7.1 Appropriate testing of the atmosphere of a
space should be carried out with properly calibrated equipment by
persons trained in the use of the equipment. The manufacturers' instructions
should be strictly followed. Testing of the space should be carried
out before any person enters the space and at regular intervals thereafter
until all work is completed. Where appropriate, the testing of the
space should be carried out at as many different levels as is necessary
to obtain a representative sample of the atmosphere in the space.
In some cases it may be difficult to test the atmosphere throughout
the enclosed space without entering the space (e.g. the bottom landing
of a stairway) and this should be taken into account when assessing
the risk to personnel entering the space. The use of flexible hoses
or fixed sampling lines, which reach remote areas within the enclosed
space, may allow for safe testing without having to enter the space.
7.2 For entry purposes, steady readings of all
of the following should be obtained:
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.1 21% oxygen by volume by oxygen content meter;
Note National requirements may determine the safe atmosphere
range.
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.2 not more than 1% of lower flammable limit (LFL)
on a suitably sensitive combustible gas indicator, where the preliminary
assessment has determined that there is potential for flammable gases
or vapours; and
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.3 not more than 50% of the occupational exposure
limit (OEL)footnote of any toxic vapours
and gases.
If these conditions cannot be met, additional ventilation should
be applied to the space and re-testing should be conducted after a
suitable interval.
7.3 Any gas testing should be carried out with
ventilation to the enclosed space stopped, and after conditions have
stabilized, in order to obtain accurate readings.
7.4 Where the preliminary assessment has determined
that there is potential for the presence of toxic gases and vapours,
appropriate testing should be carried out, using fixed or portable
gas or vapour detection equipment. The readings obtained by this equipment
should be below the occupational exposure limits for the toxic gases
or vapours given in accepted national or international standards,
in accordance with paragraph 7.2. It should be noted that testing
for flammability or oxygen content does not provide a suitable means
of measuring for toxicity, nor vice versa.
7.5 It should be emphasized that the internal
structure of the space, cargo, cargo residues and tank coatings may
also present situations where oxygen-deficient areas may exist, and
should always be suspected, even when an enclosed space has been satisfactorily
tested as being suitable for entry. This is particularly the case
for spaces where the path of the supply and outlet ventilation is
obstructed by structural members or cargo.