Plasma gas is normally argon. The torch also uses a secondary  gas, such as argon/helium or argon/nitrogen, that assists in  shielding the molten weld puddle and minimizing oxidation  of the weld.  Like GTAW, the PAW process can be used to weld most  commercial metals, and it can be used for a wide variety  of metal thicknesses. On thin material, from foil to  1 ⁄ 8 -inch,  the process is desirable because of the low heat input. The  process provides relatively constant heat input because arc  length variations are not very critical. On material thicknesses  greater than  1 ⁄ 8 -inch and using automated equipment, a  keyhole technique is often used to produce full penetration  single-path welds. In the keyhole technique, the plasma  completely penetrates the work piece. The molten weld metal  flows to the rear of the keyhole and solidifies as the torch  moves on. The high quality welds produced are characterized  by deep, narrow penetration and a small weld face. When PAW is performed manually, the process requires  a high degree of welding skills similar to that required for  GTAW. However, the equipment is more complex and  requires a high degree of knowledge to set up and use. The  equipment required for PAW includes a welding machine, a  special plasma arc control system, the plasma welding torch  (water-cooled), the source of plasma and shielding gas, and  filler material, when required. Because of the cost associated  with this equipment, this process is very limited outside of  manufacturing facilities. This is the primary fuel for oxy-fuel welding and cutting. It  is chemically very unstable, and is stored in special cylinders  designed to keep the gas dissolved. The cylinders are packed  with a porous material and then saturated with acetone. When  the acetylene is added to the cylinder, it dissolves; in this  solution, it becomes stable. Pure acetylene stored in a free  state explodes from a slight shock at 29.4 pounds per square  inch (psi). The acetylene pressure gauge should never be set  higher than 15 psi for welding or cutting.  Argon is a colorless, odorless, tasteless, and non-toxic inert  gas. Inert gas cannot combine with other elements. It has a  very low chemical reactivity and low thermal conductivity.  It is used as a gas shield for the electrode in MIG, TIG, and  plasma welding equipment.  Helium is a colorless, odorless, tasteless, and non-toxic  inert gas. Its boiling and melting points are the lowest of the  elements and it normally exists only in gas form. It is used  as a protective gas shield for many industrial uses including  electric arc welding. Hydrogen is a colorless, odorless, tasteless, and highly  flammable gas. It can be used at a higher pressure than  acetylene and is used for underwater welding and cutting.  It also can be used for aluminum welding using the oxy- hydrogen process. When a plasma cutting torch is used, the gas is usually  compressed air. The plasma cutting machine works by  constricting an electrical arc in a nozzle and forcing the  ionized gas through it. This heats the gas that melts the metal  which is blown away by the air pressure. By increasing air  pressure and intensifying the arc with higher voltages, the  cutter is capable of blasting through thicker metals and  blowing away the dross with minimal cleanup.  Plasma arc systems can cut all electrically conductive metals,  including aluminum and stainless steel. These two metals  cannot be cut by oxy-fuel cutting systems because they have  an oxide layer that prevents oxidation from occurring. Plasma  cutting works well on thin metals and can successfully cut  brass and copper in excess of two inches thick.  Plasma cutting machines can rapidly and precisely cut  through, gouge, or pierce any electrically conductive metal  without preheating. The plasma cutter produces a precise  kerf (cut) width and a small heat-affected zone (HAZ) that  prevents warping and damage. Oxygen is a colorless, odorless, and nonflammable gas. It is  used in the welding process to increase the combustion rate  which increases the flame temperature of flammable gas.  A pressure regulator is attached to a gas cylinder and is used  to lower the cylinder pressure to the desired working pressure.  Regulators have two gauges, one indicating the pressure in  the cylinder and the second showing the working pressure.  By turning the adjustment knob in or out, a spring operating  a flexible diaphragm opens or closes a valve in the regulator.  Turning the knob in causes the flow and pressure to increase;  backing it out decreases the flow and pressure. Pressure Regulators Oxygen Plasma Arc Cutting  Hydrogen Helium Argon Gas Welding and Cutting Equipment Welding Gases  Acetylene 5-7