ASTM E681-09(2023)

This international standard was developed in accordance with internationally recognized principles on standardization established in the Decision on Principles for the
Development of International Standards, Guides and Recommendations issued by the World Trade Organization Technical Barriers to Trade (TBT) Committee.
Designation: E681 − 09 (Reapproved 2023)
Standard Test Method for
Concentration Limits of Flammability of Chemicals (Vapors
and Gases)
This standard is issued under the fixed designation E681; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision. A number in parentheses indicates the year of last reapproval. A
superscript epsilon (´) indicates an editorial change since the last revision or reapproval.
1. Scope 1.6 The values stated in SI units are to be regarded as
standard. No other units of measurement are included in this
1.1 This test method covers the determination of the lower
standard.
and upper concentration limits of flammability of chemicals
having sufficient vapor pressure to form flammable mixtures in 1.7 This test method should be used to measure and describe
air at atmospheric pressure at the test temperature. This test the properties of materials, products, or assemblies in response
method may be used to determine these limits in the presence to heat and flame under controlled laboratory conditions and
of inert dilution gases. No oxidant stronger than air should be should not be used to describe or appraise the fire hazard or fire
used. risk of materials, products, or assemblies under actual fire
conditions. However, results of this test method may be used as
NOTE 1—The lower flammability limit (LFL) and upper flammability
elements of a fire risk assessment that takes into account all of
limit (UFL) are sometimes referred to as the lower explosive limit (LEL)
and the upper explosive limit (UEL), respectively. However, since the the factors pertinent to an assessment of the fire hazard of a
terms LEL and UEL are also used to denote concentrations other than the
particular end use.
limits defined in this test method, one must examine the definitions closely
1.8 This standard may involve hazardous materials,
when LEL and UEL values are reported or used.
operations, and equipment. This standard does not purport to
1.2 This test method is based on electrical ignition and
address all of the safety concerns, if any, associated with its
visual observations of flame propagation. Users may experi-
use. It is the responsibility of the user of this standard to
ence problems if the flames are difficult to observe (for
establish appropriate safety, health, and environmental prac-
example, irregular propagation or insufficient luminescence in
tices and determine the applicability of regulatory limitations
the visible spectrum), if the test material requires large ignition
prior to use. Specific precautionary statements are given in
energy, or if the material has large quenching distances.
Section 8.
1.3 Annex A1 provides a modified test method for materials
1.9 This international standard was developed in accor-
(such as certain amines, halogenated materials, and the like)
dance with internationally recognized principles on standard-
with large quenching distances which may be difficult to ignite.
ization established in the Decision on Principles for the
Development of International Standards, Guides and Recom-
1.4 In other situations where strong ignition sources (such
mendations issued by the World Trade Organization Technical
as direct flame ignition) is considered credible, the use of a test
Barriers to Trade (TBT) Committee.
method employing higher energy ignition source in a suffi-
ciently large pressure chamber (analogous, for example, to the
2. Referenced Documents
methods in Test Method E2079 for measuring limiting oxygen
concentration) may be more appropriate. In this case, expert
2.1 ASTM Standards:
advice may be necessary.
E171 Practice for Conditioning and Testing Flexible Barrier
Packaging
1.5 The flammability limits depend on the test temperature
E582 Test Method for Minimum Ignition Energy and
and pressure. This test method is limited to an initial pressure
Quenching Distance in Gaseous Mixtures
of the local ambient or less, with a practical lower pressure
E1445 Terminology Relating to Hazard Potential of Chemi-
limit of approximately 13 kPa (100 mm Hg). The maximum
cals
practical operating temperature of this equipment is approxi-
E1515 Test Method for Minimum Explosible Concentration
mately 150 °C.
of Combustible Dusts
This test method is under the jurisdiction of ASTM Committee E27 on Hazard
Potential of Chemicals and is the direct responsibility of Subcommittee E27.04 on
Flammability and Ignitability of Chemicals. For referenced ASTM standards, visit the ASTM website, www.astm.org, or
Current edition approved May 1, 2023. Published May 2023. Originally contact ASTM Customer Service at service@astm.org. For Annual Book of ASTM
approved in 1979. Last previous edition approved in 2015 as E681 – 09 (2015). Standards volume information, refer to the standard’s Document Summary page on
DOI: 10.1520/E0681-09R23. the ASTM website.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
E681 − 09 (2023)
E2079 Test Methods for Limiting Oxygen (Oxidant) Con- methods aim to result in a conservative representation of
centration in Gases and Vapors flammability parameters. For example, in this standard, LFL is
the calculated average of the lowest go and highest no-go
2.2 NFPA Standard:
NFPA 69 Standard on Explosion Prevention Systems concentrations while the European test methods report the LFL
as the minimum of the five highest no-go concentrations.
3. Terminology
NOTE 2—For hydrocarbons, the break point between nonflammability
and flammability occurs over a narrow concentration range at the lower
3.1 Definitions:
flammability limit, but the break point is less distinct at the upper limit.
3.1.1 lower limit of flammability or lower flammable limit
For materials found to be non-reproducible per 13.1.1 that are likely to
(LFL)—the minimum concentration of a combustible sub-
have large quenching distances and may be difficult to ignite, such as
ammonia and certain halogenated hydrocarbon, the lower and upper limits
stance that is capable of propagating a flame in a homogeneous
of these materials may both be less distinct. That is, a wider range exists
mixture of the combustible and a gaseous oxidizer under the
between flammable and nonflammable concentrations (see Annex A1).
specified conditions of test.
3.1.2 propagation of flame—as used in this test method, the
6. Interferences
upward and outward movement of the flame front from the
6.1 This test method is not applicable to certain readily
ignition source to the vessel walls or at least to within 13 mm
oxidized chemicals. If significant oxidation takes place when
( ⁄2 in.) of the wall, which is determined by visual observation.
the vapors are mixed with air, unreliable results may be
By outward, it is meant a flame front that has a horizontal
obtained. Flow systems designed to minimize hold-up time
component to the movement away from the ignition source.
may be required for such materials.
3.1.3 upper limit of flammability or upper flammable limit
6.2 Measured flammable limits are influenced by flame
(UFL)—the maximum concentration of a combustible sub-
quenching effects of the test vessel walls. The test vessel
stance that is capable of propagating a flame in a homogeneous
employed in this test method is of sufficient size to eliminate
mixture of the combustible and a gaseous oxidizer under the
the effects of the flame quenching for most materials (and
specified conditions of test.
conditions).
3.2 Additional terms can be found in Terminology E1445.
NOTE 3—There may be quenching effects, particularly on tests run at
subambient pressures. For materials that may be difficult to ignite (see
4. Summary of Test Method
Note 2), tests in a larger vessel or different ignition sources (see Annex
4.1 A uniform mixture of a gas or vapor with air is ignited
A1, 12 L flask) may show flame propagation that is not seen in the 5 L
flask with spark or exploding wire igniters. This test method is a small
in a closed vessel, and the upward and outward propagation of
scale test and this possible limitation must be considered in hazard
the flame away from the ignition source is noted by visual
assessments.
observation. The concentration of the flammable component is
6.3 The oxygen concentration in the air has an important
varied between trials until the composition that will just sustain
effect on the UFL. Typically, room air is used. If cylinder air is
propagation of the flame is determined.
used to simulate room air it must have an oxygen concentration
5. Significance and Use
of 20.94 % 6 0.1 %. Reconstituted air in cylinders has vari-
ability in the oxygen concentration and must be verified for
5.1 The LFL and UFL of gases and vapors define the range
oxygen concentration.
of flammable concentrations in air.
5.2 This method measures the LFL and UFL for upward
7. Apparatus
(and partially outward) flame propagation. The limits for
7.1 Fig. 1 is a schematic diagram of the apparatus; details
downward flame propagation are narrower.
and dimensions are presented in Appendix X1. The apparatus
5.3 Limits of flammability may be used to determine guide-
consists of a glass test vessel, an insulated chamber equipped
lines for the safe handling of volatile chemicals. They are used
with a source of controlled-temperature air, an ignition device
particularly in assessing ventilation requirements for the han-
with an appropriate power supply, a magnetic stirrer, and a
dling of gases and vapors. NFPA 69 provides guidance for the
cover equipped with the necessary operating connections and
practical use of flammability limit data, including the appro-
components.
priate safety margins to use.
7.2 If tests are to be conducted at an elevated temperature,
5.4 As discussed in Brandes and Ural, there is a fundamen-
the test vessel may be heated as described in Appendix X1. The
tal difference between the ASTM and European methods for
heating system must be capable of controlling the gas tempera-
flammability determination. The ASTM methods aim to pro-
ture inside the test vessel to within 63 °C both temporally and
duce the best representation of flammability parameters, and
spatially. An appropriate device such as a thermocouple must
rely upon the safety margins imposed by the application
be used to monitor the gas temperature within the test vessel.
standards, such as NFPA 69. On the other hand, European test
Active (connected) volumes beyond the test vessel itself should
be held above the condensation temperature of all components
in the material being tested. Electrical heating tapes must be
Available from National Fire Protection Association (NFPA), 1 Batterymarch
Park, Quincy, MA 02169-7471, http://www.nfpa.org.
employed for heating components to the desired temperature.
Brandes, E., and Erdem, A. U., “Towards a Global Standard for Flammability
Determination,” 42nd Annual Loss Prevention Symposium, New Orleans, LA, April NOTE 4—Certain bare wire thermocouples may cause catalytic oxida-
2008. tion of test vapors, as evidenced by a persistent high-temperature
E681 − 09 (2023)
FIG. 1 Schematic Diagram of Test Apparatus
excursion of the temperature reading. If this occurs, other thermocouple
8.2.2 Energetic explosions may be produced if tests are
materials should be employed.
made at concentrations within the flammable range, between
7.3 Pressure Transducer—A low-range pressure transducer
the LFL and UFL. The glass test vessel, equipped with a lightly
may be used for the purpose of making partial pressure
held or loose cover, vents most explosions adequately.
additions of gases and vapors to the test vessel. The transducer
Nevertheless, shielding is required to protect against vessel
and its signal conditioning/amplifying electronics should have
rupture. Methods for estimating initial test concentrations,
an accuracy, precision and repeatability sufficient to accurately
discussed in Appendix X2, Appendix X3, and Appendix X4,
resolve the required changes in the gas partial pressure for the
may be employed to ensure that initial trials are conducted at
component used in lowest concentration at the appropriate test
concentrations less than the LFL or greater than the UFL.
temperature. The transducer should be protected from defla-
8.2.3 In rare instances, particularly in the upper limit tests,
gration pressures by means of an isolation valve. An error
self-ignition may be encountered when air is rapidly introduced
analysis must be performed to demonstrate that the internal
into the partially evacuated test vessel containing the vaporized
volume of the pressure gauge and piping will not significantly
sample. Valves permitting remote operation, changes in sample
affect the test mixture.
and air introduction sequences, simple shields, and other
techniques may be employed to ensure safe operations.
8. Safety Precautions
8.2.4 The test area should be equipped with electrical
8.1 Tests should not be conducted in this apparatus with
interlocks to prevent activation of the ignition source unless
oxidizers stronger than air, since explosion violence increases
adequate shielding is in place.
as oxidizer strength increases. Do not use oxygen, nitrous
oxide, nitrogen dioxide, chlorine, etc., in this glass apparatus.
8.3 Tests should not be conducted on thermally unstable
Extra care must be used when working with compounds that
materials that might undergo explosive decomposition reac-
are potential oxidizers.
tions.
8.2 Adequate shielding must be provided to prevent injury
8.4 Tests should be conducted in a fume hood or other
in the event of equipment rupture due to both implosions and
ventilated area to prevent personal exposure to toxic chemicals
explosions. A metal enclosure, such as that recommended in
or combustion products.
Appendix X1, is one method suitable for this purpose.
8.5 Precautions must be taken to ensure that the high-
8.2.1 Implosion of the test vessel at high vacuum levels is
voltage spark ignition source does not contact temperature or
possible; therefore, all evacuations must be made with the
required shielding to protect against flying fragments. pressure-measuring devices or other conductive paths that
E681 − 09 (2023)
could create an electrical hazard to personnel or instrumenta- necessary to heat or insulate cover components and feed lines
tion outside the shielded area. Careful attention to electrical separately to prevent vapor condensation.
insulation integrity can reduce the possibility
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