Figure  4-5.—Acetylene  cylinder. 225 cubic feet of acetylene. Just because a cylinder has a  225-cubic-foot  capacity  does  not  necessarily  mean  it has  225  cubic  feet  of  acetylene  in  it.  Because  it  is dissolved  in  acetone,  you  cannot  judge  how  much  acety- lene is left in a cylinder by gauge pressure. The pressure of the acetylene cylinder will remain fairly constant until most of the gas is consumed. An example of an acetylene cylinder is shown in figure 4-5. These cylinders are equipped with fusible plugs that relieve excess pressure if the cylinder is exposed to undo heat. The standard Navy acetylene cylinder contains 225 cubic feet of acetylene and weighs about 250 pounds. The acetylene cylinder is yellow, and all  compressed-gas  cylinders  are  color-coded  for  iden- tification.  More  on  the  color  coding  of  cylinders  is covered later in this chapter. MAPP GAS MAPP  (methylacetylene-propadiene)  is  an  all-pur- pose industrial fuel having the high-flame temperature of acetylene but has the handling characteristics of propane. Being a liquid, MAPP is sold by the pound, rather than by the cubic foot, as with acetylene. One cylinder containing 70 pounds of MAPP gas can accom- plish the work of more than six and one-half 225-cubic- foot acetylene cylinders; therefore, 70 pounds of MAPP gas is equal to 1,500 cubic feet of acetylene. Cylinder Design Total weight for a MAPP cylinder, which has the same physical size as a 225-cubic-foot acetylene cylin- der, is 120 pounds (70 pounds which is MAPP gas). MAPP cylinders contain only liquid fuel. There is no cylinder packing or acetone to impair fuel withdrawal; therefore, the entire contents of a MAPP cylinder can be used. For heavy-use situations, a MAPP cylinder deliv- ers more than twice as much gas as an acetylene cylinder for  the  same  time  period. MAPP  Characteristics Because of its superior heat transfer characteristics, MAPP  produces  a  flame  temperature  of  5300°F  when burned  with  oxygen.  MAPP  equals,  or  exceeds,  the performance  of  acetylene  for  cutting,  heating,  and  braz- ing. MAPP is not sensitive to shock and is nonflamma- ble in the absence of oxygen. There is no chance of an explosion if a cylinder is bumped, jarred, or dropped. You can store or transport the cylinders in any position with no danger of forming an explosive gas pocket. The characteristic odor, while harmless, gives warn- ings of fuel leaks in the equipment long before a dan- gerous condition can occur. MAPP gas is not restricted to a maximum working pressure of 15 psig, as is acety- lene. In jobs requiring higher pressures and gas flows, MAPP can be used safely at the full-cylinder pressure of 95 psig at 70°F. Because of this, MAPP is an excellent gas  for  underwater  work. Bulk MAPP Gas Bulk MAPP gas facilities, similar to liquid oxygen stations, are installed at some activities where large supplies of the gas are used. In bulk installations, MAPP gas is delivered through a piping system directly to the user  points.  Maximum  pressure  is  controlled  centrally for  efficiency  and  economy. Cylinder-filling facilities are also available from bulk installations that allow users to fill their cylinders on site. Filling a 70-pound MAPP cylinder takes one man about 1 minute and is essentially like pumping water from a large tank to a smaller one. MAPP Gas Safety MAPP gas vapor is stable up to 600°F and 1,100 psig when exposed to an 825°F probe. The explosive limits of MAPP gas are 3.4 percent to 10.8 percent in air or 2.5 percent to 80 percent in oxygen. As shown in 4-4