Gas-free certificate

A gas-free certificate is the written confirmation by a competent person, often a marine chemist, that a tank or enclosed space has a safe atmosphere for entry or hot work: oxygen at 20.9 percent by volume, hydrocarbon vapor below 1 percent of the lower flammable limit, and toxic gases below their occupational exposure limits. It is issued after the space is cleaned, ventilated, and tested, and it is the precondition for hot work in cargo and ballast tanks under SOLAS and flag-state rules. The certificate states the spaces covered and a validity window, since conditions change; pockets, sludge, and adjacent-tank diffusion can re-establish hazardous vapor. Entry without a valid certificate is the leading cause of enclosed-space fatalities.