Regulators

You can buy regulators from a great number of suppliers.

Function

Main function of the regulator is regulating the air flow from the cylinder to the diver. On one hand we have the cylinder with pressurized air. The pressure of this air ranges from 200 bar to about 50 bar, depending on the amount of air left in the cylinder. On the other hand we have the diver who wants to stay alive under water and thus needs to breath air. Since the divers lungs are extremely sensitive to over pressure, the diver needs to breath are at the same pressure as the water pressure around him: this pressure ranges from one to a few bars, depending on the depth he is diving. At the surface the pressure of the air the diver breathes is one bar, for every ten meters of depth he is diving one bar is added. Between the cylinder and the diver comes the regulator.

Modern regulators consist of two stages: the high pressure stage (or first stage), which is located on the top of the cylinder, and the low pressure stage (second stage), which is the mouthpiece of the regulator. The high pressure stage reduces the high pressure of the air flowing from the cylinder to an intermediate pressure of around 10 bar. So the air in the tube from the first stage to the second stage flows at this pressure.

The unbalanced high pressure stage

The pictures below show a cross section of the most simple high pressure stage: the unbalanced high pressure stage. The high pressure from the cylinder enters at the top entry. This entry is sealed off by a piston (light blue). The chamber at top of the piston and the one below the piston are filled with air at the intermediate pressure. They are connected by the channel through the center of the piston. To the upper chamber the tube to the divers mouthpiece is connected. The chamber in between is filled with water, which can enter this chamber freely by channels through the regulator housing. In this chamber there is a spring (green) pressing the piston down.

There are four forces acting on the piston:

1. The spring pushing the piston down
2. The air underneath the piston (at intermediate pressure) pushing the piston up
3. The water pushing the piston down
4. The high pressure at the valve seat, pushing the piston down

At equilibrium all forces cancel out. When the diver takes a breath, the intermediate pressure goes down. Because of it the air under the piston cannot keep the piston up, so it is pushed down by the other forces, openening the high pressure inlet. Because of this air flows from the inlet, increasing the intermediate pressure until the equilibrium is estableshed again and the inlet is closed.

One can show that the intermediate pressure value is given by:

Here is A_piston the area of the piston, A₂ the area of the piston in the water chamber, A_highport the area of the high pressure inlet, P_water the water pressure (depending on depth), P_high the high pressure, P_im the intermediate pressure and F_spring the force the spring excerts on the piston.

This formula shows that when the cylinder becomes more and more empty (P_high decreases) the intermediate pressure decreases as well. For the diver it becomes more difficult to breath.

The balanced high pressure stage

The problem of decreasing intermediate pressure with decreasing high pressure is solved in the balanced stage. The picture below shows the principle: the piston (light blue) is pushed down by the spring and the water pressure (water can enter the spring chamber by channels through the housing). High pressure air enters the stage so the intermediate pressure in the chamber under the piston rises until it becomes large enough to push the piston into it's seat at the top (dark grey). This closes the intermediate pressure chamber. When the diver takes a breath the intermediate pressure decreases. The piston is pushed down by the spring and the water pressure, opening the intermediate pressure chamber and letting high pressure air in until the equilibrium is established again.

As one can see there is no way the high pressure can excert a force on the piston in the direction of its movement: the intermediate pressure value is given by:

Dry sealed balanced high pressure stage

The dry sealed balanced high pressure stage is a balanced stage in which none of the moving part is in contact with water. However, the water pressure is transmitted to the piston by a ingenious system of sealed membranes and pressure transducers. Though the dry sealed stage seems rather complex, the principle is virtually the same of the balanced stage.

The small center compartment is the high pressure compartment. High pressure air from the cylinder enters at the leftside. The piston (light blue) rests on the valve seat and seals of this compartment. If the piston opens the valve (the piston is pushed upwards by the large spring) air flows in the intermediate pressure compartment below. The pressure rises, so the upper membrane (purple) is pressed downwards. Since the mebrane is attached to the piston by a force transducer (yellow collored), the piston moves downwards, closing the valve. If the diver breathes, the intermediate pressure drops so the force pushing the upper membrane down decreases. The spring pushes the piston upwards, opening the valve, letting air flow from the high pressure compartment into the intermediate pressure compartment until equilibrium is established again.

The water pressure excerts a force on the lower membrane. This force is transduced to the piston by the pressure transducers (colored yellow). If the diver dives deeper, the water pressure increases. The force pushing the piston upwards (opening the valve) increases. This makes the intermediate pressure rise (pushing the upper membrane down) until it cancels out the force due to increased water pressure.

The intermediate pressure acts on the lower and upper side of the piston. Air flows from the high pressure compartment into the intermediate pressure compartment below. From this compartment the air flows through channels in the regulator to the most upper compartment. Usually the intermediate pressure hoses are connected to this compartement (often this compartment is a rotating turret). From the upper compartment the intermediate pressure air can reach the upper side of the piston. The forces on the upper and lower side of the piston cancel out, so the net force is zero. So the same equation of forces acting on the piston as for the balanced first stage holds for this one:

A_{UpperMembrane} is the area of the upper membrane. A_{LowerMembrane} is the area of the lower membrane. Since the forces on the upper (inner) membrane are larger than those on the lower (outer) membrane, the inner membrane is tougher.

As an example I present the Apeks TX40, which is the regulator I use. In the drawing below all the parts that make up the first stage are drawn.

A photograph of the first stage is showed below. You can find more info about this particular regulator and its operating on the Apeks site.

The great advantage of keeping water out of the regulator is preventing corrosion and freezing when used in cold water.