The tank is filled by a pipe connected between a loading union on the tank. A differential pressure monitor has its high pressure chamber connected to the bottom of the tank where the gas is liquid, and its low pressure chamber to the top where it is gaseous. Transducers work with the monitor (12) and shut off devices, so as to stop filling when hydrostatic pressure in the liquid column reaches a predetermined level. The transducers can be a threshold switch. Electrically operated valves can be incorporated in the liquid and gas circuits.