System Flow. Open Circuit

TM 5-4220-225-14 a. System Flow. Open Circuit (figure 1-3). Diver air is delivered through the umbilical. The air hose is terminated at the left side of the diver's waist and is connected to the air supply whip which leads under the left arm to the air adapter at the right rear corner on the helmet. In the air supply adapter, the air passes through a filter screen and a cartridge-type one way valve which prevents depressurization of the helmet through the hose should supply pressure be lost. From the adapter, the air flows through the tubing assembly to the air supply valve mounted in the right side of the helmet. The air supply valve varies the flow linearly through its range and has a control knob indexed by an extended vane to indicate position. From the supply valve, the air flows to the supply diffuser located across the lower edge of the front viewport. The diffuser serves the dual function of evenly discharging the air to the helmet and helping to suppress air noise. Should the diffuser become clogged or frozen, a relief valve mounted in the elbow of the diffuser adapter will open at a nominal 20 psi to prevent rupturing the diffuser. The helmet air flow is discharged and diffused through the exhaust valve in the lower left side of the helmet. The exhaust valve can be assembled in two configurations. In the adjustable configuration, the diver can vary the helmet P from 0.3 psig to 2.0 psig overbottom pressure. Operating the exhaust valve together with the supply valve allows the diver to control his buoyancy while maintaining adequate ventilation. A chin button is provided which permits the diver to open or shut the adjustable exhaust valve, manually overriding the valve setting. The ambient exhaust valve configuration is installed when the helmet is used with a neckdam. In the ambient configuration, the P across the exhaust valve is fixed at 0.125 psi, which is low enough to prevent neck squeeze at the neckdam. b. Helmet Assembly (figure 1-4). The helmet assembly consists of the shell and base together with the components and parts affixed, including the lower breech ring assembly. (1) The shell is laid up with fiberglass cloth and polyester resin for a strong, light, impact-resistant structure. The shell is coated with a highly visible yellow gel coat to provide the best underwater definition. A cast lead weight is cut to fit into the crown of the helmet, then laminated with fiberglass and resin. The weight is sized and located as to make the helmet neutral in buoyancy and to counterbalance the weight of the base and breech rings. (2) There are two breech rings in the helmet mating assembly; the helmet breech ring and the lower breech ring. The helmet breech ring is secured to the base with machine screws, with an preformed packing forming the seal between the base and helmet breech ring. The lower breech ring assembly consists of the lower breech ring, the neckdam or drysuit neckpiece, the retainer, two locking device lugs, and three jocking harness brackets. The neckdam is sealed by compression with a neoprene material and has a highly corrosion-resistant finish. The helmet assembly is secured to the lower breech ring assembly by two locking device assemblies, one on each side. Each locking device consists of a helmet lug mounted on the helmet base which mates with a lug mounted on the lower breech ring. A locking device w/safety pin, locks the two lugs together and secures the helmet. (3) There are four viewport assemblies in the helmet which provide visibility comparable to that of Self Contained Underwater Breathing Apparatus (SCUBA). A groove is molded into each viewport recess to receive a shaped-to-fit preformed packing. A retainer is placed over each lexan viewport and secured into place with socket head cap screws that mate with female threaded inserts fitted into the helmet shell. 1-8