Have you ever wondered how regulators make it possible for scuba divers to breathe pressurized air underwater? Well, here's an illustrated explanation on half the equation: how a scuba diving regulator's second stage works.

We’ve come a long way since the early 1940s when Jacques Cousteau and Emile Gagnan co-invented the Aqua-Lung, an underwater regulator modified from Gagnan’s demand regulator that fed cooking gas to a car’s carburetor in the exact amount needed. Remarkably, today’s regulators rely on the same design principles used by Cousteau and Gagnan. You cannot breathe directly out of your tank because the high pressure would damage your lungs. Just like the Aqua-Lung, today’s second stages take pressurized air from the first stage and provide it to a diver at ambient pressure, allowing him or her to breathe normally underwater. Through the use of precision manufacturing, high tech materials and intensive testing, modern regulator design is efficient and reliable, and with proper maintenance and care, modern second stages will provide years of dependable operation.

1. The Parts: Mouthpiece, housing, cover incorporating a purge valve, flexible silicone diaphragm to separate the external water from the housing, demand valve assembly, demand lever and exhaust valve.

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2. How It Works: Air from the first stage enters the second stage housing through an inlet fitting. At the inlet, an orifice with a circular “knife edge” provides a sealing surface for the main valve assembly. This assembly consists of a poppet with a hard-rubber seat at one end, a bias spring and the valve body. The bias spring applies pressure on the poppet, pressing the seat against the edge of the orifice and creating an airtight seal. The demand lever is attached to the valve body and compresses the bias spring to pull the poppet away from the orifice, allowing air to flow into the valve.

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As you inhale, pressure in the second stage is reduced and the diaphragm is pushed inward by the surrounding water pressure. The diaphragm then pushes on the demand lever, opening the valve. With the valve open, air then enters the second stage through an opening in the valve housing until it equalizes with the surrounding water pressure, making it possible for a diver to breathe air at ambient pressure. When the diver stops inhaling, pressure inside the second stage increases, causing the diaphragm to relax and release the demand lever, closing the valve.

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As the diver exhales, air is expelled through the exhaust valves. The design of the second stage keeps ambient pressure in the housing at the same pressure as the surrounding water, ensuring that the reg enables consistent breathing effort regardless of conditions and depth.

3. Additional Features: Many regs have cracking pressure adjustment knobs, Venturi adjustment controls, and a balancing chamber in the demand valve. In a balanced second stage, a balancing chamber is added to the end of a modified poppet with a drilled-out center chamber and a hollowed-out seat. When a balanced second stage valve is closed, air travels through the opening in the seat and poppet into the balance chamber, pushing the poppet back against the orifice. This means a lighter bias spring can be used and less effort is required to open the valve. A cracking pressure adjustment manually changes the tension on the bias spring to increase or decrease the amount of effort required to open the valve, and the Venturi adjustment moves a rotating vane to direct air flow toward or away from the diver’s mouth. Both features potentially reduce breathing effort and aid in managing freeflows.