A safety system for a breathing apparatus for delivery of an anesthetic agent having an anesthetic agent vaporizer having a reservoir for containing a liquid anesthetic agent has a reservoir for containing a liquid medical agent, a port for filling the reservoir with the liquid anesthetic agent, and a sensor device for sensing a filling action of the port. A change of position of a lid covering the port can be detected, or insertion of a fill vessel into the port can be detected. Thus safety of the equipment is improved by being able to de-pressurize the reservoir before opening a filling valve for communication of the port with the interior of the reservoir.
