• cylindrical gauges may be used provided they are fitted with
self-closing valves at their lower end as well as at their upper
end if the latter is below the maximum liquid level
• in the case of tanks not subject to filling by power pumps, with
the exception of fuel oil service tanks, the valves need not be of
the self-closing type. Such valves are, however, to be readily
accessible and instruction plates are to be fitted adjacent to
them specifying that they are to be kept closed.