This document specifies requirements specific for portable units which are part of a transportable liquid oxygen system and used as a supply source for oxygen therapy particularly for patients in the home-care environment. Portable units are intended to be used without professional supervision, carried by patients whilst moving around and during their off-site activities and refilled from a base unit via a transfilling connector through the portable unit s filling port connector. NOTE: Requirements that are common to both portable units and base units are specified in ISO 18777-1.