FROM DA KINE REPAIR BENCH

Many people who scuba dive do not really understand how the equipment they own actually works. This is through no fault of their own; most people who sell scuba equipment would be hard-pressed to give a detailed explanation of how air is delivered to the diver from an air cylinder. This is the first of several installments that will discuss how regulators work. It is my hope that after this series of discussions, you will have a more thorough understanding of scuba regulator systems and how they work.

The scuba regulator delivers high-pressure air from a scuba cylinder to the diver at a lower breathable pressure, on demand. This air must be delivered at ambient, or surrounding, water pressure. Modern scuba regulators utilize a single hose, two-stage design to accomplish this: a first stage, which attaches to the cylinder valve, and a second stage, which is attached to the diver’s mouth.

The regulator first stage reduces the high-pressure air from the scuba cylinder to approximately 120-150 pounds per square inch (psi) above ambient pressure, which we call intermediate pressure. It must be able to compensate for decreasing cylinder pressures, as well as changing ambient pressure as the diver descends and ascends in the water column. There are two basic first stage designs (with variations) in use today; they are piston and diaphragm. Piston and diaphragm regulators can be further classified as balanced or unbalanced.

The Piston First Stage: These first stages use a piston and main spring to deliver air to the second stage. When a diver inhales, a low-pressure area is created in the second stage; this is connected to the first stage via a low-pressure hose. The pressure in the compression chamber (behind the piston head) drops, which allows the main spring to push the piston into the open position, allowing air to flow into the second stage. When the diver ends the inhalation cycle, air pressure builds up behind the piston head (the compression chamber) until it equals the opening force of the main spring, allowing the piston to close, ceasing airflow. Changes in ambient pressure are sensed by the piston first stage in a special chamber, which is open to water. This ambient chamber (which also houses the main spring) allows ambient water pressure to be directly applied to the spring side of the piston.

The Diaphragm First Stage: The diaphragm first stage separates the compression chamber from the spring/ambient chamber (and outside water) by using a flexible diaphragm. When the diver inhales, a low-pressure area is created in the second stage; this is connected to the first stage via a low-pressure hose. As the intermediate pressure drops, the main spring outside of the diaphragm deflects the diaphragm inward, which pushes a pin to move the high-pressure seat from its seating surface. This allows air to flow to the second stage. When the diver ends the inhalation cycle, the air pressure in the compression chamber builds until it is equal to the opening force of the main spring. This deflects the diaphragm outward, allowing the high-pressure seat to seal onto the seating surface, ceasing airflow.

The Second Stage: The second stage reduces the intermediate pressure from the first stage to ambient, or surrounding pressure, and delivers it to the diver upon demand. When a diver inhales, the pressure inside the case drops. This vacuum creates a pressure imbalance across the second stage diaphragm (the outside pressure is greater than the internal pressure), deflecting the diaphragm inward. This causes the demand lever to move downward, which in turn retracts the seat and spring assembly, allowing air to flow into the case. When the diver ceases to inhale, the air pressure inside the case increases, pushing the diaphragm outward, until the water pressure outside the case is equal to the air pressure inside. The demand lever moves upward, causing the spring to push the seat toward its seating surface, until the valve closes. The mating of the poppet seat to the sealing orifice stops the airflow into the case. When the diver exhales, the exhaled air (and any water) exits the case through the exhaust valve.

In the next installment, I will discuss the differences between balanced and unbalanced first and second stages, as well as venturi assist designs incorporated into second stages.

In my last installment, I briefly discussed how air is delivered from the scuba cylinder to the diver, using piston and diaphragm style first stages, and basic second stage function. I will now discuss the difference between balanced and unbalanced first and second stages.

Balanced vs. Unbalanced: We often hear these two terms used when describing a regulator system. What exactly do they mean? You may have a member of your family that you consider unbalanced; does this also mean that your unbalanced regulator is emotionally unstable, unable to hold drink or job?

The main difference between a balanced and unbalanced first stage is that a balanced first stage is not affected by diminishing cylinder pressure throughout the dive; the intermediate pressure output remains relatively the same from full cylinder to empty. This is accomplished by allowing all surfaces exposed to incoming cylinder pressure, also known as linear, or downstream force, to be symmetrical; that is, for the pressure that attempts to close the valve, there is an equal opposing pressure that attempts to open the valve. Let’s use the typical balanced flow-thru piston first stage as an example. The main spring (the opening force) may have an opening pressure of 140 pounds per square inch (psi). Incoming cylinder air is allowed to travel through the hollow bore of the piston until it builds enough closing pressure behind the piston head to overcome the opening force (140 psi) of the main spring. Since the piston surfaces exposed to the incoming cylinder air is relatively symmetrical, cylinder pressure has no effect on the movement of the piston, allowing for a stable, constant intermediate pressure of 140 psi. The main advantage of a stable intermediate pressure is the finer tuning of the second stage; the second stage spring does not have to accommodate varying intermediate pressures.

In the unbalanced first stage, the intermediate pressure output changes in direct proportion to the diminishing cylinder pressure. This is because the surfaces exposed to incoming cylinder air are not symmetrical. Let’s use the typical unbalanced piston first stage as an example. The main spring (the opening force) may have an opening pressure of 110 psi. Since there is an uneven piston surface (the high pressure seat in the end of the piston) for incoming downstream force to push against, the opening pressure of downstream force may be 30 psi. The combined spring/downstream force pressure is 140 psi at a full cylinder (3000 psi). Cylinder air is allowed to pass through an access hole and travel through the hollow bore of the piston until it builds enough closing pressure behind the piston head to overcome the combined opening pressure of the main spring and downstream force (140 psi). As cylinder pressure diminishes during the dive to 500 psi, so does the effect of downstream force on the piston. This opening pressure of downstream force has decreased to 10 psi; the opening force of the main spring is a constant 110 psi; the combined opening pressure is now 120 psi. The air traveling through the piston until it reaches the chamber behind the piston head now only has to build a closing pressure of 120 psi to overcome the combined opening pressure of the main spring and downstream force. The intermediate pressure to the second stage is also reduced to 120 psi. This reduced intermediate pressure affects the breathing performance of the second stage. If the second stage spring is stiff enough to withstand the higher intermediate pressure at a full cylinder, the same second stage spring is now stronger at the lower intermediate pressure of a near empty cylinder, since it has less downstream force to push against.

In the balanced 2nd stage, the movement of the low-pressure seat carrier, called the poppet, is not affected by incoming downstream force of intermediate pressure. This is accomplished by several different methods. The most common method is to allow air pressure on both sides of the poppet to become equal. The poppet has a soft sealing material (the seat) which mates to the sharp conical edge of the orifice. Air from the low-pressure hose passes through the orifice and pushes against the poppet. This opening pressure is resisted by the poppet spring, which pushes the poppet seat against the sharp orifice. A small hole in the face of the seat allows incoming air to travel through the poppet and build pressure behind the poppet (spring side) in a balance chamber, until it equals the pressure in front of the poppet (seat side). This equal pressure on both sides of the poppet allows for the use of a much lighter spring to close the poppet against the orifice, resulting in an easy breathing regulator. If the 2nd stage were perfectly balanced, the pressure on both sides of the poppet would remain the same, regardless of incoming intermediate pressure; this would be undesirable (and dangerous!). In the event of a first stage failure, the poppet spring would not allow excessive intermediate pressure to vent (resulting in a free-flow), possibly allowing the hose to rupture or the second stage to be blown off the hose end. This is why balanced second stages are not perfectly balanced; excessive intermediate pressure will override the poppet spring at approximately 250-300 psi. This is accomplished by making the balance chamber opening slightly smaller than the orifice opening.

In an unbalanced second stage, spring force alone is used to counteract the opening downstream force of intermediate pressure from the low-pressure hose. A much stiffer spring is required to do this. For those of you who have been paying attention (I hope you’re not all asleep!), you may have been wondering how the relatively small poppet spring is able to counteract 140 psi of intermediate pressure from the hose. Is the closing force of the poppet spring 140 psi? If this were true, the amount of inhalation effort to retract the poppet spring to allow air into the second stage case would be near impossible! The answer is the amount of downstream force is reduced significantly by the size of the orifice opening. If the orifice opening were 1 square inch in surface area, then 140 pounds per square inch of intermediate pressure would be applied against the poppet seat. The orifice opening is much, much smaller than 1 square inch, reducing downstream force so a much lighter spring can be used.

Now that you know the difference between balanced and unbalanced regulator systems, why do scuba manufacturers build both types of regulators? The main reason is economy. Unbalanced regulator systems have fewer parts, and are less expensive to manufacture. Is one design superior to the other? The answer to this question depends on the situation. For the beginning entry-level diver looking for a dependable, less-expensive system to start their diving career, an unbalanced system may be perfect for them. They are also frequently found in rental and instructional use of many dive shops and dive charter companies due to low initial cost and maintenance. For the advanced diver, diving professional and "technical" diving, whose typical dives are more challenging due to depth, current, or environment, the balanced regulator system may be more appropriate.

The o-ring is a common item that most divers are aware of, but know very little about. We are told in our open water training to inspect the o-ring found in the face of the valve before installing our regulator; this is usually the extent of our knowledge concerning o-rings. However, the lowly o-ring is an extremely diverse part of scuba equipment that I will discuss in more detail here.

An o-ring is a torus (doughnut) shaped seal made from an elastomer, used to prevent the transfer of air or water from one part to another. How an o-ring seals is determined by many factors: the design of the o-ring groove, referred to as the gland; the type of elastomer used in the construction of the o-ring; the size of the o-ring, which include the inner and outer diameters, and the cross-sectional width; the relative hardness, referred to as the durometer; the installation stretch; the temperature the o-ring will be subjected to; and the amount of pressure applied to the o-ring. O-rings are generally divided into two groups, called static and dynamic.

Static o-rings have little or no relative motion between the sealing surfaces. A static o-ring seal includes seat seals, which close the passageway by distorting the face of the o-ring against the opposite surface. This is the most common seal in scuba, which include the valve face and base o-rings, port-plug o-rings, and high-pressure, low-pressure, and inflator hose o-rings at the first stage. Another type of static seal is the crush seal, which literally crushes the o-ring into a space with a cross section different from the standard o-ring groove. This type of seal is not normally found in scuba equipment, as the o-ring is permanently distorted and must be discarded after every use.

Dynamic o-rings create seals between surfaces where there is definite relative motion. The most common dynamic seal is the reciprocal seal, which seal reciprocal motion like that found in first stage piston o-rings. Oscillating seals are o-rings that seal a surface that moves in an arc relative to the other surface, as in a valve stem o-ring on your cylinder valve. Rotary seals are o-rings that seal surfaces that revolve around a shaft axis in one direction (the direction may be reversed), like those found in the low-pressure swivel between the second stage and hose.

The o-ring gland (groove) design determines the minimum and maximum clearance tolerance of the seal. If the o-ring is too small for the gland, the o-ring may extrude into the clearance gap, allowing an air or water leak. If the o-ring is too large for the gland, the o-ring may not allow the proper mating of the sealing surfaces.

The durometer of an o-ring is the measurement of relative hardness. This measurement is expressed as Shore hardness. Most o-rings are manufactured with a Shore A durometer between 60 and 75. The lower the Shore number, the softer the o-ring. O-rings that require more resistance to abrasion, compression set, or extrusion, usually have a Shore hardness of 90 or greater.

There are many types of elastomer compounds used in the manufacture of o-rings. An elastomer is a polymer in which the molecules are tightly coiled to give the property of elasticity. The following are some of the most common types used in scuba equipment:

Nitrile: This is the most common elastomer found in o-rings used in scuba equipment. Commonly called Buna-N, it is the copolymer of acrylonitrile-butadiene. It has excellent resistance to petroleum products, silicone greases and oils, and water. It also possesses good compression set resistance, cold flow, tear, and abrasion resistance properties. It does not have good resistance to ozone, sunlight, or weather. The temperature range (depending on compound composition) is -65F to +300F.

Silicone: Although this elastomer is not commonly found in scuba o-rings, I have listed it because many divers mistakenly refer to the amber-colored o-ring in some valve faces as silicone when in fact it is polyurethane. Silicone has poor tensile strength, abrasion and tear resistance, but has excellent flexibility properties over a wide range of temperatures. The temperature range is -80F to +450F.

Fluorocarbon: A copolymer of vinylidene fluoride and hexafluoropropylene, fluorocarbon elastomers are highly resistant to deterioration when exposed to many fluids and gases. It also exhibits low compression set, and is highly resistant to heat; it is this property that makes it a common elastomer in Nitrox applications. It is most widely known as Viton, which is a registered trade name of the DuPont Dow Chemical Company. The temperature range is -20F to +450F.

Polyurethane: Polyurethane, or urethane, is highly resistant to petroleum products, ozone, and oxidation. It exhibits high tensile strength and is very abrasion resistant. This is the amber-colored o-ring found in the valve face of some cylinder valves. The temperature range is -65F to +200F.

Ethylene Propylene: Commonly referred to as EP or EPDM (ethylene propylene diene monomer), it is a copolymer of ethylene and propylene. It has excellent resistance to ozone and sunlight, compression set, and most solvents. It is non-compatible with petroleum fluids and oils. It is considered inert in the presence of oxygen, and has become increasingly the elastomer of choice for EAN/Nitrox applications. The temperature range is -65F to +300F.

As you can see, there is more to an o-ring than meets the eye. That is why it is important not to replace a manufacturer’s o-ring with any "generic" o-ring purchased at the local hardware store. Quite often custom elastomer "recipes" and durometers are used in scuba equipment. If you have any scuba o-ring needs, make sure you have them replaced by a knowledgeable dive store professional.

You may have heard or read about the phenomena of neck cracks in high-pressure aluminum scuba and scba cylinders. In our industry, we refer to this as Sustained Load Cracking, or SLC. Several factors can contribute to SLC (formerly known as Room Temperature Grain Boundary Creep Cracking). The primary source of SLC’s was the use of 6351 aluminum alloy in the manufacture of the cylinder. Luxfer, the largest aluminum cylinder manufacturer in the world, used this alloy in the United States from 1972 through mid-1988, in England from 1967 through 1995, and in Australia from 1975 through 1990. Currently, Luxfer and Catalina use the aluminum alloy 6061-T6, which apparently is far less susceptible to SLC. Other factors include unusually high levels of lead occurring in the alloy, the method used in forming the crown of the cylinder, and elevated ambient temperatures (SLC’S are far more frequent in tropic and sub-tropic climates, like Hawaii).

Sustained Load Cracking manifests itself as a small radial crack, usually originating from a fold in the interior shoulder area of the aluminum cylinder, and may travel up through the threaded region of the neck. The crack may become large and deep enough to allow gas or air to leak directly through the cylinder wall in the neck area! The potential for cylinder rupture exists. Although this is rare in scuba cylinders, it has happened, taking both life and limb. The potential for rupture in high-pressure (4500 psi) scba cylinders with a SLC is far greater.

It is imperative to identify cylinders that have SLC’s and remove them from service. This can only be done by trained, qualified cylinder inspectors during the annual visual inspection of the cylinder. The first line of defense is the use of a magnifying inspection mirror and inspection light. Thorough visual inspection of the crown and threaded neck areas will usually uncover most cracks. The use of Non-Destructive Testing methods, like eddy-current technology, allows the discovery of sub-surface cracks and flaws in the alloy. Luxfer recently released a technical bulletin (July 17, 2000) requiring all Luxfer cylinders manufactured with 6351 alloy be tested with an eddy-current device such as Visual Plus or Visual Eddy machine. In an eddy-current test, a probe is threaded into the cylinder neck, transmitting an electromagnetic current into the adjacent metal. Interruptions in the current caused by a crack or flaw are displayed on a LCD screen as a spike across a threshold line. By rotating the probe up and down the neck area, a SLC location can be determined and documented.

All aluminum cylinders, regardless of make or manufacture date, are required to be inspected with a Visual Plus machine at Da Kine Scuba Repair. No cylinder will be filled at Da Kine Scuba Repair without proof of this eddy-current test, which is noted on the visual inspection sticker. This is to insure the safety of all involved, especially those most at risk of injury: the fill operator!