5 - Testing the atmosphere in the tank
5.1 After a tank has been cleaned, ventilated
and prepared for entry, it should be tested for oxygen content,
and finally, as appropriate, for toxic gases at various levels
from top to bottom.
5.2 The atmosphere can only be accepted as
suitable for entry when all the relevant hazards have been
identified and removed.
5.3 Appropriate testing of the atmosphere of a
tank 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 tank
should be carried out before any person enters the tank, and at
regular intervals thereafter until all work is completed. Where
appropriate, the testing of the tank should be carried out at as
many different levels as is necessary to obtain a representative
sample of the atmosphere in the tank. In some cases, it may be
difficult to test the atmosphere throughout the tank without
entering the tank and this should be taken into account when
assessing the risk to personnel entering the tank. The use of
flexible hoses or fixed sampling lines which reach remote areas
within the tank, may allow for safe testing without having to
enter the tank.
5.4 All ventilation must be stopped prior to and
during the atmosphere tests and resumed prior to any person
entering the tank.
5.5 Criteria for Tank Entry
5.5.1 For entry purposes, steady readings of
all the following should be obtained:
- 21% oxygen by volume by oxygen content
- 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
- not more than 50% of the occupational exposure limit
(OEL) of any toxic vapours and gases.2
5.5.2 A responsible person should ensure
that all measuring instruments in use have been properly
calibrated and are maintained in accordance with the
respective manufacturer's instructions.
5.5.3 If these conditions cannot be met,
additional ventilation should be applied to the tank and
re-testing should be conducted after a suitable
1National requirements may determine the safe atmosphere
2It should be noted that the term Occupational Exposure Limit
(OEL) includes the Permissible Exposure Limit (PEL), Maximum
Admissible Concentration (MAC) and Threshold Limit Value (TLV)
or any other internationally recognized terms.