Open Circuit Scuba

General

Open circuit scuba is so called because the circuit is broken by the diver exhaling his air into the surrounding atmosphere, rather than re-breathing it.

The open circuit scubas used by sport divers consist of a tank of compressed air with a valve, a harness equipped with quick release buckles, and a regulator, which supplies air to the diver upon demand only. The basic premise of scuba is that it supplies the diver his breathing medium at ambient pressure, whatever that pressure may be.

Single Stage Regulator

The single stage regulator is so called because it reduces tank pressure to ambient pressure in one pressure reduction. In order that we may have a basic knowledge of the function of this equipment, let us refer to the schmatic diagram of a single stage regulator on the facing page;

The regulator consists primarily of a housing, a flexible diaphram (C) which is exposed to ambient water pressure on one side, a lever (B), and a high pressure valve (D).

As the diver inhales through the inhalation hose, he creates a partial vacuum in the pressure reduction chamber (G). As this partial vacuum is created, ambient water pressure causes diaphram C to bow inward, engaging lever B and thereby opening valve D. Tank pressure flows into the pressure reduction area until such time as the diver ceases his inhalation, when the pressure in chamber G equals the ambient pressure. When the pressure in chamber G euqals ambient pressure the diaphram returns to a neutral position and the high pressure valve D is closed.

The exhalation of the diver is carried back through the exhalation hose to the level of the diaphram, in order to prevent a "free-flow" effect.

The mouthpiece assembly is equipped with two non-return valves. When the diver inhales, valve A opens and valve B closes. Upon exhalation, valve B opens and valve A closes. Many regulators have an additional check valve, or so-called duck-billed valve at the point where the exhalation hose enters the regulator housing. These valves are all designed to maintain "dry" hoses, so that the diver may inhale and exhale with a

minimum of effort.

Two Stage Regulator

The essential difference between the two stage regulator and the single stage lies in the pressure reduction areas. In the two stage regulator, there are two pressure reductions from tank pressure to ambient pressure. The first pressure reduction is from tank pressure to approximately 100 pounds over ambient pressure. The second reduction is to ambient pressure.

The first stage pressure is maintained by a high pressure diaphram assisted by a pre-set spring of approximately 100 psi. (figure on facing page).

So sensitive are these demand type regulators to pressure that simply raising the mouthpiece assembly a few inches above diaphram level will cause air to flow, due to a lesser pressure at the mouthpiece than at the diaphram. This sensitivity is utilized by the diver to "clear" his mouthpiece by the simple expedient of rolling upon his back, or raising the mouthpiece above diaphram level and allowing the air to "free-flow", thus clearing the mouthpiece or hose of water.

Single Hose Regulators

The single hose regulator differs from the two hose in that the first and second stages are separated by a length of high pressure hose; with the second pressure reduction taking place in the mouthpiece assembly. The component parts play the same roles that they do in the two hose regulator. However, because the second stage with the diaphram, is at the same level as the mouthpiece, the "free-flow" principle has been replaced by a purging button, which manually depresses the diaphram.

Valves General

The following valves are those currently being used in scuba tanks. They are of principally three types, non-reserve (K), depth-compensated, and spring loaded. The latter two are reserve type valves, and should be differentiated between.

K-Valve

The K valve is simply an on-off valve, and contains no reserve features. Unless used with a regulator which features an incorporated reserve, this valve is not recommended.

J-Valve

The J, or spring loaded valve operates through the utilization of a spring loaded check valve. The spring is calibrated to approximately 300 psi. As long as the air pressure in the tank is in excess of 300 psi, the valve is held off its seat. However, as the tank pressure approaches 300 psi, the valve begins to close, thus restricting the flow of air to the diver. If the pressure is allowed to drop to 300 psi the valve closes and the diver must then manually pull a lever, which bypasses the check valve and enables the diver to breathe the remaining air in the tank.

Depth-Compensated Reserve

The depth-compensated reserves rely upon a calibrated orifice for their function. That is, the amount of air that will flow through a mathematically calibrated orifice is dependent upon the pressure differential across the orifice. In other words, if tank pressure is high, enough air will flow through the orifice to supply the diver with an adequate supply of air. However, as the tanks pressure drops, the diver will not be able to get enough air from the tank at depth. He must lower ambient pressure, or in other words, ascends, in order to get an adequate supply of air. These depth compensated reserves are calibrated so that a diver will have an adequate supply of air to enable him to breathe normally all the way to the surface. However, care should be taken when diving at shallow depths, for under some conditions a diver at shallow depth can "breathe through" this type of reserve, and find himself deprived of the safety which a reserve normally affords.

Today many manufacturers incorporate a reserve into their regulators, thus enabling the diver to utilize the relatively inexpensive "K" valve tanks.

QUICK RELEASE SAFETY HITCH

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