The presence of toxic gases such as sulphur dioxide, carbon
monoxide of nitrogen, can be ascertained only by measurement. However,
provided that the hydrocarbon gas content of an inerted tank exceeds
about 2% by volume before gas-freeing is started, the dilution of
the toxic components of flue gas during the subsequent gas-freeing
can be correlated with the readings of an approved combustible gas
indicator or equivalent. If by ventilating the compartment, a reading
of 1% of the lower flammable limit or less is obtained in conjunction
with an oxygen reading of 21% by volume, the toxic trace gases will
be diluted to concentrations at which it will be safe to enter. Alternatively,
and irrespective of initial hydrocarbon gas content, ventilation should
be continued until a steady oxygen reading of 21% by volume is obtained.