pressures in excess of 3,750 psi. The gas then flows through the priority valve (PRIV). This valve is set  to  stop  the  flow  of  gas  from  the  supply cylinders  when  cylinder  pressure  falls  below 200 ± 10 psi. From the priority valve, gas flows to  the  pressure  regulating  valve  (PRV).  Pressure indicated on the high-pressure gauge (PG-1) is reduced to 50±5 psi by the pressure regulating valve. The reduced pressure is shown on the low- pressure gauge (PG-2). Gas then flows to the relief valve (RV-2), which is preset to 75+ 0, – 10 psi. From  the  relief  valve,  gas  passes  through  the heater assembly (HIR), where the gas is heated to 285°F. The heated gas flow is then directed through the flow control orifice (ORF), which maintains the flow of gas at 0.20±0.017 pounds per minute at the regulated pressure of 50 ± 5 psi. The gas that exists in the filler valve (FLRV) will be  at  a  temperature  of  225±25°F. Electrical power for the heating element in the heater (HTR) is controlled by the pressure switch (PSW)  and  two  temperature  switches.  When  gas pressure  of  50±5  psi  is  reached,  the  pressure switch energizes the temperature control circuit and  allows  the  heater  assembly  to  warm.  The temperature  switch  breaks  contact  within  the circuit  when  the  heater  assembly  temperature exceeds 285°F, and re-establishes contact when the heater assembly cools to 270°F, thus main- taining a temperature of 250°F to 260°F at the heater^- assembly   discharge. The high temperature switch functions as a safety switch. This switch is set to break the heater circuit when heater assembly temperature exceeds 300°F,   and   re-establish   the   circuit   when temperatures fall below 273°F. For operational use of the purging unit, seethe users manual sup- plied with the unit or the  Liquid  Oxygen,  System Gas Purging  Set, NAVAIR  19-25D-27. GASEOUS OXYGEN SERVICING  TRAILERS Learning Objective: Identify components and  operating  procedures  for  gaseous oxygen servicing trailers. There are several different models of gaseous oxygen servicing trailers currently in use by naval activities.  They  are  all  similar  in  operation; therefore, only one, the type NO-2, manufactured by  Aeroil  Products,  Incorporated,  is  described here. The trailer is shown in figure 5-13. Equipment provided on the trailer includes six manifold control valves with pressure gauges; an upper   and   lower   manifold;   two   pressure regulators; a recharge valve; four shutoff valves; a drier assembly, six cylinders, and connecting flexible hoses; and a servicing hose fitted with a line servicing valve fitted with a high-pressure charging adapter. The function of each of these components is described in the following text. COMPONENTS Complete familiarity with the following trailer components   is   a   basic   prerequisite   for   safe operation. MANIFOLD CONTROL VALVES. The six manifold control valves serve to shutoff the flow of oxygen from the cylinders to the system being charged. These valves are lever-type valves. The manifold control valves should not be used for long-time storage. Always use the handwheel type valves located on the cylinders. UPPER  MANIFOLD.  The  upper  manifold provides   connections/mounting   for   the   six manifold control valves with pressure gauges (each connected to a supply cylinder), a recharge valve, and two upper/inlet shutoff valves that connect to the inlet side of the regulators. PRESSURE  REGULATORS.  The  pressure regulator controls the charging pressure when the trailer is being used to service aircraft oxygen systems.  Only  one  pressure  regulator  is  used during operation. The spare is provided to ensure uninterrupted operation should one fail. RECHARGE VALVE. The recharge valve is provided  as  a  means  of  recharging  the  trailer cylinders  directly  through  the  upper  manifold without  the  necessity  of  removing  the  cylinders. When  not  in  use,  the  valve  adapter  should  be fitted with a dust cap. SHUTOFF VALVES. There are four shutoff valves,  one  on  the  inlet  side  of  each  pressure regulator  and  one  on  the  outlet  side  of  each regulator. These shutoff valves control the flow of oxygen from the upper manifold to the lower manifold, via the regulator. When the shutoff valves on the inlet and outlet sides  of  the  regulator  are  open,  the  pressure regulator is ready for use. By turning the regulator control-handle clockwise, the pressure (as read on 5-16