Also known as the demand valve, the primary purpose of the second stage is to reduce intermediate or hose pressure to ambient pressure and deliver it to the diver on demand. In addition to the primary purpose the second stage also provides an exit for exhaled air and air when the purge button is pressed.

The diffrent types of second stage:  

• Upstream valves
• Downstream valves
• Pilot valves

We will also be looking at

• Materials and design
• Design of exhaust ports
• The venturi effect
• Cracking pressure or inhalation resistance

Upstream valves:

This is the old tilt valve that is still used when the hose pressure is not constant. Activation is by a lever in contact with the water pressure measuring diaphragm. If the depth is increased or the diver inhales, the diaphragm distorts inwards pushing the lever. The lever tilts the upstream valve off it’s seat allowing the air to flow. The manual purge simply acts on the diaphragm. A mushroom exhaust valve opens during exhalation allowing the air out but not the water in. The exhaust T guides the bubbles away from the face.

In the event of first stage failure and hihg pressure air comes down the hose, the valve will close tighter resulting in a hose rupture. To prevent this the first stages of these valves must be equiped with a pressure relief valve.

More information on upstream valves

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Downstream valves:

This is the most common system used today. Modern first stages can now deliver a very constant hose pressure and so the second stage valve can be set precisely. The operation is almost the same as the upstream valve, except that the lever lifts a small rubbet poppet off it’s seat, allowing the air to flow. Spring pressure closes the poppet once the diaphragm has been straightened.

The “cracking pressure” or amount of force needed to open the valve can be precisely adjusted, using a adjustable orifice (seat) or by adjusting the lever height or spring tension.

In the event of a first stage failure, the downstream valve simply free flows.

Find out more about downstream valves.

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Pilot valves:                                          (View diagram)

In order to get a marketing edge many manufacturers have brought out various models of pilot valves, some with limited success.

The characteristics of a pilot valve are:

• Noticeable delay on inhalation and shut off
• High air volume delivery
• Complicated
• Temperamental
• The pilot valve allows diffrent configurations resulting in various shapes and sizes.

How does it work?

The operations system is based on a small pilot valve being opened by the breathing action on the on a diaphragm. Opening this valve will drop the pressure in the chamber and allow the hose pressure to push open the main valve. On completion of the inhalation the pilot valve closes the pressure in the chamber builds up and closes themain valve. The main valve is usually a small rubber pad about 2 cm in diameter. The poseidon Jetstream uses a collapsing silicone bag that when inflated seals the exit holes.

Although popular with some divers pilot valves have met with limited success.

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Materials and Design

Like all diving equipment the type of materials used in the manufacturing, makes a great diffrence to the quality of the product. Most second stages today are made from plastics. Although lighter, cheaper and easier to make than the metal type but, they are not as robust and long lasting. In addition plastic housings are easily damaged during maintenance. Carbon fibre is also used today.

There are two main shapes:

Conventional, symmetrical with exhaust at the bottom:

• Most popular
• Easy to strip and repair
• Most familiar to most divers
• Easy to clear

Asymetrical with side exhaust:

• Often more compact
• Need to have the exhaust side lower in order to clear. This has caused several incidents with buddy breathing
• Can be used right or left sides
• Bubbles often interfere with divers vision.

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Design of exhaust ports

As the size of the exaust port increases, so the exhalation resistance decreases. This can be significant at greater depths where the air is much thicker. Unfortunately the larger the exhaust ports the more difficult it is to control water ingress. Techniques to overcome this include having two exhaust ports and bevelling of the exhaust valve.

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The Venturi effect

Moving air will pull still air with it, increasing the flow without further effort.

Manufacturers use this principle in order to increase the performance of their regulators. Many have internal vanes to increase the venturi effect, several have adjustable vanes that can increase or decrease the free flow effect with a switch. Others have increased this effect by spiralling the air flow. Harnessing the venturi effect significantly increases the performance.

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Cracking pressure or inhalation resistance

This is the amount of inhalation force needed to open the valve and start the air flow. This should be as low as possible. As the hose pressure on a good regulator is constantthis can be set exactly. It is adjusted by the factory or by a technician.

In an effort to “offer more” certain manufacturers have brought out models with a splined knob that changes spring pressure on the opening valve. By adjusting the knob the diver can adjust the cracking pressure. This may be usefull in heavy surf to avoid a free flow on entry or diving in a current. The down side is a more complicated unit and reduced performance. The advantages are more market related than realistic.