Features of a Diving Regulator

Internal parts of a Diving Regulator first stage

The Features of a Diving Regulator

Your scuba cylinder isn't very useful without a way to control the gas flow so that you can get exactly as much air as you need when you inhale.  Your respiratory system only tolerates a differential of about 0.14 atm/bar between the breathing air pressure and the pressure surrounding the lungs.  To breathe, you need air delivered to you at a pressure that's about the same as the pressure surrounding you.  That's what an open-circuit scuba regulator does.

A Regulator consists of a first stage and second stage mechanism, in simple terms the First Stage is the bit that attaches to the cylinder and the Second stage is the bit you put in your mouth.

First Stage

In choosing your regulator, you'll find two basic first stage designs, as well as balanced versus unbalanced versions of these designs, shapes and ports, environmental sealing and connector system

Piston and Diaphragm.  First-stage pressure-sensing mechanisms fall into two categories: piston and diaphragm.  In piston regulators, external water pressure acts on a piston, which is the main moving part in the type of first stage.  In diaphragm first stages, the pressure acts on a flexible diaphragm.  A push rod on the inside of this diaphragm transmits the diaphragm's movement to the valve mechanism

Balanced versus unbalanced.  Both piston and diaphragm first stages may use either balanced or unbalanced valves.  The difference has to do with a design that offset variations in cylinder pressure.  In an unbalanced valve, the cylinder pressure presses on the valve and assists opening it.  As you use up your air, the pressure lessens and therefore doesn't assist with opening the valve.  This causes a minor variation in performance.

A balanced valve uses a design that routes high pressure cylinder air around both sides of the valve opening.  This cancels out or balances the cylinder pressure so that the performance stays the same throughout your dive.

Today the majority of regulators have balanced first stages, however there are a few low end models that don't.  These don't perform as well, but are adequate for shallow recreational diving and may be a suitable option if you're for an inexpensive spare for your dive kit.

Shape and ports.  In choosing your regulator an important consideration is the ability to attach all your necessary hoses and route them so you have a streamlined setup.  Most top end regulators have at least four low pressure ports that deliver air at the intermediate pressure (used for your second stage, alternate air source second stage and low pressure inflators) and one or two high pressure ports that deliver air at the cylinder pressure (used for gauges and transmitters).  Some less expensive models may have few ( a consideration if dry suit diving, since that adds a hose) or you  may not be able to direction the hoses as you wish.

High pressure and low pressure ports have different sizes to avoid putting an accessory in the wrong place.

Some regulator first stages have portions that swivel.  The advantage of this is that it makes it easier to get the hoses where you want them.  However, when comparing two models, if they have adequate ports for your needs and route the hoses where you need them to go, the swivel is immaterial

Environmental Seal.  Some first stages have an environmental seal, in these first stages, water pressure doesn't act directly on the piston or diaphragm.  Instead it acts on a silicone or alcohol based fluid that seals inside a watertight pressure-sensitive barrier and transmits the pressure to the piton or diaphragm.  This seal offers two primary benefits... first it prevents salt, sediment and other contaminants from entering the first stage, thus reducing internal corrosion or contaminate buildup.  Second, it helps isolate the valve mechanism from freezing temperatures.  This is important in cold-water environments, where excessively low temperatures may cause the first-stage mechanism to malfunction by freezing.

Connector System.  There two basic types of cylinder to regulator fitting, DIN (up to 200 or up to 300 Bar) or A-Clamp (also referred to as Yoke) which is up to 232 Bar only. DIN is a much newer fitting which has been gaining popularity over the  last few years, particularly within the technical sector. Where once DIN was confined to Europe and A-Clamp the rest of the world, DIN cylinders are becoming much more readily available especially thanks to convertible valves that are fitted to the majority of new 232 Bar cylinders these days.The DIN fitting provides a safer coupling to the cylinder, successfully trapping the o-ring between the cylinder valve the the first stage. Permanent converters can be fitted by a qualified technician at a later date if a diver needs to change from A-Clamp to DIN but the added benefit of a DIN fitting is that screw on adapters are available to conveniently convert the fitting if the diver travels to an area where only A-Clamp cylinders are available.

Second Stage

Perhaps you notice an interesting little knob on the second stage labeled "Dive/Pre-Dive," "On/Off," or "+/-." This switch or knob alters the airflow inside the regulator, making breathing either easier or more difficult.
Turning the knob enables and disables something called the Venturi Effect, which regulator designers take advantage of to assist breathing. Click through the following pages to find out how it works, and when you should disable the Venturi Effect.
The second stage converts the intermediate pressure (approximately 10 Bar) provided by the first stage and reduces it further to ambient pressure for the diver to breathe.

Second stage options generally include configuration, downstream or pilot valves, balanced or unbalanced, adjustable or non adjustable and Venturi assist.

Downstream or pilot valve, second stages have two basic valve types.  Downstream valves are most common.  With these, when you inhale, the diaphragm pushes against a demand lever connected to a one way valve.  This causes the valve to open, supplying you with air.  Because the valve opens with the air flow, its called a downstream valve.

Downstream valves have several advantages.  First they are mechanically simple and therefore highly reliable.  Second, the air flows pushes against them, so that they are easy to set so they just barely closed.  This makes it easy to breathe.  Third if a downstream valve does fail,, it almost always fails in the open position.  While this means your air free flows, you can still breathe from it while ascending to safety (remember this skill as part of your open water training?)

Second stages that have the diaphragm on the rear / bottom or on the side usually have pilot valves.  With a pilot valve, when you inhale the diaphragm depresses a lever on a small valve.  The small valve releases air pressure that in turn opens the larger main valve.  The benefit of this design is that it provides greater airflow with less effort.  The drawbacks to pilot-valve second stages include complexity and expense.  They typically cost more and servicing generally costs more.  Pilot valves tend to free flow easily, so most have a dive / predate switch.  The predive setting puts tension on the diaphragm so it doesn't free flow as easily when it is not in your mouth.

Balanced or unbalanced.  As with first stage, second stages are available in either unbalanced or balanced variants with very similar advantages and disadvantages of those for first stages.   Up to the late 1990s, virtually all second stages had an unbalanced design. Basically balanced second stages will provide consistent performance and flow rates at any depth whilst unbalanced second stages will not be able to maintain the same performance as the depth increases.

Adjustable or non adjustable.  All second stages have an internal adjustment that your service technician sets when assembling the regulator after servicing and on initial manufacture.  Higher end second stages have an external adjustment which allows you to slight detune the regulator, temporarily to avoid free flowing which may occur if swimming into a current.  The other benefit is between services your regulator performance may decline slightly, but with an adjustable model you can set it so the performance remains high.

Venturi Assist​.  Some second stages you will notice an interesting little knob or lever labelled "Dive/Pre-Dive," "On/Off," or "+/-." This switch or knob alters the airflow inside the regulator, making breathing either easier or more difficult either on the surface or underwater.  When in the “off” setting it helps avoid free-flows at the surface.

Learn More about Regulators

You can find out more about regulators by attending one of our PADI Equipment Speciality or SSI Equipment Techniques courses where we take one apart and take you through the internals of a regulator - this is not a service technician course but a great course to understand what you can do to protect and look after your own dive equipment

We also have other articles that you can read to find out more, such as 


v1.01 Reviewed November 2020 / v1.00 Original May 2018