The first stage is probably one of the most important parts of a scua divers equipment. Without this small part of equipment breathing underwater is almost impossible. Here is all you need to know on how this piece of equipment works.

We also have a look at sintered bronze filters, environmental sealing and the materials used to make first stages

Function:

• To reduce the high pessure in the cylinder to a steady intermediate pressure of 8-12 bar.
• Acts as a distribution and attachment point for high pressure and low pressure hoses.

The different types of first stages:

Unbalanced diaphragm, Unbalanced piston, Balanced piston, Balanced diaphragm

Comparing the different types of first stages:

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Unbalanced Diaphragm:              (View Diagram)

This was the design of the original regulators that pioneered Scuba diving. This design is not really in use, in the unbalanced form, anymore.

How does it work?

The water and intermediate chamber is separated by a thick flexible rubber diaphragm (hence the name). Inhaling causes air to flow out of the intermediate chamber, lowering the pressure in the chamber. The lower pressure causes the diaphragm to flex inward. The flexing diaphragm pushes the rod connected to the poppet valve upwards, allowing high pressure air into the intermediate chamber. The increase in pressure will then straighten the diaphragm and allow the poppet valve to close.

Poppet opening force:

• Tension on adjustable spring in wet chamber
• The ambient pressure of the water

Poppet closing force:

• The small spring pressure
• The high pressure from the cylinder- because this drops during the dive the closing pressure will drop allowing the intermediate pressure to rise during the dive.

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Unbalanced Piston:                       (View Diagram)

This design was introduced in the 1950’s and is still in use today. It is ideal for dive schools because, they are easy to maintain and not a lot can go wrong with them.

How does it work?

In order to understand the principles of this design, we need to have an understanding of forces and pressures. The forces exerted on an object depends on the pressure and the surface area the the pressure the acts on. This means that a low pressure on a big surface area can exert a force greater than high pressure on a small surface area.

The piston is hollow and T-shaped. It moves back and forth in a smooth cylinder and has an O-ring as a seal along the circumference of the piston. The wide part of the piston has a large surface area. This allows the low pressure or intermediate pressure to have a strong influence, compared to the small area and high pressure of the valve seal area.

when the diver inhales the intermediate or hose pressure drops, this will cause the piston to move down. Remember the hose pressure is connected to the chamber at the bottom of the piston by the hollow piston stem. Air now flows down the hose from the high pressure side until the diver stops breathing. The hose pressure now builds up and pushes the piston from the bottom. Remember the effect of low pressure air on a large surface.

The opening forces are:

• The spring - If you increase the spring pressure you will need a higher hose pressure to close the valve.
• The high pressure air acting on the valve - As the pressure in the cylinder drops so will the hose pressure as it now needs less force to close the valve.

The closing force is:

• The build up of hose pressure on the large piston face.

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Balanced Diaphragm:                          (View Diagram)

The design is almost the same as the unbalanced diaphragm, there is however the addition of a small balancing mechanism. This is a small low pressure chamber that fits over the hollow stem of the poppet valve. This low pressure chamber is known as the balancing chamber.

How does it work?

The low pressure air comes from the low pressure chamber and is at hose pressure. This enters the balance chamber through a hollow poppet stem. The result is a balancing of pressure on both sides of the valve, through the hollow poppet stem. The result is a balancing of pressure on both sides of the valve.

The technical complexities of manufacturing a balanced diaphragm first stage, means that there are very few brand names that actually design and make there branded regulators. Most first stages are unique to the brand name and the research and development cost result in the jealous guarding of the patent.

The balanced knife edge piston design is however manufactured by several factories.

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Balanced Piston:                                  (View Diagram)

This is the most widely spread first stage and is often called the knife edge piston design.

How does it work?

The piston has its edge sharply machined. This sharp edge then seals against a Teflon seat pressed into the end of the high pressure chamber. The high pressure air has no effect on the valve as it surrounds the stem and does not act on an end. This design is very easy to manufacture.

This very simple design suffers however from a few problems:

• Moving parts are in contact with water and sediments.
• The Teflon seat wears, resulting in changed hose pressures
• The knife edge is very susceptible to damage

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Materials used for first stages:

The majority of first stages are made of chrome plated brass. A high quality regulator should have a uniform thickness of chrome and close tolerances with the machining. A few first stages are made of stainless steel which is much harder and resistant to damage than brass. The latest technique is to have titanium plated brass and titanium inside parts. Titanium is very hard and corrosion resistant.

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Environmental sealing:

Many regulators come with a environmental exclusion kit that prevents water and sediments from entering the first stage. The sealer is usually in the form of a flexible silicon sealer that is injected into the water chamber. Because there is a drop in the responsiveness of the regulator, this is usually only done to prevent ice up in cold water diving.

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Sintered bronze filters:

A cylinder often has foreign particles that can damage a regulator if penatrated. To prevent this there is a sintered bronze particle filter at the interface with the cylinder. These are usually flat but some ae conical.

After use a filter may show a different colour indicating likely contaminates:

• Reddish brown - Rust from a steel cylinder
• Black - Carbon from compressor filter breaking down OR Oil from a faulty compressor
• Turquoise, green or white - Salt water
• Paint like flakes - Certain cylinders are lined and this means that the lining is breaking down