TECHNICAL PRINCIPLES OF OPERATION -cont

TM 5-4220-225-14 (4) The air supply assembly handles the air flow through the helmet from the umbilical to the air supply diffuser. The air supply adapter is secured to the helmet base with three socket head cap screws, and sealed by a barrel preformed packing set into the adapter body. Contained in the adapter is a 100-mesh stainless steel filter screen, upstream of a cartridge-type one way valve. A preformed packing in the body of the one way valve seals the bore of the adapter. The 3/8-inch stainless steel air tubing is connected to the air supply adapter by a double male fitting which threads into the air supply adapter and sealed with a preformed packing. The opposite end of the tubing is terminated in the same fashion with the double male fitting threaded into the air supply valve body assembly. The elbow of the valve has a threaded relief valve boss to accept the air supply relief valve, which opens at a nominal 20 psig to protect the air supply diffuser against possible rupture should the filter clog or freeze up. The supply diffuser threads into the elbow and serves the dual purpose of diffusing the air into the breathing area and suppressing air noise. The supply valve varies the air flow through its full range. The valve handle is secured to the valve stem by self-locking set screws threaded into two of the handle vanes. Valve handle position is indexed by an extended third vane. The helmet air is discharged through the exhaust valve in either the adjustable or the ambient exhaust valve configuration. (5) The exhaust valve (figure 1-5) is easily disassembled and reassembled for inspection or maintenance without need for special tools. In the adjustable configuration air exhaust is controlled by a spring-laced poppet-type valve. It is adjustable to maintain helmet pressure in the range of 0.3 � 0.05 psig to 2.0 � 0.3 psig above ambient pressure with 6 ACFM flow. Turning the valve handle increases or decreases the spring tension which produces a corresponding effect on internal helmet P. A spring tension adjustment screw, externally accessible at the valve handle center, is used to obtain the high/low settings of the valve operating pressure range. Since the poppet valve is restricted only by spring tension, the diver can press against the chin button or pull it shut with his mouth to manually override helmet P settings. The valve body is mounted to the helmet through an extension of the helmet base and sealed in the penetration with a preformed packing. (6) When the helmet is used with a neckdam, the ambient exhaust valve (figure 1-5), configuration must be employed. The ambient exhaust valve maintains a low helmet P of 0.125 psig with a flow of 6 ACFM. In the ambient configuration the helmet P is not variable and the valve consists basically of a diffuser with a rubber diaphragm one-way valve. Mounting is the same as the adjustable exhaust valve. (7) The MK12 SSDS earphones are mounted in molded rubber cups which are cemented into recesses on either side of the helmet shell. The microphone is mounted in an openback rubber cup held by a stainless steel spring clip which snaps onto the air supply valve body. With the communications station, the earphones are connected in parallel, and are in parallel with the microphone. The earphones will then function as transceivers in the event of a microphone failure. (8) The interior surfaces of the helmet are lined with 1/2-inch thick open-cell polyurethane foam, attached to the helmet with Velcro tape. The liner serves to absorb and deaden helmet noise and to reduce condensation on the helmet interior. 1-11