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