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4 The mixer is perhaps misnamed, since it doesn’t actually mix the gases, but merely meters them into the mixing chamber. Two types of ”mixers” are widely used: the medium-pressure type, to which the gases are usually supplied at approximately equal pressures, and the injector type, to which the oxygen is supplied at relatively high pressure (up to 55 psi or more) and the acetylene is supplied at rather low pressure (down to less than 1 psi). In the injector type, the oxygen passes through a very small orifice in the injector, and the expansion of the oxygen as it leaves that orifice pulls the acetylene into the mixing chamber. An advantage of the injector is that minor fluctuations in oxygen pressure and flow cause changes in the amount of acetylene drawn into the mixing chamber and maintain the normal ratio of the gas mixture. This is especially helpful when oxygen is supplied to the torch from a plant piping system, where changes in demand placed on the system may cause pressure fluctuations. As already stated, the mixer, whether medium-pressure or injector type, is normally a part of the welding head. The reason is this: There must be a relationship between the sizes of the orifices in the mixer and the size of the orifice in the welding tip. A single mixer cannot serve a wide range of tip sizes. Further, all the passages in the welding head must be designed so that if the flame is forced back into the head, as by momentary contact of the torch tip against the work, it will not continue to burn just ahead of the mixer, but will be extinguished without damage to head or torch. Fig. 5-2. Simplified sketch of a typical welding torch, with details showing the construction of a throttle valve and the flow of gases through a ”mixer” and an ”injector”. HEAD NUT HANDLE OXYGEN ACETYLENE TIP MIXING THROAT MIXER OR INJECTOR THROTTLE VALVES PACKING NUT PACKING WASHER VALVE STEM BALL SEAT OXYGEN ACETYLENE OXYGEN MIXER INJECTOR ACETYLENE OXYGEN ACETYLENE