The Second Stage of a regulator takes the intermediate pressure from the first stage and reduces it to the ambient pressure that corresponds to the depth that you are diving in.

There are several factors that influence the performance of a second stage including:

1. Configuration
2. Downstream or Pilot Valves
3. Balanced or Unbalanced
4. Adjustable or nonadjustable
5. Venturi assist or not

Configuration

Configuration describes the basic design of the second stage. In most regulators the diaphragm is on the front with the purge button and the exhaust valves on the bottom and side of the second stage. This means that air is exhaled out of reg and then bubbles up the side of divers faces.

Some second stages have the diaphragm and the purge button on the side or at the rear on the bottom. This means that there is no right side up or upside down. However, second stages with this configuration can drizzle a bit which can be annoying.

Bottom line: Having the purge and diaphragm in the front with the exhaust valves at the bottom is the best choice.

Downstream or Pilot Valve
Downstream valves are the most common. With this design when you breath in the diaphragm pushes against a demand lever connected to a one way valve. This causes the valve to open, giving you air.

These are mechanically simple and highly reliable. They are also very easy to breath from. Also, if it does fail, it fails in the open position causing a free flow regulator which as you know from you Open Water course, you can still breath from.

Pilot valves are normally with regulators that have the diaphragm on the side or bottom. When you inhale the diaphragm pushes a lever on a small valve. This releases air pressure that then opens a larger main valve. This design means that it provides greater airflow with less effort. However, there is a “flutter” effect which happens in shallow water where there is a noticeable delay between the pilot valve opening and the main valve opening.

Bottom line: Downstream regulators are simple and effective. Pilot Valve second stages can be more expensive and complex and the flutter effect can be rather disconcerting.

Balanced or Unbalanced

Second stages can also be balanced or unbalanced, just like first stages. Ultimately it doesn’t have too much effect on performance, however, by balancing the valve the manufacturer can use lighter materials and reduce the wear and more finely tune the performance.

Bottom line: More and more high end second stages are balanced, but it doesn’t effect performance substantially.

Adjustable and non adjustable

All second stages have an internal preset adjustment that sets the amount of air that is released with every inhalation. Some second stages have an external ajdustment that divers can also use. This allows the diver to increase and decrease the ease at which it is to breath.

The benefit of having an adustable second stage is that on the surface you can reduce the flow, decreasing the possibility for it to free flow on the surface. Also, you can set the alternate air source so that it is less sensitive and then increase the flow when it is actually being used.

Bottom line: Adjustable second stages are better.

Venturi Assist or Not
Another way of reducing breathing resistance in a second stage regulator is by taking advantage of the Venture principle. This is the principle that a moving gas creates low pressure. By using Venturi, as you inhale the gas flow low pressure reduces the effort you need to pull down the demand lever. To trigger Venturi, the second stage outlet aims down the mouthiness opening. This works so well that some Venturi-assisted second stages free flow very easily. This is why you always need to put your reg into the water mouthpiece down!

To offset this there is a predive and dive control that swings a vane into the air flow to disrupt the Venturi principal, keeping it from flowing so easily.

Bottom line: Venturi-assisted second stages free flow easily, but if pre-dive and dive controls are there, it can be prevented.

• R 295 Second Stage