Before regulators were invented, divers faced a serious problem. Air had to be supplied on a freeflow basis. Early helmet divers relied on hand driven air pumps, and later motor powered compressors that supplied air continuously. Unbreathed air simply escaped via a valve into the water and was wasted. For divers at work harvesting pearl beds or salvaging wrecks, mobility was not a great issue. They'd be dropped on the work site and a short length of hose let them move around as much as they needed to. If air was wasted, it wasn't a concern. The planet was full of it. But if you wanted to swim around unfettered, you had to lose the air hose and replace it with a small air tank you could take with you. Early cylinders that were pressed into service for diving worked at much lower fill pressures than today. So they held comparatively little air. A freeflow system would have been unusable as all the air would have been lost almost immediately. To get around this early self contained underwater breathing apparatus (SCUBA) used a simple on/off tap that the diver turned on to inhale and off when he'd had enough air. Clearly walking around the seabed while synchronizing turning a valve to your breathing cycle was very inefficient.

A handful of engineers worked on a solution - a valve that would automatically deliver air with each inhalation, and then shut off the air at the end of the breath to prevent any wastage. The first commercially successful "on demand" system was the Aqualung developed in 1943 by engineer Emile Gagnan and the undersea explorer and filmmaker Jacques Yves Cousteau. The aqualung was designed in France and marketed by gas control specialists Air L'iquide. In 1947 Air L'iquide created a new company with the exclusive task of developing and selling self contained diving equipment. La Spirotechnique took partners overseas who manufactured and marketed equipment under their own label. The Spiro group eventually included US Divers, Sea Quest and Wenoka of the United States, Nihon of Japan, Technisub of Italy, Fenzy of France, Draeger of Germany and Apeks of the United Kingdom. Today these companies' products are largely sold under one label - Aqualung.

The regulator revolutionized diving. It gave us the freedom to swim without links to the surface. Selecting yours is one of the most important buying decisions you will ever make as a new diver. Understanding a little about them will help you choose wisely. This guide will, we hope, help you get started.

The original Cousteau-Gagnan demand valve or regulator (the terms are used interchangeably and mean the same thing) was a two-stage design. It dropped tank pressure to a level that was comfortable to breathe in two stages via two reduction valves. But the technology at the time proved unreliable. A simpler single stage regulator was developed. Single stage regulators dropped the tank pressure, which was high, to that exactly matching the water pressure surrounding the diver, which was low, in one bound.

Early regulators used two hoses. Air was fed to the diver through one and waste air exhausted through the other. As technology improved and reliability increased, many manufacturers went back to a two stage design for better breathing performance. In the fifties a new idea was tried. Instead of placing both reduction stages behind the divers neck in a single casing joined by two hoses, why not place one stage at the divers mouth and join it to the other via a single hose? The idea worked very well and all modern recreational regulators are single hose designs. It also got over the Cousteau - Gagnan patent. Recreational twin hose regulators are now found only on rebreathers, which recycle exhaled air. These are very different to normal open circuit regulators. We believe conventional twin hose regulators are now only being manufactured for preliminary training of military divers in preparation for using rebreathers.

So that's a brief look at the history, let's have a look at how the modern regulator really works.

The first stage and second stage both act as reducing valves. The first stage takes the air in the diving cylinder and reduces it to a pressure the second stage can handle. Tank pressure may start as high as 300 bar / 4500 psi and drop to near zero during the dive in line with the diver's breathing. The role of the first stage is to deliver air to the second stage at a stable pressure, usually 8-10 bar / 130-145 psi, regardless of how lightly or heavily the diver inhales.

There are three commonly sold first stage designs - unbalanced piston, balanced piston and balanced diaphragm. Each has its pros and cons. All depend upon using a moving piston or rod to open and shut off the air supply to the diver. How this part moves will be a major factor affecting how easily your regulator breathes.

At its most basic this piston or rod is opened by a combination of pressure exerted by the water around the diver and a spring. When opened, air leaves the tank and heads towards the divers mouth and lungs. It's closed by back pressure created when the diver stops breathing in. To stop the regulator either freeflowing madly or not supplying any air at all, the forces have to be balanced. Otherwise the valve would always be open or closed. This is done by making one end of the rod or piston wider than the other. Physics then take over - a small amount of high pressure can be equaled by a larger amount of lower pressure if that pressure is applied over a larger area. Inhaling and exhaling effectively unbalances the valve allowing the piston or rod to move forwards and backwards opening and shutting off the air in time to the divers breathing pattern. Breathing in gives the high pressure air acting on the small end of the rod or piston the upper hand by reducing opposing pressure on the larger end. When breathing stops air pressure on the low pressure end of the rod or piston builds up on the large end and is able to move the rod or piston backwards to seal off the air supply. This action happens every time you take a breath.

The unbalanced piston is popular with diving schools because of its low purchase price and simplicity, which reduces maintenance time and lowers servicing costs. Deep divers sometimes use them for making shallow decompression stops where high performance is not an issue. Unbalanced piston valves get their name from the fact that both ends of the moving piston are not opposed by equal pressure. Opening forces vary with tank pressure. At the beginning of the dive when tank pressure is high the valve has the full force of this air supply to help open it against the medium pressure at the far end of the piston. Although the medium pressure trying to close the piston against the incoming tank pressure and the balance spring is much less than the tank pressure, it acts on a much greater surface area. So it can close it quite easily. As tank pressure drops so does the opening force on the high pressure (tank) side of the piston. Air pressure on the medium pressure (second stage) side of the piston remains the same. At low tank pressure the diver has to work harder to open the valve and get the air flowing. Traditionally unbalanced piston valves become noticeably harder to breathe from as the tank gets near to empty, providing a warning that the diver is low on air. In theory unbalanced piston valves cannot deliver air as efficiently as balanced models. This has implications for deep diving, strenuous diving and donating air via a safe second. However during research for Running on Empty, a Scubapro unbalanced first stage supported two heavily breathing divers at 50 m / 164 ft at tank pressures below 50 bar / 750 psi. Even so, the divers involved in that test all own balanced first stage regulators themselves! Balanced piston regulators work slightly differently. The high pressure side (tank side) of the piston is surrounded on all sides by air, rather than having tank air forced against one end exerting an opening action. Without having high pressure air bearing directly against it, the pistons opening and closing action is unaffected by changes in tank pressure until the diver is virtually out of air. The opening and closing forces on both ends of the piston are engineered to be equal until the diver inhales tipping the balance slightly in favour of the high pressure end and allowing the piston to move off its seat. This lets air flow through the piston to the second stage and on to the diver.

Both unbalanced and balanced piston regulators have a spring that surrounds the piston and that is open to the water. The spring sets the medium, or intermediate, pressure of the air that will be delivered to the second stage. Spring pressure is constant throughout the dive and works to open the valve and let air flow through to the second stage. Pressure building up on the medium pressure side of the piston inside the hose is used to close the piston against the spring. To account for pressures encountered at different depths of the dive, water is allowed to enter the spring chamber and press against the wetside of the piston. The valve always operates with an opening force equivalent to its intermediate pressure regardless of depth. Without this feature the valve would become stiffer to breathe from with increasing depth.

The internal diameter of the piston reduces tank pressure to a pressure that the second stage can handle. This is similar to attaching a garden hose pipe to the water supply. However far you open the tap, the water comes out with less force at the hose end than it does from the tap. To increase the volume of air to a diver working at depth or breathing hard, the piston rises higher off the high pressure seat. More air can then flow through the gap and into the piston and then onto the second stage and the diver.

Balanced diaphragm regulators work on a similar principle. In place of a piston they use a rod, which passes through the high and medium pressure chambers. At one end the rod sits against a flexible diaphragm. Water pressing on this diaphragm acts to transmit water pressure to the rod. An intermediate spring set to 8-10 bar / 130-145 psi works with the diaphragm to try and open the valve and allow air to flow through the first stage to the second stage. Medium pressure, and sometimes a spring, oppose the opening force and try to keep the valve closed. Opening and closing forces are in balance. Inhaling drops the intermediate pressure allowing the diaphragm and intermediate spring to overcome the closing force and open. This action lets air flow from the tank, through the first stage and on to the mouthpiece.

Balanced first stages of either type are associated with high performance regulators that deliver the large volumes of air needed for deep diving, high activity or supporting two divers in a sharing situation at normal recreational diving depths. Ideally they should provide an additional safety margin and have some performance left in reserve for unforeseen demands.

With any first stage as the diver stops inhaling pressure builds up in the hose and then acts on the medium pressure side of the piston or diaphragm rod. This back pressure is enough to close the valve against its seat and shut off the air supply until the next breath.

The first stage will have outlets for pressure gauges, which tell you how much air you have left, and for the hoses used to add air to drysuits and buoyancy compensators or supply your alternate air source, sound your air horn or operate air powered tools. Pressure gauges are fed from the high pressure side of the regulator and measure the pressure of air remaining in your tank. This is typically as high as 300 bar / 4500 psi. To slow down loss of air in case of a hose failure or a gauge blow out (both extremely rare occurrences) a restrictor is fitted. Medium pressure outlets supply air to everything else. The air pressure delivered to the second stage and other accessories is normally only 9-10 bar / 135-147 psi. Screwing a medium pressure rated hose into the high pressure outlet would burst it, so usually the HP and MP threads are different to prevent mismatching.

Often one medium pressure outlet is designated as the feed to the primary second stage. This ensures that if a diver is breathing very heavily, the second stage will receive the lion's share of the air. Sometimes wide diameter hoses are used that can carry larger volumes of air to the diver than standard hoses can. Leading the hose from the end of the first stage valve can also improve performance by reducing turbulence. This happens when the demand for air is high and causes the air to swirl inside the regulator passageways creating drag. The friction actually impairs delivery of the air. By leading the air through the more gently curving hose at the end of the piston, rather than sending it through a sharp right angle, a smoother laminar flow is created which is more efficient. This outlet may also be linked to special features inside the first stage that improve performance and decrease breathing resistance, especially at depth where air is denser. This is sometimes done by slightly increasing the intermediate pressure to create a greater driving force to push the denser air along the hose.

Outlets may be duplicated on both sides of the first stage to allow the diver to route his hoses in the most comfortable and streamlined way. It's also common to mount the MP outlets on a swivel collar to help get the best layout. Having a choice of first stage orientations can also help with laying out your hoses so that they run close to your body. This makes them less likely to snag. How a diver sets up his hoses may depend on how many hoses he uses, which side his alternate air source is and whether he uses one or two cylinders.

The hose to the second stage normally leads over the shoulder. A few slip under the arm to reduce the risk of snagging. Recovering a lost mouthpiece requires different techniques with either type.

An over-balanced diaphragm first stage is a development of the balanced diaphragm first stage mechanism. The goal of the over-balanced design is not only to supply high volumes of gas easily, but also to compensate for increased air density. On deep dives beyond 30-35 metres, using air, you can easily detect the air feeling thicker or more viscous to inhale or exhale. This is caused by the air becoming denser with increasing pressure found at depth. At 40 metres, not only are you using five times the amount of gas per breath that you did on the surface, but that air is also now five times thicker or denser.

A conventional regulator first stage accounts for the increasing pressure on a deep dive. It does this by exactly matching the intermediate air pressure delivered to the second stage to the increasing water pressure. For every 10 metres you descend it boosts intermediate pressure by one extra bar. But because the air is becoming more dense, it is increasingly hard for it to pass through the narrow passages of the regulator. This leads to greater breathing resistance for the diver.

The over-balanced diaphragm first stage works a little differently. Instead of exactly matching intermediate pressure to increasing water pressure, this type of first stage adds a little extra: about 20% in the case of Aqua Lung regulators. At 40 metres a conventional first stage would have matched the water pressure surrounding the diver and intermediate pressure would be increased by 5 bar. An over-balanced design would have increased this to 6 bar. The additional back pressure helps force the denser air through the regulator compensating for its viscosity and resulting in an easier breathe for the deeper diver.

There are three main regulator to tank connections: A-clamp, DIN 200 and DIN 300. The A-clamp is commonly used in the UK, North America, the Caribbean, Australia and countries bordering the Red Sea. The A-clamp has several disadvantages. It is not suitable for high pressure cylinders, limiting it to maximum working pressures of around 240 bar / 3300 psi. A sharp bang to the first stage may dislodge the seating allowing the O ring to extrude causing the cylinder to quickly drain of air.

The DIN connection is safer. It's used extensively in Scandinavia, Germany and at German run resorts worldwide. DIN valves screw into the cylinder valve. They are able to withstand higher pressures allowing 300 bar / 4500psi cylinders to be used and they are less easily damaged by impact. For these reasons wreck and cave divers prefer them as collisions with cave walls or wreckage are an ever present danger. The O-ring is effectively captured within the DIN valve system making it much harder to blow an O-ring and slowing air loss if you do.

DIN valves designed for 200 bar / 3000 psi service have 7 threads. 300 bar / 4500 psi models have 9. This means a 300 bar model can be used safely with lower pressure cylinders, but 200 bar versions won't seal on a 300 bar tank.

Mavericks regulators are all suited to 300 bar use. We supply our first stages with a 300 bar DIN fitting. An A-clamp adapter is included and screws onto the DIN valve in seconds without tools. This lets you use your regulator on any rental tank.

The second stage is held in the mouth or sometimes fitted to a full face mask or mouth mask. Full face masks are used by deep divers to allow easier switches between breathing mixtures. In commercial diving they are used for safety - the diver cannot lose his mouthpiece and gas supply, so losing consciousness for example is not quite so dangerous. They also better protect divers working in contaminated water. Full face masks also make it easier to speak with underwater communications equipment. Recreational divers using radiophones usually use mouth masks. The mouthmask has a cavity into which the diver can speak clearly.

The task of the second stage is to take the incoming medium pressure from the first stage and reduce it to low pressure that is easily breathed by the diver. This is equivalent to the water pressure surrounding the diver. For a diver at 40 m / 130 ft this is 5 bar or 75 psi. At the surface it is one bar or 15 psi.

The second stage has a valve that sits against a seat next to the hose. The valve closes off the incoming air by using a spring set to balance the opposing medium pressure air coming from the first stage. The valve and spring are linked to a lever. In turn the lever rests against a flexible diaphragm. One side of the diaphragm is open to the water. The other is inside the second stage air space. When the diver is not inhaling the diaphragm is in the out position and the lever remains high, keeping the valve closed against the seat and shutting off the air supply from the first stage and tank. Inhaling causes the diaphragm to collapse inwards, towards the diver. The lever drops. Acting like a fulcrum, the lever overcomes the closing action of the second stage spring and the valve opens allowing air to reach the diver.

If the diver had to use lung pressure alone to open the valve, each breath would become more tiring than the last. So regulator designers engineer second stage valves to be easy to open. The force needed to open the valve is often called cracking effort. Long levers add to the fulcrum effect to minimize cracking effort. Another solution is to pneumatically balance the valve. This is similar to the how a balanced first stage works. Air is used to help keep the valve closed and makes up some of the force normally provided by the spring alone. A weaker spring is then used that requires less human effort to overcome.

If a diver also had to use lungpower to keep the valve open and delivering air, it would also be very tiring. So once the valve has been cracked or opened, another force is harnessed to do this work for the diver. Instead of delivering all of the incoming air directly into the divers mouth, some is rerouted. The rerouting is designed to create a vacuum. This vacuum keeps the diaphragm depressed, the lever down and the valve open without any further effort by the diver. This is called the Venturi effect.

Servo or pilot second stages generally yield extremely high performance. They typically use a small air chamber linked to the diaphragm via a needle. Inhaling depresses the diaphragm and the needle opens the pilot valve. The valve needs minimum effort to crack. Opening the pilot valve lets air escape from a second chamber . Air in this second chamber is used to hold closed the main valve. As it escapes the pressure lowers and the main valve can open sending air to the diver.

Many second stages have user adjustable cracking and venturi controls. Toning down the cracking effort can prevent freeflows when facing strong currents or riding a propulsion vehicle. Reducing the venturi effect will stop a freeflow when a regulator is out of your mouth. Some divers detune their regulators assuming this will help them save air and prolong their dive. In fact an easy breathing regulator reduces how hard you have to work to breathe and decreases air consumption.

Detuning safe seconds to prevent them freeflowing by accident can be problematic. During tests for Running on Empty it was noted that passing a detuned safe second might alarm the recipient who could find the regulator hard to breathe from. Proper stowage of the safe second will usually eliminate problems with freeflowing.

Most second stages are downstream models. If a fault develops in the first stage and the intermediate pressure rises above the manufacturers settings, the second stage spring will not be able to hold back the air. The regulator will freeflow and the diver will still be able to breathe. Upstream models have a relief valve fitted. The relief valve prevents the hose from bursting. It also keeps the valve from locking up so you can still breathe.

Your lungs are the main switch. As you inhale pressure and volume in your airways drop. Without back pressure to hold the second stage diaphragm rigid, it collapses. This lifts the second stage valve from its seat unsealing the valve opening or orifice. Air stored in the hose to the first stage flows into the second stage. As pressure in the hose drops, it can no longer overcome the opening forces of the spring and water pressure in the first stage. Their combined strength lift the first stage valve from its seat and air from the tank flows through the hose, through the second stage and into your lungs.

When you stop inhaling pressure and volume rise in your airways and your mouth and inflate the second stage diaphragm. As the diaphragm returns to its inflated position, it takes the second stage lever with it, allowing the spring to close the second stage valve against its orifice. With air flow into the second stage capped, pressure quickly builds in the hose. This back pressure forces the first stage valve back onto its seat against the opening forces of the first stage spring and water pressure. This in turn seals off the air from the tank.

As you exhale, the over pressure in the second stage opens one or two simple mushroom valves and lets the air bubble out. As you stop exhaling, pressure inside and outside reach equilibrium and the mushroom valves close.

So that's the basics of regulator technology. In part two we'll look at how regulators are affected by cold water and ice diving, precautions for using enriched air, oxygen and mixed gases, examine breathing performance in more detail and dispel a few myths about breathing underwater.

Diving in cold water presents special problems. As air passes through your regulator on its way to your lungs the pressure drops as the volume expands. This causes the temperature of the air to also drop. This can be by as much as 15 degrees Fahrenheit. The cold air can cause components in the regulator mechanisms to freeze open, leading to a freeflow of air; or cause them to freeze closed, causing the air supply to stop.

Freezing is usually a result of ice forming as water inside the first or second stage is chilled by air flowing through them. For example air passing through the first stage piston initially cools the piston itself from within. As the outside surface of the piston gets cold ice forms on it. With diaphragm first stages water in contact with the spring can also freeze. With each design the ice can physically block the movement of the mechanism preventing the valve from opening. This stops the air supply to the second stage. More often the valve freezes open leading to an uncontrolled freeflow, which can swiftly empty the air tank.

Freezing in the second stage occurs because of water droplets turning to ice inhibiting operation of the spring or lever. The moisture may come from the diver's own exhaled breath condensing or from water that has found its way into the second stage. A little leakage is common from around the mouthpiece and through the mushroom valves as the diver exhales.

Freezing is more common in fresh water, with its higher melting temperature, than salt water. High demand for air exacerbates the problem, so deep diving and heavy breathing including using an octopus, tend to provoke freeze ups.

To avoid freezing special cold water diving techniques are used. In addition regulators should be modified or special regulators purchased that are designed to counter the problems. The mechanisms inside regulator first stages often rely on isolating the water so it is not in direct contact with the main diaphragm, piston or springs. Diaphragm first stages often use a second diaphragm or cap over the first. Silicone or alcohol is placed between the two. Pressure is transmitted between the diaphragms, but neither silicone or alcohol ice easily. The spring is immersed in this anti freeze. It's almost impossible for ice to form on it. Piston first stages often have the first stage chamber through which the piston runs packed with silicone grease. This prevents water being in direct contact with either the piston or the main spring, while still transmitting water pressure. Again, excluding the water prevents ice from forming on the spring itself or inside the balance chamber. Another solution is to use non stick surfaces to prevent ice forming on moving parts. Sherwood regulators use their patented Dry Bleed system to prevent icing in their piston operated first stages. Water is totally excluded from the spring and piston balance chamber and replaced by air. A small one way bleed valve senses changing water pressure and transmits it to the first stage mechanism.

Second stages also often use non stick components to stop ice adhering to the valve seat, spring or lever. Heat sinks that capture warmth from the divers exhalations are sometimes used to maintain the second stage above freezing point. Aqualung regulators utilize heat exchangers. These draw heat from the warmer surrounding water outside the regulator to heat the inside and stop icing of the second stage mechanism.

Wet air fills can also cause icing. This occurs when improper compressor maintenance allows moist air to enter the tank and then the regulator. As the air cools the moisture freezes and ices up the mechanisms from within. This is much harder for a regulator manufacturer to combat. Fortunately this is an uncommon situation.

Air is not the only breathing gas used by divers. Nitrox offers advantages for diving down to around 40 m / 130 ft and is also used for decompressing. It contains more oxygen than normal air. For shallow decompression stops pure oxygen might be used. For deep diving helium may also be added to oxygen or nitrox. This helps minimize problems with narcosis.

When gases other than normal air are chosen, the chemicals in the seals and lubricants of most regulators can react and cause a fire risk. To prevent this the regulator is carefully cleaned and non reactive components fitted. Many regulators can be adapted for diving with special gas mixes. However titanium models are not recommended.

Choosing your first regulator requires some research. Good regulators can last ten years or more. So you need to consider both your present needs and what you will want to do in the future. Issues to consider include the performance of the regulator as measured objectively by the US Navy Navy Experimental Diving Unit (NEDU) or another independent authority. You also need to consider a large safety margin to allow for a worst case scenario that could overload an inferior regulator, such as having to share air with another diver at depth with little air remaining in your tank. If you intend to use gas mixes other than normal air or dive in cold water then these requirements must also be thought through.

Once you select your regulator you may also want to make some modifications. It's common to change mouthpieces as everyone's jaw is a little different and some mouthpieces are more comfortable than others. Hose layouts might need to be altered and the length of the hoses themselves changed for better routing.

Aftersales is also vital. Most regulators get binned by their owners due to spares being phased out. Occasionally a brand gets passed through a string of distributors. This can lead to problems with spare parts. It can be safer to choose your regulator from a range with a long track record for good back up.

Your regulator is something that your life depends upon with every breath you take.. To protect yourself it should be maintained annually by a qualified professional. This is usually a small cost.

During entry level training you have should been taught how to handle a regulator that freeflows. Once you have breathed from a simulated freeflow handling the real thing is merely an irritant that should not develop into an emergency. Air stoppages, usually from running out of air rather than an equipment failure, are more serious and may depend upon your being properly trained to make a controlled emergency swimming ascent or hoping your buddy is close enough, well trained enough and has sufficient gas to assist you safely to the surface. These survival skills should be practiced regularly. You should also give serious thought to packing an independent alternate air source, such as a pony bottle. This virtually removes your dependence upon another diver for assistance and makes resolving an out of air situation as easy and fast as switching second stages.  

See also:

The Health and Safety Executive research report Breathing performance of 'Octopus' demand diving regulator systems 2005: RR341 (pdf).