Measuring instruments, detectors, analysers for use in connection with gases, off-gases, gas mixtures and synthesis gases; switching, regulating and controlling devices for melting furnaces, in particular those for switching and controlling gas flows in connection with melting furnaces and melting processes; machines and apparatus for controlling the gas-supply to appliances/apparatus for melting, welding and cutting, and which consist essentially of valves, switching devices, control, mixing, security and indicating instruments