Piston vs Diaphragm Regulator First Stages

Scuba diver at the surface of a tropical dive site.
Both piston and diaphragm regulators work well. The choice is yours!. (Getty Images)

There are essentially three design types of modern scuba regulators commonly sold by all the major manufacturers: balanced piston, unbalanced piston, and balanced diaphragm. All of these designs refer to the first stage.

Why Is First Stage Design So Important?

The first stage of a scuba regulator does most of the hard work by reducing the high-pressure air in the tank (sometimes higher than 3000 PSI) to a stable intermediate pressure of around 135 PSI above ambient pressure.

The first stage is subjected to extreme pressure from the tank and must flow an adequate amount of air to supply as many as two second stages at any depth and at any tank pressure.

Piston First Stages

Piston first stages use a hollow metal piston in combination with a heavy spring to operate the high-pressure valve that separates tank pressure from intermediate pressure.

The piston consists of a head about 1 inch in diameter and a shaft about ¼ inch in diameter. The end of the piston shaft seals against a hard plastic seat, separating the two chambers in the first stage and sealing tank pressure from intermediate pressure.

When the regulator is not pressurized, the heavy spring keeps the piston shaft separated from the seat. As air flows in from the tank, it flows into the first chamber, through the piston shaft, into the second chamber. As air pressure in the second chamber increases, it pushes against the piston head on the opposite side of the shaft. When pressure in the chamber reaches intermediate pressure, it forces the piston against the seat and high-pressure air from the tank stops flowing. This process repeats with every breath!​

  • Balanced piston designs supply the same intermediate pressure regardless of tank pressure.
  • Unbalanced piston designs supply a slightly lower intermediate pressure as the tank empties.

There are advantages to both designs, although balanced piston first stages are considered higher performing and are typically more expensive than unbalanced piston first stages.

Advantages and Disadvantages of Piston First Stages


  • Simplicity, especially in unbalanced piston designs
  • Durability
  • The potential for very high air flow in balanced piston designs


  • The potential for freezing and free-flow: Part of the piston is exposed to surrounding water. In very cold conditions it can freeze open, resulting in a strong free-flow. Those who dive in extremely cold water often prefer diaphragm first stages for this reason. There are ways to seal the piston from water using silicone or PTFE grease, but this adds expense to servicing the regulator.

Diaphragm First Stages

Diaphragm first stages use a thick rubber diaphragm with a heavy spring to operate the valve between the two chambers in the first stage. This involves a slightly more complex design, as there are more parts used in the valve mechanism than in a piston-style first stage.

There is a pin and secondary spring on the inside of the regulator that operates the high-pressure valve. When the regulator is not pressurized, the heavy spring on the outside of the diaphragm pushes the diaphragm inward, which in turn pushes on the pin that separates a hard plastic seat from a metal orifice. When connected to a tank and pressurized, air flows into the regulator and pushes the diaphragm outward, which allows the hard plastic seat to seal against the orifice and stop the air flow when the pressure reaches intermediate pressure. This process also repeats with every breath.

One interesting detail of this design is that it is very easy to balance the valve so that the intermediate pressure does not change with tank pressure; in fact, all modern diaphragm first stages are balanced.

Advantages and Disadvantages of Diaphragm First Stages


  • Good cold water reliability: Most of the working parts of a diaphragm first stage are sealed from water, making the valve less likely to freeze open and reducing the risk of free-flow when diving in very cold water.
  • Easier to keep clean: As the working parts of a diaphragm first stage are sealed from the water, a diaphragm first stage is easier to keep clean and free of salt water corrosion than a piston first stage.


  • More parts to replace during servicing: Most diaphragm first stages are a little more complex than most piston first stages and a little more finicky to service. But good service techs can easily handle both piston and diaphragm designs.
  • The potential air flow is not as high as with the highest performance piston first stages: While this is factually correct, it’s a little like the difference between a car that can go 100 mph vs one that can go 200 mph. All good quality first stages flow plenty of air for recreational diving.

What to Buy

You tell me, what’s better: Ford or Chevy? Budweiser or Miller? Chicken or fish? The Spurs or the Lakers? (Well, that one is easy!) The point is, both designs work extremely well. There are some inherent advantages to each design, and these are small and hotly contested among regulator nerds. If you ever have trouble sleeping, consider doing an internet search for arguments for and against each type of first stage. Before you know it, you’ll be happily snoozing.

Keep in mind that the classic first stage designs have been around for several decades, almost unchanged since the days of the old double hose regulators. Jacques Cousteau used this style of regulator on thousands of very deep, very demanding dives. Remember this when a salesman tries to convince you that only the latest and greatest regulator design is good enough for you!

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