Common Types of Second Stage Regulators
The operating principle behind all common second stage regulator designs is essentially the same: negative pressure created during inhalation causes the diaphragm to press inward, pushing a lever that pulls open the low-pressure valve seat, allowing air to flow from the hose into the interior of the second stage regulator. This includes downstream valve designs and balanced piston designs. This article introduces the detailed functions of the second stage regulator, with the hope of giving beginner divers a deeper understanding of diving knowledge.
Downstream Valve Design
Simple in construction and highly reliable, this design uses a spring to directly counteract the intermediate pressure from the hose. When inhaling, a lever uses the principle of torque to pull the spring and open the valve.
Disadvantage: A certain amount of suction is required to overcome the spring tension in order to open the valve.
Balanced Piston Design
Inside the second stage regulator there is an elongated balancing chamber. This balancing design partially offsets the intermediate pressure coming from the hose, allowing the spring tension to be reduced and achieving minimum inhalation resistance.
Venturi Effect (Vacuum Assist):
When inhaling, once the valve opens, the high-speed flow of air creates a negative pressure inside the second stage that helps the diaphragm continue to retract, maintaining a continuous supply of air. When inhalation stops, the excess air pushes the diaphragm back to its original position and closes the valve. This means you don't have to constantly exert effort while inhaling, increasing overall comfort.
Venturi Lever (Airflow Deflector):
Commonly labelled Dive / Pre-Dive. When the purge button is pressed in air causing a large free-flow of air, the high-speed gas flow generates negative pressure that draws the diaphragm inward. By changing the angle of the airflow deflector to create airflow resistance, the negative pressure is reduced, forcing the diaphragm back into position.
Inhalation Effort Adjustment Knob:
Uses the force of an assist spring to increase inhalation resistance or to prevent free-flow.
One-Way Exhaust Valve:
There is typically a rubber diaphragm on the underside of the second stage. When exhaling, if the gas pressure inside the second stage exceeds the ambient pressure, the gas is released through the exhaust valve. Because it only opens in one direction, water cannot flow back in. However, be aware that any foreign matter adhering to the valve could potentially allow water ingress.
The second stage also incorporates certain hose routing designs intended to improve inhalation flow — the goal is to direct incoming air straight into the mouthpiece rather than filling the interior of the second stage and counteracting the inward force of the diaphragm.
Regulator Fail-Safe Design
In the event of a first stage regulator failure causing a large amount of air to be released, the excessive intermediate pressure will force the second stage valve to remain open, allowing high-pressure air to flow freely from the mouthpiece. A well-trained diver should be able to breathe from a freely flowing regulator, providing enough air to reach their dive buddy or the surface.
Regulator Performance and Selection
We should prioritise first stage regulators that can deliver stable intermediate pressure and high flow rates. When the intermediate pressure of a first stage fluctuates too widely — for example, a downstream piston first stage with intermediate pressure ranging between 8–10 bar — to prevent the second stage from free-flowing due to excessively high intermediate pressure, the spring must be set to its maximum tension for a 10 bar environment. This significantly increases inhalation resistance. The difference becomes even more pronounced during deep diving, as increased gas density raises flow resistance. Additionally, a higher output volume can supply sufficient air during heavy exertion or air sharing. A good regulator, beyond breathing comfort, can also help prevent shortness of breath and hyperventilation.
After reading this article, you should now have a general understanding of regulators. But once you've purchased a regulator, how should you go about routine maintenance and servicing?
Further reading: Regulator Routine Maintenance.
