The Self Contained Underwater Breathing Apparatus was invented by Jacques-Yves Cousteau in 1943. He called it the 'Aqualung' and prior to its invention divers used what was known as the Standard Diving Suit; consisting of a suit made of rubberised leather, lead boots and a brass helmet. Air was fed to the diver through a hose under pressure from the surface. The diver was raised and lowered to the seabed on the end of a rope and consequently was restricted in his movements. The invention of the Aqualung gave divers the freedom to move about on the seabed, almost like a fish, and thus the sport of SCUBA diving was developed.

Single Stage Regulators

The earliest Aqualung equipment consisted of a cylinder with a single stage regulator clamped to the pillar valve at the top. From the regulator came two convoluted rubber hoses, one over each shoulder to meet at the mouthpiece. The mouthpiece contains two simple butterfly valves that allowed air to enter the mouthpiece from the left-hand hose and exhaled air to go down the right-hand hose. The regulator consisted of a diaphragm linked to a spring-loaded valve. One side of the diaphragm was open to water pressure.

As the diver descended into the water the increasing water pressure acted on the diaphragm easing the pressure on the spring, thus allowing the air to be delivered at an ever-increasing pressure. When the diver inhaled, it had the effect of reducing the pressure on the inside of the diaphragm, thus opening the valve and delivering air through the left hand hose to the mouthpiece. As the diver exhaled through the right hand hose, the exhaled gasses increased the pressure on the inside of the diaphragm and closed the valve. The gases where then vented through a one-way valve to the water.

The problem with this arrangement was that the response to increasing pressure was not linear. This meant that if the regulator was adjusted to deliver air at the correct pressure at a shallow depth, by the time the diver got down to any depth, air was being delivered at an excess pressure. This didn't do the diver any harm as excess air was vented through the exhaust hose, but it meant a lot of air was being wasted. If the regulator was adjusted to provide air at the correct pressure at a deeper depth, it meant that breathing at shallow depths was difficult, the diver having to suck that little bit harder to get air. Also, as the pressure in the cylinder reduced, breathing got increasingly harder.

Two-stage Regulators

Enter the two-stage regulator. The first stage is again clamped to the pillar valve on top of the cylinder, connected by a single hose, called an intermediate pressure hose, to the second stage, which is incorporated into the mouthpiece. A piston valve replaces the diaphragm in the first stage. This is set up to deliver air to the intermediate pressure hose at approximately six bars above ambient pressure. The second stage contains a diaphragm and lever arrangement similar to the old single stage regulator described above. Exhaust gasses are vented straight to the water.

As the diver descends, the absolute pressure in the intermediate hose increases accordingly, but always remains six bars above ambient. This means that the second stage, also called the demand valve, is working to a constant pressure differential between the air being demanded and the air being delivered. Regardless of the pressure in the cylinder or variations in depth, the diver gets air at the same relative pressure. This works fine until the pressure in the cylinder is reduced to six bars plus ambient pressure, by which time the cylinder can be considered to be empty.

Modern cylinders are filled to a pressure of 3000 psi or approximately 200 bars. Safe diving practice requires that the diver begins to surface when the pressure in the cylinder gets down to 30-50 bar depending on depth.

Go here for a discussion on what pressure is.