A gas cylinder for the storage of compressed gas includes a shut-off valve in engagement with a mouth of the cylinder; an external pressure sensor device for sensing pressure in the cylinder, the pressure sensor device being in continuous communication with the interior of the cylinder; a temperature sensor device for sensing temperature of the gas in the cylinder, the temperature sensor device being in thermal communication with the interior of the cylinder; a programmable computing device for calculating information from the temperature and pressure signals concerning the amount of contents in the cylinder; and a display screen operatively associated with the programmable computing device for displaying said information.