A regulator system is required to reduce the pressure of the compressed air contained in the cylinder to ambient pressure to supply breathable air when needed. It is also possible to connect pressure gauges (analog or digital), IP inflators to provide buoyancy compensators, dry suits and other devices to this system. The regulator system is composed of a pressure reduction device and one or more breathing devices. In this article, the pressure-reducing device and the breathing device will be indicated, respectively, by the terms “first stage” and “second stage.”

First Stage

Regulators can use either a piston assembly or a diaphragm assembly. The piston or diaphragm controls and reduces the air pressure from high pressure to moderate pressure in the first stage. Either design may be equally good and equally sensitive to the diver’s inhalation needs except under conditions where high flow is required: there the piston regulator delivers much more air than the diaphragm regulator. The first stage uses a standard piston, balanced piston or diaphragm mechanism. Air is compressed and stored in the tank under high pressure. To reduce this pressure, the first stage is a valve or piston that lowers the pressure to about 140psi to let air into the hose. The valve opens to allow some air into the hose and then closes again. A regulator compensates the pressure as it decreases in the tank when the air is used and as the diver changes depth which causes change the ambient water pressure.

Standard Piston

Standard piston first stages are the simplest mechanism with minimum moving parts to control the pressure drop from a tank to feed the second stage. Which translates to high reliability and durability, combined with minimal maintenance requirements.

Balanced Piston

First stages with air balanced pistons deliver significantly more air to the second stage than any other first stage, while their performance is unaffected by the changing tank pressure. A balanced system allows the use of lighter and more sensitive components, resulting in ultra fast breathing response, instant delivery of air on demand and extra high airflow.

Balanced diaphragm

The inner mechanism in a diaphragm first stage is isolated from the surrounding water. This feature makes it especially suitable for diving in cold-water conditions or murky water. In this system, the air flows through a seat & pin assembly, controlled by a loaded diaphragm. The seat receives equal inter-stage pressure from both sides, making it react independently of tank pressure. Balanced diaphragm systems deliver a slightly lower flow than piston regulators, due to the smaller diameter air passageways. However, these differences in performance are only detectable at great depth. Cold-water divers typically prefer diaphragm regulators because they are less likely to free flow in cold water than are piston regulators.

First Stage Body 

This metal cylinder contains the mechanisms that reduce the high-pressure air in the scuba tank to an intermediate pressure. High-pressure air flows in one side of the first stage body undergo pressure reduction and then flows out through the low-pressure hoses.


The regulator first stage body is held against the scuba tank’s valve through one of two methods: a Yoke or a DIN fitting. This diagram illustrates a yoke fitting, also called an international fitting. The “yoke” is the metal oval that fits over the tank valve to hold the regulator in place. Two types of coupling fittings are used to attach a regulator’s 1st stage to the tank: DIN and Yoke.

Yoke couplings are more commonly found worldwide and are generally always used by the ever-popular aluminum 80 tanks.

DIN fittings are safer than yoke fittings and are the only fittings that can couple with high-pressure tanks. The advantage of DIN fittings is that they screw into the tank valve and trap the high-pressure o-ring so it cannot protrude, this can occasionally happen on a Yoke.

Yoke Screw

The regulator’s yoke is equipped with a yoke screw–a metal screw that runs through the regulator yoke and tightens the regulator first stage body onto the tank. To tighten the yoke screw, the diver turns the black, plastic handle attached to the screw.

Dust Cap

Keeps water from entering the regulator first stage body when not connected to the tank. The dust cap is a rubber cap that can be placed over the regulator first stage opening and tightened down using the regulator yoke screw. This seals closed the opening on the first stage.

Port/ Port Plug

Regulator first stage bodies have multiple openings, or ports, that regulator hoses and transmitters can bed screwed into. Most, regulators have more ports than the standard number of hoses, which allows divers to position their hoses in a variety of configurations. These openings are called ports, and the plugs that close the regulator ports when they are not in use are called port plugs.

Second stage

The second stage of a regulator is basically the part that goes in your mouth and delivers air upon demand. It contains a mechanism that reduces the intermediate pressure in the hose coming from the first stage to the surrounding water pressure making it comfortable and easy to breathe. The second stage also contains a piston or diaphragm construction which starts and stops the airflow. The mouthpiece, an exhaust valve, and an emergency purge valve/button are all parts of the second stage. The exhaust valve lets the air escape into the water when you exhale. It is a one-way valve and does not allow water in. When the purge button is pushed, it forces air to flow continuously into the second stage chamber forcing any water out of the mouthpiece through the exhaust valve.

This unit is supplied, with the intermediate pressure coming out of the first stage through the low-pressure hose. It reduces pressure further to balance air with the ambient pressure. The second stage may be balanced or unbalanced and equipped with a Venturi effect control (V.I.V.A.) and/or with an inhalation resistance control.

Second Stages 

There are two main types of the Second stage, Air Balanced, and Downstream


Air-balanced valve technology provides the optimum breathing comfort preferred by demanding divers. The air-balanced valve technology of SCUBAPRO’s second stages fine-tunes the pressure of the air delivered by the first stage to decrease inhalation resistance to the lowest possible level. The result is an ultra-high airflow that remains exceptionally stable under all breathing conditions.


The classic downstream valve is the best solution for resorts and rental facilities worldwide, as well as many recreational divers. These second stages are particularly noted for their legendary safety and reliability. A specific inhalation effort is always required to overcome the spring tension and opens the valve that lets the air flow in.

Parts of the Regulator

1. Purge Button
The purge button is located on the face of the regulator second stage. The purpose of the purge button is to flood the second stage with air, forcing water out of the second stage. Divers use the purge button when the second stage has been allowed to fill with water–for example, when a diver removes the regulator from his mouth during the regulator recovery skill.

2. Ease of Breathing Adjustment
Most regulators has a lever or knob that allows divers to adjust breathing resistance. This feature helps to prevent regulator free flow (a state when air flows rapidly out of the regulator second stage without the diver breathing from it), which typically occurs when the breathing resistance has been lowered too much. A free flow can quickly empty a tank.

Many second stage adjustments have a setting labeled “pre-dive” to help prevent free flow at the surface, and one labeled “dive” for easy breathing once underwater. As a diver descends, he can adjust the ease of breathing to compensate for the increased difficulty of breathing as he descends.

3. Exhaust Valve
The second stage exhaust valve is the plastic unit that channels exhaled air bubbles away from a diver’s face. The exhaust valve is usually located below the regulator’s mouthpiece to channel air down and to the sides. Helping to keep a diver’s field of vision clear of bubbles.

4. Mouthpiece
The Mouthpiece is the part of the regulator that a diver bites down on. High-quality mouthpieces are made of silicon or soft rubber and comes in a variety of shapes and sizes to fit divers’ mouths. Mouthpieces are removable and replaceable. A diver should check to make sure that his mouthpiece is secured to the regulator second stage with a SCUBAPRO Quick release mouthpiece clamp. Try not to use zip-ties. They are not designed for long-term water exposure. The SP Quick clips are designed for long-lasting UV and saltwater exposure.


The Octopus /alternate second stage, does the same thing as a primary second stage. The Octopus second stage is not intended to be used, except in the case of an out-of-air emergency. A diver with an alternate second stage can allow a diver that is having an out-of-air emergence to breathe from their tank without putting themselves at risk.

1. Mouthpiece
The mouthpiece is the part of the regulator second stage that a diver bites down on. Alternate second stage mouthpieces should be a standard size to fit any diver’s mouth–not a custom mouthpiece. The idea is that any diver should be able to use the mouthpiece in an emergency.

2. Low-Pressure Hose
Low-pressure hoses (LP hoses) transport air from a regulator first stage to its second stages. An alternate second stage’s LP hose is usually longer than the LP hose attached to the primary second stage. This extra length makes it easy for an out-of-air to use an alternate second stage connected to a tank he is not wearing. The LP Hose attached to an alternative second stage is frequently a bright color, such as yellow, to make it easy to see.

3. Purge Button
The purge button on the alternate second stage, has the same function as a purge button on the primary second stage–to remove water that has entered the second stage. Alternate second stage purge buttons are usually brightly colored–this one is neon yellow. The bright color makes it easy for an out-of-air diver to locate the alternative second stage in an emergency.

4. Ease of Breathing Adjustment
Just like the ease of breathing adjustment on a primary second stage, the ease of breathing adjustment on an alternate second stage can be used to increase or decrease breathing resistance during a dive. If ease of breathing adjustment is present, a diver should adjust it so that the breathing resistance of the alternate second stage is increased. The diver should also turn any pre-dive/ dive adjustment to “pre-dive.” The regulator will still work if needed, but this adjustment will ensure that the alternate will not free-flow during the dive.

5. Diver adjustable inhalation resistance control

Second stages equipped with this system have an over-sized external control knob acting directly on the spring tension, allowing the diver to adjust the inhalation resistance to adapt it to the needs of the dive conditions. Adjusting the control knob (clockwise rotation) causes an increase in inhalation resistance. Adjusting with a counterclockwise rotation reduces the spring tension for lower inhalation effort. All depends on the diving conditions, such as in strong currents, when the diver spends some time with his head down and when the second stage is used as an alternate air source (octopus).


The Air 2 is a second stage regulator and a balanced inflator for your BCD in a single, compact housing. SCUBAPRO introduces the inflator concept to diving almost 30 years ago with the introduction of the AIR2 ( Alternate Inflation Reg). The Air 2’s air-balanced power inflator valve gives a steady stream of air, regardless of the pressure in your tank. The Air 2 has a fixed Venturi Initiated Vacuum Assist (VIVA) Flow Vane for safe and uncomplicated use now has a dive/pre-dive switch. This switch allows the regulator mechanism to be set more like that of a standard second stage regulator. It is CE-certified for waters 50°F (10°C) or warmer.

Post dive

Close the cylinder valve and drain the system by pushing on the purge button of each second stage. Once the system has been depressurized disconnect the first stage regulator from the valve. All inlets must be closed with the provided protective caps to avoid the entry of debris, dirt or moisture. If the cylinder valve is equipped with a reserve system, the rod should be put in the “open” position (fully lowered) to indicate that the cylinder needs to be filled.

Care And Maintenance

Regulators are precision devices that are essential to the diver’s safety. After every dive and especially if in chlorinated water (pools), rinse the regulator with fresh water, preventing water from entering the system by following these steps:

1.    Ensure that the high-pressure inlet of the first stage regulator is closed with the special protective cap.

2.    Should water accidentally enter the low-pressure hose, connect the regulator to the cylinder, open the valve and press the second stage purge button down until all water has been expelled.

3.    Dry your regulator entirely in a dry ventilated place, away from heat and direct sunlight. So not in your garage.

4.    Store in a cool, dry place.

This is from our older site but it is still good.

4 Responses to “SCUBAPRO SUNDAY -Regulators”

  1. PPGMD says:

    “The inner mechanism in a diaphragm first stage is isolated from the surrounding water. This feature makes it especially suitable for diving in cold-water conditions or murky water.”

    Only if it has a sealed diaphragm, for example Scubapro makes both, the Mk11 is unsealed while the Mk17 is sealed.

    “The advantage of DIN fittings is that they screw into the tank valve and trap the high-pressure o-ring so it cannot protrude…”

    Extrude, not protrude, as all oring protrude or else they would be useless. But even DIN orings can fail. TBH the best advantage of DIN is that you aren’t relying on a tank o-ring being perfect, which on many rental tanks are only replaced when the leak is really bad. And even if you own your own tanks you have much fewer regulators than you have tanks.

  2. ER says:

    You are right SCUBAPRO makes sealed and unsealed first stages. It does say an “Isolated ” it doesn’t say they all make that way. So thanks for the help.

    As for extrude vs protrude
    verb (used with object), ex·trud·ed, ex·trud·ing.
    to thrust out; force or press out; expel:
    to extrude molten rock.
    to form (metal, plastic, etc.) with a desired cross section by forcing it through a die Extrude is mainly used for liquids.

    extend beyond or above a surface

    the o-ring will protrude about the surface.

    Most Tec divers use DIN. One isn’t better then the other most of it is a personal choice. DIN can hold more pressure and there is a reason TEC divers. What was posted isn’t the law it is the consensus of most divers.

    • PPGMD says:

      Extrude is the correct term because it is describing what caused the o-ring to fail, as the pressure on the o-ring is extruding the o-ring through the gap, like pasta extruding through a die.