a subsurface safety valve for use in subterranean oil and gas well which is actuated by pressure fluctuations in the tubing - casing annulus is disclosed . the safety valve assembly consists of a safety valve mounted in a packer which is actuated by a subsurface control located above the packer . annulus pressure variations are controlled by a surface unit . the subsurface control utilizes a reference pressure chamber in which the pressure is initially adjustable to compensate for hydrostatic pressure in the annulus . leakage of the gas in a dome pressure chamber in the subsurface control is impeded by the presence of a barrier fluid impervious to the passage of gas therethrough , with the barrier fluid being maintained at a higher pressure than the reference pressure in the dome charged chamber .