4.0 GENERAL

The variety of missions and functions requiring the services of a diver call for the availability of many items and types of equipment. All diving equipment has its special function, and all must be of high reliability and maintained in good working order. The use of untried equipment must be limited to those special situations where precautions and proven backup systems are available. This section discusses the equipment that is in general use and has the proven reliability to support a diver and assist him in accomplishing his assigned task.

Basic equipment includes life support units, which supply the diver with the breathing gas at the pressure and temperature that satisfy his respiratory requirements and provide for thermal protection. Also included are those items which, under normal conditions are not essential to life support, but rather assist a diver in overcoming or adapting to the unique problems encountered in the underwater environment. Fins and wet suits are examples of these items of equipment. They are not normally required to support a diver's life under water, but do enable him to perform more efficiently. Also discussed are those specialized diver's tools or instruments designed for the performance of underwater tasks.

Various types of life support equipment are available to meet the many demands of the underwater environment and the specialized tasks to be performed. Each has its advantages and limitations. A diver equipped with scuba carries his breathing gas with him, independent of the surface. Umbilicalsupplied equipment provides the diver with breathing gas from a source either on the surface or submerged, through an umbilical. The primary advantages of self-contained diving include equipment portability, diver mobility, and reduced surface support requirements. Primary disadvantages are limited gas duration and related

depth limitations, limited ability to perform heavy work, and lack of effective voice communications. In addition, the freedom of the scuba equipped diver, which provides the major advantage, gives rise to certain negative physical, physiological, and psychological considerations, such as a feeling of insecurity in cold, dark waters, freedom to move too far from a habitat or surface support, and a diver's almost complete dependence upon himself to recognize and correct difficulties.

Scuba equipment is available in three basic configurations: Open-circuit scuba, closed-circuit scuba, and semi-closed-circuit scuba.

Open-circuit scuba is by far the most widely used configuration. This unit provides for "one time" use of each volume of the breathing gas supplied by the high pressure (HP) cylinders. The breathing gas flows from the cylinders through a regulator to the diver's mouthpiece during inhalation. Exhaled gas is discharged into the water. Air is the most common breathing medium used with open circuit scuba. The cylinders can be charged with a mixture of gases other than air, but, due to the complexity and expense associated with mixed gas diving, it is not generally undertaken with open-circuit scuba.

Closed-circuit scuba, as the name implies, involves a recirculation of the breathing gas after exhalation and continuous rebreathing, with no intentional loss of gas to the surrounding water. A diver inhales gas from a container such as a breathing bag and exhales through a purifying canister back into the breathing bag. A high-pressure gas cylinder or flask replenishes the gas in the breathing bag as it is expended through respiration. Carbon dioxide generated by the body and released during respiration is removed by an absorbent in the purifying canister. The breathing medium used in closed-circuit scuba diving may be pure oxygen or mixed gas, depending on the system used.

Open Circuit

Scuba Equipment

Photo: U.S. Divers

Semi-closed-circuit scuba is a modification of the closed-circuit system, which allows a partial rebreathing but provides for a continuous purge to prevent buildup of inert gas (nitrogen, helium, etc.) in the breathing bag.

Closed and semi-closed-circuit scuba provide for more efficient utilization of the compressed breathing medium through rebreathing than does the opencircuit system. Closed- and semi-closed systems, therefore, permit efficient utilization of the costly gas mixes necessary for diving to depths not safely attainable on air. The closed-circuit scuba further enables a diver to mingle less obtrusively with marine life because of the absence of exhaust bubbles. The primary disadvantage of both closed- and semi-closed-circuit scuba is system complexity, necessitating correspondingly rigid maintenance and diver training requirements.

4.1 OPEN-CIRCUIT SCUBA

The open-circuit scuba system is shown in Figure 4-1. It is a one-piece funtional unit consisting of several individual components. The first is a cylinder

assembly, which stores the breathing gas under pressure and provides a diver with a supply of breathing gas. The second is a regulator assembly, which reduces the pressure of the high-pressure breathing gas to ambient pressure and provides the gas to the diver upon demand. The diver then exhausts the gas directly into the water.

4.1.1 Demand Regulators

The demand regulator is used to reduce the pressure of breathing gas in high pressure cylinders to ambient pressure and provide the gas to a diver on demand, using the pressure differential created by the respiratory action of the diver's lungs as the metering signal. Most regulators automatically adjust to changes in depth and the diver's respiration rate, and conserve the gas supply by delivering only the quantity of breathing gas required. To understand the operation of a regulator, one must be familiar with the function of "upstream" and "downstream" valves. An upstream valve (Figure 4-2) is one that is forced closed by the high pressure gas in the cylinder. Conversely, the downstream valve (Figure 4-2a) is one that is forced open by the high pressure cylinder gas. As the pressure in the cylinders drops, less ambient pressure is required to open the upstream valve. The downstream valve is configured with springs, which can keep the valve closed at maximum cylinder pressure and is, therefore, more resistant to opening as cylinder pressure de

creases.

Although several different types of demand regulators may still be in use, the one-stage regulator is no longer in common use. Several different types of two stage regulators are commercially available.

Two stage regulators are designed to reduce the gas in a cylinder to ambient pressure in two stages. The first stage reduces the pressure to approximately 110 to 130 psi above ambient pressure. The second or demand stage further reduces the pressure to ambient pressure for diver breathing. The major advantage of this additional stage is that air is supplied to the demand stage at a nearly constant pressure, thus allowing a reduction in breathing resistance, and reduced fluctuations in breathing resistance resulting from changes in depth and decreasing cylinder pressure. Breathing resistance is further reduced because the demand valve is working against a controlled pressure (110 to 130 psi above ambient from the first stage).

Two stage regulators are available in two different styles, and with three different types of first stage reduction valves. The two styles of the two stage regulators are double and single hose models; the three types of first stage reduction valves are the standard, the balanced, and the piston types.

The double hose regulator combines both pressure reduction stages into one assembly mounted on the high pressure cylinder valve. A low pressure hose leads from the regulator valves to the mouthpiece, which contains two one-way valves. Another low pressure hose returns exhaled air to the regulator body where it is exhausted into the surrounding water (Figure 4-3).

The single hose regulator is configured with the first pressure reduction stage attached to the high pressure (HP) cylinder valve. The second pressure reduction stage is connected to the mouthpiece. Both first and second stages are connected by a

length of medium pressure hose. Exhaled air is exhausted from the second stage (mouthpiece connected) into the water (Figure 4-4).

The standard first stage valve (Figure 4-4a) is an upstream valve in which the high pressure cylinder acts to close the valve. A heavy spring applies force to compensate for the high cylinder pressure and acts on a flexible diaphragm. The flexible diaphragm is directly connected to a valve stem which unseats (opens) the high pressure valve. Water or air pressure on the flexible diaphragm compensates for pressure changes resulting from changes in depth. The heavy spring can be manually adjusted to maintain a constant medium pressure between the first and second stages. As this constant medium pressure between stages is reduced by respiration, the diaphragm flexes, unseats the high pressure valve, and restores the medium pressure to its desired level. The standard first stage is used with double hose as well as single hose regulators.

The balanced first stage valve (Figure 4-4b) was developed to eliminate the effects of HP cylinder gas pressure on seating the first stage valve. The balanced first stage eliminates the requirement for a small valve orifice, thereby increasing the maximum air flow capacity of the unit. The valve stem of the balanced first stage is exactly the same diameter as the valve orifice and extends through the high pressure chamber into the mid-pressure (MP) chamber opposite the flow orifice. As a result, the high pressure air does not exert a closing force on the valve stem, and the gas pressure in the mid-pressure chamber acts to balance the forces acting on the valve stem. If the valve stem were not of the same diameter as the orifice, an unbalanced surface area would be presented to the MP air at one end of the valve/valve stem assembly, and an opening or closing force would exist. With the effect of cylinder air pressure neutralized, only the mechanical force of the spring effects the operation of the valve. The springs can be adjusted to give exactly the desired medium pressure, and this will remain constant regardless of the pressure in the HP cylinder. As the valve is unaffected by fluctuations in cylinder pressure, large orifice diameters can be used to deliver large volumes of air flow with no increase in respiration effort.

The piston first stage valve (Figure 4-4c) represents an alternative to the balanced valve. The piston actuated first stage valve is opened and closed as a result of forces generated by the pres

sure chamber builds up above ambient pressure due to exhalation, a one-way mushroom valve is unseated, allowing the exhaled gas to be exhausted into the surrounding water. A properly constructed second stage has a minimum of dead space, thereby limiting the air that will be rebreathed.

4.1.1.2 Breathing Hoses

In double hose scuba, the breathing hoses (Figure 4-5) are flexible, large diameter rubber ducts, which provide passageways for air from the cylinder to the diver. They are usually corrugated rubber hoses but may be rubberized fabric with metallic ring or spiral stiffening. In order to provide minimum resistance to breathing, the hose should have an inside diameter of at least 1 inch, and must be long enough, in the "relaxed" state, to allow full freedom of body movement. The hose must be capable of stretching to twice their relaxed length without collapsing or buckling.

Single hose scuba, with the second stage of the demand regulator mask-mounted or mouthpiecemounted, does not require the large bore, ambient pressure breathing hose described above as the gas in the hose is at a medium pressure (110 to 130 psi above ambient) rather than ambient pressure. The second stage, or demand valve, is connected to a cylinder-mounted first stage regulator by a single, smooth base, medium pressure hose of relatively small diameter. Exhaled gases are discharged directly into the water through an exhaust valve in the mask or mouthpiece.

4.1.1.3 Mouthpiece

The mouthpiece (Figure 4-6) provides a relatively watertight channel for the flow of breathing gas

between the diver and his life support system. The size and design of the mouthpiece differs between various manufacturers, but it is generally molded of rubber, neoprene, or plastic. Typically, it consists of a flange which fits between the lips and teeth. Bits, one on either side of the mouthpiece opening, serve to space the teeth restfully apart. The mouthpiece should fit comfortably and should be held in place by a slight pressure exerted by the lips and teeth. The novice diver often forgets that the bits are spacers and are not to be used as grips under normal conditions. In an emergency, the bits will provide a reliable grip, but continuous force exerted through the teeth will weaken the bits and cause considerable fatigue to the muscles around the jaws.

The mouthpiece assembly incorporates a system of "one-way" check valves. Clamps are provided for a pair of breathing hoses in the case of the double hose scuba. The mouthpiece is incorporated into the second stage demand valve housing of the single hose scuba regulator. In unusual cases, the mouthpiece assembly may be replaced entirely by a full face mask. The use of the full face mask in place of the mouthpiece facilitates voice communications by freeing the diver's mouth.

4.1.1.4 Check Valves and Exhaust Valves

Check valves and exhaust valves (Figure 4-7) are designed to permit fluid flow in one direction only. Check valves direct the flow of inhaled and exhaled gases through the breathing system. During inhalation, the mouthpiece chamber experiences a decrease in pressure (now lower than ambient), which seats the exhalation check valve, but opens the inhalation check valve. During exhalation, the air is directed out through the mouthpiece and