To fully understand the principals of the regulator we need to understand how valves work. Valves controls the supply of air baced on the influence of changing pressures.

We will briefly discuss the two main types of valves in regulators, namely upstream and downstream valves and have a look at balanced valves.

Valves consists out of two main parts:

• The seat - The stationary part
• Valve - The moving part

Upstream valves:                                  (View diagram)

This is a very simple method of controlling the air flow. The most common form was the tilt valve found in early second stages. Today only a few regulators are designed with the tilt valve.

Disadvantages:

• To minimise the closing force the orifice must be kept small. This reduces air flow.
• Air flow is further reduced by the opening rod passing through the orifice
• The tilt valves wear unevenly increasing the chance for leaks
• In the event of a first stage failure, the pressure can not be released and the hose can burst. To combat this problem a over pressure relief valve is nessacary on the first stage  

Advantages:

• Upstream valves are simple and cheap to make.
• They need no adjustment or tuning
• they can handle varied inter-stage pressure.

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Downstream valves:                             (View diagram)

Downstream valves are used in most modern regulator second stages. The force of air opens the valve and this is balanced by an adjustable closing spring. Varying the tension on the spring will vary the opening pressure.

Disadvantages:

• Downstream valves are more difficult to design
• More expensive to manufacture
• Only operates in a tight range of pressure
• Requires more care and maintenance

Advantages:

• A large orifice can be used for a better air flow
• The pressure is adjustable, allowing fine tuning
• In the event of over-pressurisation the valve will blow off

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Balanced valves:

With unbalanced valves the falling cylinder pressure will cause the intermediate or hose pressure to change during the dive. By having a balanced valve the cylinder pressure will have no effect.

More important however is that the designers of unbalanced valves are forced to have a smaller orifice to reduce the opening or closing effect of the high pressure air. The small orifice acts as a restriction and decreases the performance of the regulator at depth. With a balanced valves, high pressure air plays no part in the opening or closing of the valve. this means that wider orifices can be used and the valve can cope with higher cylinder pressures.