A RECENT series of explosions aboard supertankers during tank cleaning operations (using high pressure water jets) has raised the possibility that the sudden electrical discharge of an electrostatically charged mist may ignite an explosive mixture of hydrocarbon and air1. Although the discharge configuration (cloud–electrode) is somewhat different from the usual electrode–electrode situation used to obtain minimum ignition energies, it is to be expected that the discharge energy and hence the volume charge density of the mist will be an important factor in determining the likelihood of an explosion. It is known that a mist is produced during tank cleaning operations and that its charge density stabilizes at a value ∼10−8 C m−3. The mechanisms responsible both for charging and for then limiting the charge density to such a value are, however, not fully understood2.