A portable fill cart is used, and all of the same precautions  must be observed as when servicing a high-pressure gaseous  oxygen system. Additionally, protection from cold burns is  necessary. Due to the amount of gaseous oxygen released  during the process, refilling should be accomplished outside.  The servicing cart is attached to the aircraft system through  a fill valve. The buildup/vent valve on the LOX container  assembly is placed in the vent position. The valve on the  service cart is then opened. LOX flows into the aircraft  system; some vaporizes and cools the entire setup. This  gaseous oxygen flows overboard through the vent valve  while the system fills. When a steady stream of LOX flows  from the vent valve, the system is filled. The valve is then  switched to the buildup position. The aircraft refill valve  and cart supply valves are closed, and the hose is removed.  Note that back seated valves can freeze in the open position  due to the low temperature involved while LOX is being  transferred. Valves should be opened completely and then  closed slightly so as to not be back seated. The wide varieties of oxygen masks used in aviation require  periodic inspection. Mask and hose integrity ensure effective  delivery of oxygen to the user when it is needed. Sometimes  this is in an emergency situation. Leaks, holes, and tears are  not acceptable. Most discrepancies of this type are remedied  by replacement of the damaged unit. Some continuous-flow masks are designed for disposal after  use. Be sure there is a mask for each potential user on board  the aircraft. Masks designed to be reused should be clean,  as well as functional. This reduces the danger of infection  and prolongs the life of the mask. Various mild cleaners and  antiseptics that are free of petroleum products can be used.  A supply of individually wrapped alcohol swabs are often  kept in the cockpit. Built-in microphones should be operational. Donning straps  and fittings should be in good condition and function so  that the mask is held firm to the user’s face. Note that the  diameter of mask hoses in a continuous-flow system is quite  a bit smaller than those used in a demand-flow system. This  is because the inside diameter of the hose aids in controlling  flow rate. Masks for each kind of system are made to only  connect to the proper hose. Smoke masks are required on transport aircraft and are used  on some other aircraft as well. These cover the eyes, as well  as the user’s nose and mouth. Smoke masks are usually  available within easy grasp of the crew members. They are  used when the situation in the cockpit demands the increased  level of protection offered. Smoke mask hoses plug into  demand regulators in the same port used for regular demand  type masks and operate in the same manner. Most include a  built-in microphone.  [Figure 16-33]  Some portable oxygen  systems are also fitted with smoke masks. The replacement of aircraft oxygen system tubing, valves,  and fittings is similar to the replacement of the same  components in other aircraft systems. There is, however, an  added emphasis on cleanliness and compatible sealant use. Any oxygen system component should be cleaned thoroughly  before installation. Often tubing comes with leftover residue  from the bending or flaring processes. Cleaning should  be accomplished with nonpetroleum-based cleansers.  Trichlorethylene, acetone, and similar cleaners can be used  to flush new tubing. Tubing should be blown or baked dry  before installation. Follow the manufacturer’s procedures  for cleaning oxygen system components. Some oxygen components make use of tapered pipe  fittings. This type of connection is usually sealed with  the application of thread lubricant/sealant. Typical thread  sealers are petroleum based and should not be used; only  oxygen compatible thread lubricant/sealers should be used.  Alternatively, Teflon$^{®}$ tape is also used on oxygen pipe fitting  Figure 16-33.  Smoke masks cover the eyes as well as the nose and  mouth of the user. Replacing Tubing, Valves, and Fittings Inspection of Masks and Hoses 16-20