The present invention describes a medical, supply or transfer lock (120) for a pressure vessel (100) for human occupancy. The medical lock (120) includes a body tube (122), an outer door flange (124) and an outer door (140) in sealing face relation with an inside face (125) of the outer door flange (124). In an embodiment, the outer door (124) has upper rollers (146a) and lower rollers (146b). The upper rollers run on a horizontal rail (160) whilst the lower rollers run on a slant rail (170), which is mounted below the horizontal rail (160). The ends of the horizontal and slant rails (160,170) are curved (163, 172) so that weight component of the outer door is generated to assist the outer door to close. When pressure in the medical lock (120) is increased, the outer door of this medical lock (120) becomes self-locking and self-sealing.
