Balanced vs Unbalanced, Piston vs Diaphragm - Regulator Basics for Beginners

A female scuba diver checks her dive gear and regulators.
All modern regulator designs work well for recreational diving. © istockphoto.com

This article builds on concepts from Regulator Basics for Beginner's Part 1: How Does a Scuba Regulator Work?

With so many different styles of scuba diving regulators available, choosing a regulator may seem daunting to a new diver. Features such as piston or diaphragm first stages, and terms such as balanced and unbalanced may seem confusing to a novice. This article aims to demystify the terminology and features of scuba regulators so that divers may make an informed decision when purchasing regulators.

What Is a Balanced Regulator?:

A balanced regulator works the same no matter what pressure remains in a scuba diver's tank.

Balanced vs Unbalanced First Stages:

What is the difference between balanced and unbalanced first stages?

• Balanced First Stages:
A regulator's first stage feeds air to the second stage at an intermediate pressure (lower than the tank pressure, but higher than the ambient pressure at which a diver breathes the air).

A balanced first stage supplies air at a constant intermediate pressure, regardless of the pressure remaining in a scuba diver's tank. This is important because first stages must function with very wide range of tank pressures, such as 3000 psi in a full tank to under 500 psi as a diver depletes his air supply.

• Unbalanced First Stages:
Unbalanced first stages will supply air to the second stage at a lower pressure as a diver's tank empties. When combined with an unbalanced second stage, a diver's breathing effort increases slightly as the tank approaches empty. In modern designs, unbalanced first stages are always piston-style (see below).

What Are the Benefits of a Balanced Regulator?:

When using an unbalanced regulator, breathing resistance increases slightly as a diver's tank pressure drops. The key word here is slightly.

I have compared balanced and unbalanced regulators and found that at recreational diving depths there is very little difference in breathing resistance between balanced and unbalanced scuba regulators until the tank is well below 500 PSI.

Most conservative divers surface with a reserve of at least 500 psi, and should be on the surface before the tank pressure drops enough to affect ease of breathing. For these divers, the benefits of a balanced regulator are questionable.

Interestingly, some older regulators and tank valves incorporated deliberate increases in breathing resistance as the tank emptied so that divers in the pre-pressure-gauge era would have ample warning that they were about to run out of air. Some diving practices have really changed!

Should You Buy a Balanced Regulator?:

It's up to you! When shopping for a scuba diving regulator, remember that balanced and unbalanced regulators deal with changes in depth in exactly the same way, and for all practical purposes there is no difference in depth performance for recreational diving. The only difference between balanced and unbalanced first stages is that tank pressure affects unbalanced regulators when tank pressures are lower than most divers will ever experience.

The take home message? If a salesman tells you that an unbalanced regulator is only acceptable for very shallow dives, don’t believe it!

Piston vs Diaphragm First Stage Regulators:

Here are the basic differences, as well as the advantages and disadvantage of piston vs diaphragm first stages.

Piston First Stages:

Piston-style regulators use a rigid, hollow piston with a heavy spring to operate the valve between the two chambers in the first stage. The end of the piston shaft seals against a hard plastic seat, sealing the two chambers in the first stage from each other.

Most of the time, the piston is separated from the seat, allowing air to flow through its hollow shaft into the second (intermediate) pressure chamber in the first stage. When intermediate pressure builds up in the second chamber, the piston is forced against the seat, and high pressure air stops flowing into the second chamber.

Advantages of a Piston First Stage
• Simplicity
• Durability
• The potential for high air flow
Disadvantages of a Piston First Stage
• 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. 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-style regulators 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.

Advantages of a Diaphragm First Stage
• Less likely to freeze open

Most of the working parts of in 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.
Disadvantages of Diaphragm First Stages
• More parts to replace during servicing
• The potential air flow is not as high as with the highest performance piston first stages

Should You Buy a Diaphragm or a Piston First Stage?:

What a great question! 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, but these are small and hotly contested among regulator nerds. In fact, 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. It’s worked for my wife on many occasions.

Keep in mind that one of the classic diaphragm first stage designs has 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!

The Take-Home Message About Regulator Features:

Depending upon his needs, a diver may elect to purchase balanced or unbalanced regulators with either diaphragm or piston first stages.

His choice may be based on his available funds or on the type of diving he does. All commercially available regulators today undergo rigorous testing and will work well for recreational diving scenarios. If a diver sticks with well-known brands, he won't go wrong!

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