The Second Stage | How They Work and How To Resolve Common Problems

Second Stage Regulators: How they Work and How to Resolve Common Problems

Article by Laura Tyrrell

If you’ve splashed out on your own regulator set recently but you’re not completely sure what language the sales person was speaking while they bombarded you with facts about all the fancy knobs and levers then this article might help explain what you need to know. I focus here on second stage regulators, the preceding article ‘First Stage Regulators: Mechanical Differences and Potential Maintenance Issues’ will highlight concepts of the first stage.

Why do you sometimes experience a loud embarrassing pre-dive free-flow for no apparent reason? What should you do if this happens at depth? Why can water enter the second stage? Or even, which mouth piece is best?

Regulator sets are normally bought as a first stage and a second stage regulator sets. The first stage attaches to the tank and is the first point of contact for the high pressure air. Once the tank opens air enters the high pressure chamber where either the piston or diaphragm reduces the air to intermediate pressure on inhalation. The ‘IP’ air will then travel down the hose to the second stage for processing further into breathable ‘ambient’ (surrounding) pressure.

Second stage regulators used for scuba diving are ‘demand valves’ meaning they will supply air on demand. So they are trying to keep the terminology pretty simple for us really. This differs from other types of valves such as a ‘continuous flow’ which… well, flows continuously. You would find such a valve on an oxygen unit delivering o2 to an unconscious patient, for example. A demand valve is obviously the best choice for scuba so that you only take the air that you need. Instead of having 3000 psi blasting your mask off your face while you try and take delicate underwater photos. And your tank wouldn’t last very long either.

However, should the regulator fail at depth a gush of continuous air or a ‘free flow’ is exactly what would happen. This is all in the name of safety because you will continue to have air delivered to you even down at 30 meters. If this happens you would need to employ the free flowing regulator ‘sip’ skill perfected in your confined open water training. Sip the escaping air by tipping your head to one side and positioning half the mouth piece outside of the mouth while you either find your buddy’s alternate air source or, if necessary, make a safe slow solo ascent. Remember to keep calm and keep a close eye on your air supply as you go. It’s surprising how much air you can lose in just 60 seconds.

A regulator free flow before the dive on the surface is a more common occurrence. A surface free-flow can waste a lot of air if you don’t respond swiftly. This is unnerving, especially if you’re one of those bigger guys using the same 12litre as everyone else but with lungs twice that size who just back rolled off a boat and is suddenly faced with losing minutes of dive time thanks to a pesky mechanical complication. To stop a free flowing second stage on the surface you simply turn the mouthpiece in a downward position and gently block the air flow with you hand. Never forcefully hit the apparatus. Remember that you should have your mouth piece in before you make your deep water entry to avoid this problem in the first place. As a last resort you should ask the nearest person to quickly turn off your air and then inspect the equipment with your guide.

What are the mechanical reasons for a free flow happening? Let’s take a closer look at how the second stage delivers air.

second stage

When the diver is not breathing the bias spring pressure will equal the intermediate pressure and the valve will stay closed. When the diver breathes the flexible diaphragm moves inwards and pushes on a movable lever. This lever opens a valve that is connected to the hose to the first stage and thus delivers air.  When the diver exhales the opposite happens and exhaled air leaves through the exhaust valve.

A free flow can be explained by a drop in pressure from behind the diaphragm causing an exponential reaction as the diaphragm opens the valve more and more. The drop in pressure can be as a result of extreme cold conditions, a sudden rush of fast flowing air (such as when you press the purge button) or changing from air to water pressure (the deep water entry).

The good news is that when you purchase your own regulator set you can chose a more advanced design than generic rental second stages to help reduce free flow problems. Such designs include a pre-dive knob to create a different air flow direction. The switch operates a small internal lever near to the mouth piece on most designs, changing the air flow in much of the same way that a rudder on a sail boat would change its direction in water. Other designs also have an adjustment on the spring making the demand of the air flow easier at different depths.

Other common problems with the second stage involve a slight leaking of water. There are a couple of reasons for this. Sometimes the diaphragm can become folded over or unaligned within its housing – after being dragged through sand for example. If you feel there is water inside the second stage it’s possible to ask an experienced person to look inside and resolve this situation very easily. Check you won’t be voiding your warranty by opening up the equipment.

Water leaks are common when the mouth piece is old and needs replacing. Which mouth pieces are the best? Well a lot of it is down to personal choice and how they fit in your mouth. I am a big fan of either the more expensive mouldable mouth pieces that are specifically shaped to your teeth and never become chewed down. A great investment. I also had a lot of success with ‘long bite’ mouth pieces, they seem to last forever and easily stay in without having to bite hard. A good tip: if you are renting then buy a comfortable mouth piece and have the dive shop fit it to their regs. Then you will be guaranteed a nice comfortable dive – free of lock jaw and pre-chewed silicon!

To summarise, some second stages can be more sensitive than others and it is possible to make adjustments to how sensitive they are thanks to manufacturer innovations. Read your manual carefully to understand how yours works. Avoid free flows by keeping your equipment streamlined and protected when you make deep water entries. In the unlikely event of an underwater free flow, sip the escaping air until you find your buddy or ascend on your own. Take a refresher course to brush up on these skills. Check the mouth piece to avoid nuisance leaks: an unidentified leak may also point towards problems with the diaphragm so have a professional help you to resolve the issue. As with all dive equipment maintain your gear by washing in fresh water after diving and have it serviced regularly. Specifically when rinsing second stages; do not depress the purge button while rinsing. This will cause water to enter the entire system through and will lead to corrosion and malfunction.

Laura Tyrrell is a PADI Dive Instructor from the UK working out of Mexico. Laura writes articles for

Leave a Reply