In cases where polymerization of vinyl chloride is prevented
by addition of an inhibitor, 17.8 is applicable. In cases where no
inhibitor has been added, or the inhibitor concentration is insufficient,
any inert gas used for the purposes of 17.6 shall contain no more
oxygen than 0.1% by volume. Before loading is started, inert gas samples
from the tanks and piping shall be analysed. When vinyl chloride is
carried, a positive pressure shall always be maintained in the tanks
and during ballast voyages between successive carriages.