exhalation tube to the exhaust valve. This pair of valves within the mouthpiece assembly minimizes "dead air space" within the system and thus minimizes rebreathing of exhaled gases. The inhalation check valve also prevents water from entering the demand regulator when the mouthpiece floods.

The exhaust valve is a special check valve which permits discharge of exhaled gas from the breathing system without permitting the entrance of water. The flapper valve (also called a flutter valve) is typically employed as an exhaust valve in the double hose regulator, while a mushroom valve generally fulfills the function in the single hose model. The flapper valve is simply a soft rubber tube collapsed at one end. When ambient water pressure is greater than the air pressure within the valve, it remains in the collapsed condition. During exhalation, the increase in pressure (over ambient pressure) forces the flapper open allowing the gas to escape. Water cannot enter the valve while the higher pressure gas escapes, and when the pressure equalizes, the flapper returns to the "relaxed" or closed position.

The mushroom valve of the single hose model is made of extremely soft, flexible rubber which renders it very sensitive to changes in pressure across the check valve. A wheel-shaped valve seat is fashioned to hold the rubber mushroom in place. Rigid "spokes" of the valve seat support the mushroom valve against a closing pressure, but permit the flow of air when pressure within the mouthpiece. exceeds ambient pressure.

4.1.1.5 Preventative Maintenance Procedures

As one of the primary components of the life support system, the regulator will require careful maintenance. While all components of the regulator are

1-6

constructed of corrosion resistant materials, the introduction of foreign matter into areas where close tolerances exist or where perfect seals are required can cause a malfunction. The primary entry point for foreign matter is the high pressure inlet in the first stage. For this reason the plastic dust cap should be kept in position covering the high pressure inlet whenever the regulator is not in use. Salt water. entering the HP inlet will leave deposits of salt, which can prevent proper operation or can pit valve surfaces.

The most important maintenance to be performed on the regulator is the fresh water rinse after each use. This will remove salt and other debris (sand. dirt, etc.) from the regulator and prevent deterioration. This should be accomplished within a few hours of the completion of the dive, regardless of whether the dive was conducted in fresh or salt water. Procedures for washing the single and double hose regulators vary significantly, and are discussed below.

With the dust cap sealed in place, the first stage of a single hose regulator should be held under a stream of warm, fresh water for a period of at least 2 minutes. The water should be allowed to flow freely through any open ports. This is especially important with piston type regulators as it prevents the buildup of salts on the piston tracks. Since dust caps provided with some regulators may not be water-tight, the diver must check this before rinsing the regulator.

When rinsing the second stage, allow the water to enter the mouthpiece and exit the exhaust. This is working in the direction of the nonreturn exhaust valve and will wash sand, dirt, etc. out of the mouthpiece. Never push the purge button as this opens the air inlet valve, and could possibly allow water carrying other debris to pass through the MP hose to the HP stage. If the regulator is to be stored for a long period of time, it may be desirable to remove the band holding the two sections of the second stage and the diaphragm in place, and to rinse each separately, allowing each piece to dry prior to reassembly.

Rinsing procedures for the double hose regulator are slightly more complicated than for the single hose model. As with the single hose regulator, rinsing should be conducted with the water-tight dust cap in place. The exhaust side of the regulator will have a series of holes, and water should be allowed to flow freely through this section.

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Care must be taken when rinsing the hose and mouthpiece assembly. If water is forced under high pressure into the mouthpiece, it may bypass the soft rubber nonreturn valve and permit water to enter the intake side, resulting in corrosion. Hold the mouthpiece with the air inlet valve up, and allow water to enter the mouthpiece, flow through the exhaust valve and hose, exiting at the main body of the regulator. To remove water from the corrugations in the hose, stretch the hose slightly and blow through the mouthpiece, allowing excess water to pass out the exhaust. Never hang the regulator by the mouthpiece, as this will stretch and weaken the hose.

4.1.2 Compressed Gas Cylinders

The scuba cylinder assembly is secured to the diver's back through a combination of shoulder, chest, waist, and crotch straps, known as the harness assembly. There has been a more recent trend toward the more comfortable, form fitting back pack assembly. The back pack itself is a lightweight frame, molded to conform to a diver's back and hip contours, and secured to the diver by adjustable, nylon shoulder and waist straps. The back pack is equipped with a clamping mechanism which secures either a single scuba cylinder or a multiple cylinder unit. Regardless of which of the many available harness or back pack models is employed, all straps which secure the apparatus to the diver must be equipped with corrosion-resistant, quick release buckles to permit rapid opening under emergency conditions.

The scuba cylinders contain the compressed breathing gas to be used by a diver. Most cylinders are of steel or aluminum alloy construction, specially designed and manufactured to safely contain compressed gas at working pressures from 1800 psig to 3000 psig (211 kg/cm2) or greater. Regardless of cylinder type, data describing the cylinder should be clearly stamped into the shoulder of the cylinder (Figures 4-8 and 4-8a).

The internal volume of a cylinder is a function of its physical dimensions and may be expressed in cubic inches or cubic feet. Of more interest is the capacity of the cylinder, which is the quantity of gas at surface pressure, which can be compressed into the cylinder at its rated pressure. The capacity is usually expressed in cubic feet or liters of gas.

Cylinders of various capacities are commercially

available. Commonly encountered steel scuba cylinders have a rated working pressure of 2250 psig (158 kg/cm2) (153 atm) and contain 64.7 cubic feet (1848 liters) of gas. When these cylinders meet certain Department of Transportation standards, they may be overfilled by 10 percent of the rated capacity. This additional capacity is indicated by a plus (+) symbol adjacent to the hydrostatic test date stamped on the cylinder. Cylinders with capacities from 26 cubic feet (742 liters) to over 100 cubic feet (2857 liters) are also available commercially.

The gas capacity of any cylinder is a function of the internal volume of the cylinder, and its rated pressure. It can be determined using the following formula:

Combination Submersible Cylinder Pressure Gauge and Depth Gauge

(front and back view)

4.1.4.2 Submersible Cylinder Pressure Gauge

The submersible cylinder pressure gauge, providing a continuous cylinder pressure read-out, is an alternative to the J-valve reserve mechanism. However, it does not provide a reserve air supply in the strict sense. It is useless under conditions of zero visibility unless illuminated. Employing the J-valve reserve with the submersible cylinder pressure gauge provides the diver with a system which overcomes the individual limitations. The submersible pressure gauge, positioned at the end of a two to three foot length of hose increases the the chances of fouling on bottom debris or with other items of equipment when worn improperly. The submersible cylinder pressure gauge is illustrated in Figure 4-10.

The gauge assembly employs standard fittings, and can be secured to most high pressure fittings on the cylinder valve demand regulator.

WARNING

The Submersible Cylinder Pressure Gauge Is Vulnerable to a Great Degree of Inaccuracy if Water Gets Into It Prior to Its Use. It Should Not Be Used Until Repaired.

Do Not Look Directly Into the Face of a Submersible Pressure Gauge When Turning on the Cylinder Due to the Possibility of Blow Out.

4.1.4.3 Audible Low-Air Warning Device

The audible low-air warning device produces a sonic signal automatically when cylinder pressure reaches a predetermined level. The sound continues until the cylinder air is exhausted. These devices are usually incorporated into the first stage assembly.

4.1.4.4 Restricted (or Calibrated) Orifice

The restricted (or calibrated) orifice warning/ reserve mechanism is now seldom used and is not recommended. It operates on the principle that the flow rate through an orifice of a given size is proportional to the pressure differential across the orifice. Inserting an orifice of a given size into the system can result in an insufficient flow rate when cylinder pressure decreases to the demand pressure over ambient pressure.

4.2 SEMI-CLOSED-CIRCUIT MIXED GAS SCUBA

Closed circuit oxygen scuba was developed to take advantage of the benefits of gas conservation but is extremely limited in dive depths and duration by oxygen toxicity effects. Open-circuit scuba provides greater depth flexibility, but is limited in dive duration, especially at deeper depths by the inefficiency of gas utilization. Because an entirely closed-circuit mixed gas scuba was not technically possible at the time, due to the problem involved with inert gas buildup, semi-closed-circuit mixed gas scuba was developed (Figure 4-11) to bridge this gap between duration and depth.

The semi-closed circuit scuba operates on the same basic principles as closed-circuit scuba, but requires a continuous or frequent purge to prevent a toxic buildup of inert gas. The high pressure gas cylinders are charged with a mixture of oxygen and an inert gas which corresponds to the safe oxygen level for the maximum depth to which the diver will descend. The gas flow from the cylinders into the breathing circuit is controlled by a regulator and nozzle which admits a continuous and constant mass of gas. The diver inhales the breathing medium from a breathing bag and exhales it into an exhalation bag. Pressure in the exhalation bag forces the gas through the scrubber where carbon dioxide is removed, and from the scrubber the gas again passes into the breathing bag for consumption by the

diver. When the gas pressure in the breathing circuit reaches a pre-set level, a relief valve located in the exhalation bag lifts, purging excess gas into the surrounding water (See Paragraph 11.3).

4.3 CLOSED-CIRCUIT MIXED GAS SCUBA

All closed-circuit mixed gas scuba function on the same basic principle and have the same basic components. They are precision pieces of equipment, and safe operation is dependent on a thorough understanding of the system. Oxygen and an inert gas, referred to here as the diluent, are stored in separate high pressure cylinders. The addition of diluent gas is controlled by a regulator to maintain a constant volume. The oxygen content of the breathing gas is continually monitored by a series of sensors which automatically add oxygen to the breathing circuit as required (Figure 6-3). The mixed breathing gas of some systems is stored in a breathing bag from which the diver inhales directly. The exhaled gas is passed directly into the absorbent canister where

CO2 is removed, and returned to the breathing bag to await rebreathing (See Paragraph 11.4).

4.4 CLOSED-CIRCUIT OXYGEN SCUBA By employing carbon dioxide absorption, closedcircuit oxygen scuba permits essentially complete utilization of the available gas supply at a rate independent of depth. Most units consist of a mouthpiece, breathing valve assembly, breathing hoses, inhalation and exhalation breathing bags, a carbon dioxide absorption canister, an oxygen supply cylinder, and an adjustable gas-flow regulating assembly.

Compressed oxygen is delivered from the highpressure oxygen cylinder into the inhalation breathing bag by the gas-flow regulating assembly at a rate. at which the diver uses the oxygen. The diver inhales the gas from the inhalation breathing bag through the inhalation hose and exhales into the exhalation breathing bag through the exhalation hose. The exhaled gas displaces the gas from the exhalation breathing bag causing it to flow into the carbon

Umbilical Diver

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dioxide absorption canister, where the carbon dioxide is removed. Gas within the canister, now freed of carbon dioxide, is displaced into the inhalation bag, where it remains until the next inhalation.

4.5 UMBILICAL DIVING

While the majority of diving situations will be under circumstances which dictate the use of open-circuit scuba equipment, umbilical diving has many important advantages which self-contained diving cannot provide (Figure 4-12).

One of the major limitations of self-contained diving is the quantity of breathing gas the diver can carry. With umbilical diving, the diver has a continous air supply, thus allowing extended bottom times. Because the diver is directly tethered, only one diver is required in the water at a time, thus conserving diver manhours.

Of significant importance is the increased safety factor possible with umbilical equipment. The diver is tethered and normally has direct voice communications; therefore, he can safely operate under c drons which may be considered too hazarçons for

the self-contained diver such as zero visibility. Should the diver become fouled or disabled, his ar supply is continuous and the standby diver can pro ceed directly to his assitance by following the en trapped diver's tether. Strong currents may present a significant problem to the tethered diver. If required, he can safely use additional weights to hold him on the bottom while working.

Umbilical diving may be conducted not only from the surface but from a habitat, a personnel transfer capsule, or a lock-out submersible. An umbilical from the gas storage cylinders of the habitat. capsule or submersible provides the diver's breathing gas, hot water, if required, and a communications link.

The major disadvantage associated with umbilical diving equipment is the increased requirement for support equipment and personnel.

A wide variety of diver's masks and helmets are commercially available. They are safe and efficient and most can be adapted for use with scuba. All will provide the diver with a continuous supply of breathing gas, and some models allow the diver to select between free-flow or demand breathing. A communications system is standard equipment in the majority of modern helmets.

4.5.1 Lightweight Diving Mask

Probably the oldest and simplest form of lightweight diving mask is the "Jack Browne" mask (Fig. ure 4-13). This mask is easy to maintain and operate. and may be equipped with voice communications. It consists of a rubber seal molded to a copper frame and a large triangular faceplate. Air enters the mask through an air control valve on the right side, and is exhausted through a manually closeable one-way valve located on the left side. This helmet is compatible with scuba accessories or a weighted dry suit. The umbilical consists of the air supply hose. lifeline, and communications line, when installed.

4.5.2 Free-Flow/Demand Mask

The free-flow/demand mask (Figure 4-14) provides several features, such as variable air flow, communications and an emergency air bottle connection. This equipment is designed to be used tethered, or with scuba from the surface, from a personnel transfer caps'e PTC or from a submerged habitat. It can be used to deep depths with either air or mixed gas as the beat me drum.