address below or
your local IEC member National Committee for further information.
Droits de reproduction réservés. Sauf indication contraire, aucune partie de cette publication ne peut être reproduite ni
utilisée sous quelque forme que ce soit et par aucun procédé, électronique ou mécanique, y compris la photocopie et
les microfilms, sans l'accord écrit de l'IEC ou du Comité national de l'IEC du pays du demandeur. Si vous avez des
questions sur le copyright de l'IEC ou si vous désirez obtenir des droits supplémentaires sur cette publication, utilisez
les coordonnées ci-après ou contactez le Comité national de l'IEC de votre pays de résidence.
IEC Central Office Tel.: +41 22 919 02 11
3, rue de Varembé info@iec.ch
CH-1211 Geneva 20 www.iec.ch
Switzerland
About the IEC
The International Electrotechnical Commission (IEC) is the leading global organization that prepares and publishes
International Standards for all electrical, electronic and related technologies.
About IEC publications
The technical content of IEC publications is kept under constant review by the IEC. Please make sure that you have the
latest edition, a corrigendum or an amendment might have been published.
IEC publications search - webstore.iec.ch/advsearchform IEC online collection - oc.iec.ch
The advanced search enables to find IEC publications by a Discover our powerful search engine and read freely all the
variety of criteria (reference number, text, technical publications previews. With a subscription you will always have
committee, …). It also gives information on projects, replaced access to up to date content tailored to your needs.
and withdrawn publications.
Electropedia - www.electropedia.org
IEC Just Published - webstore.iec.ch/justpublished
The world's leading online dictionary on electrotechnology,
Stay up to date on all new IEC publications. Just Published
containing more than 22 000 terminological entries in English
details all new publications released. Available online and once
and French, with equivalent terms in 18 additional languages.
a month by email.
Also known as the International Electrotechnical Vocabulary
(IEV) online.
IEC Customer Service Centre - webstore.iec.ch/csc
If you wish to give us your feedback on this publication or need
further assistance, please contact the Customer Service
Centre: sales@iec.ch.
A propos de l'IEC
La Commission Electrotechnique Internationale (IEC) est la première organisation mondiale qui élabore et publie des
Normes internationales pour tout ce qui a trait à l'électricité, à l'électronique et aux technologies apparentées.
A propos des publications IEC
Le contenu technique des publications IEC est constamment revu. Veuillez vous assurer que vous possédez l’édition la
plus récente, un corrigendum ou amendement peut avoir été publié.
Recherche de publications IEC - Découvrez notre puissant moteur de recherche et consultez
webstore.iec.ch/advsearchform gratuitement tous les aperçus des publications. Avec un
La recherche avancée permet de trouver des publications IEC abonnement, vous aurez toujours accès à un contenu à jour
en utilisant différents critères (numéro de référence, texte, adapté à vos besoins.
comité d’études, …). Elle donne aussi des informations sur les
projets et les publications remplacées ou retirées. Electropedia - www.electropedia.org
Le premier dictionnaire d'électrotechnologie en ligne au monde,
IEC Just Published - webstore.iec.ch/justpublished
avec plus de 22 000 articles terminologiques en anglais et en
Restez informé sur les nouvelles publications IEC. Just
français, ainsi que les termes équivalents dans 16 langues
Published détaille les nouvelles publications parues.
additionnelles. Egalement appelé Vocabulaire
Disponible en ligne et une fois par mois par email.
Electrotechnique International (IEV) en ligne.
Service Clients - webstore.iec.ch/csc
Si vous désirez nous donner des commentaires sur cette
publication ou si vous avez des questions contactez-nous:
sales@iec.ch.
IEC online collection - oc.iec.ch
IEC 62990-2
Edition 1.0 2021-06
INTERNATIONAL
STANDARD
NORME
INTERNATIONALE
colour
inside
Workplace atmospheres –
Part 2: Gas detectors – Selection, installation, use and maintenance of detectors
for toxic gases and vapours
Atmosphères des lieux de travail –
Partie 2: Détecteurs de gaz – Sélection, installation, utilisation et maintenance
des détecteurs de gaz et de vapeurs toxiques
INTERNATIONAL
ELECTROTECHNICAL
COMMISSION
COMMISSION
ELECTROTECHNIQUE
INTERNATIONALE
ICS 29.260.20 ISBN 978-2-8322-1048-5
– 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
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
– 4 – IEC 62990-2:2021 © IEC 2021
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.
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.
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
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
– 10 – IEC 62990-2:2021 © IEC 2021
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)
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
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.
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 equivalent effect to a decrease in
temperature of 10 K to 17 K at constant pressure. Halving the pressure has an effect equivalent
to a similar rise in temperature.
The temperature at which the saturated volume fraction can reach 100 % at the prevailing
pressure is the boiling point.
It is only possible to have 100 % volume fraction of a vapour at or above the boiling point at
that atmospheric pressure. Below the boiling point of the liquid, the maximum possible
concentration of vapour in air or other gases will be less than 100 % volume fraction.
The actual amount of vapour will be less than the amounts predicted above if fresh atmosphere
is being continually passed over the liquid surface, or if there has not been enough time for
equilibration to be established. However, this maximum amount can be achieved in an enclosed
space, particularly if it has been closed for some time and the air space is slowly stirred by
convecti
...
Questions, Comments and Discussion
Ask us and Technical Secretary will try to provide an answer. You can facilitate discussion about the standard in here.
Loading comments...