We've all used one in training, we all know why we carry them, but are we really prepared to use our alternative air source in an emergency? And if we do, will it actually work? Running out of air is one of the primary causes of diving incidents and deaths. Out-of-air emergencies happen, regulators do fail, and there's no reason it won't happen to you. All divers need to be able to choose the right back-up system for their type of diving and they need to practice using it. We have carried out a series of tests with a large group of divers in Gibraltar - some were relatively inexperienced, many were highly qualified instructors. The results revealed that great care is needed in picking the right alternative air source, and far, far more practice is necessary for using one. You're nearing the end of your dive and suddenly your buddy swims up to you, arms flailing and eyes like saucers. He's out of air and you have to deal with it. If you're recently trained, or practice regularly, there may be no problems - but how many divers regularly practice safety exercises? If your response isn't immediate, you can be sure the situation will get worse.

We video-taped two days' footage of ten buddy pairs simulating an out-of-air emergency. On day one divers were paired up with familiar buddies, on day two we mixed them up, and in most cases we paired people from different training backgrounds. They were asked not to discuss the exercise before the dive, but just to get into the water and carry out the simulated emergency. The results help to build a clear picture of the problems that can all too easily be encountered.

Confusion over signals: The absence of a standard signal led to a communication breakdown. The BSAC divers indicated 'out-of-air' with a chopping action the the throat. The PADI divers did it by drawing their hand across the throat - this was interpreted as a danger signal by the BSAC divers.

Inadequate signals: Alarmingly, we observed inadequate signals from almost half the divers taking part in the tests. Only 40% used and exaggerated motion that would have been easy to interpret in low visibility.

Most of the divers delayed their ascent unnecessarily: As soon as two divers are breathing from one cylinder they must begin their ascent immediately. We expected to see a fairly straightforward ascent to the surface from our test divers. What we actually saw was nothing like that - 50% of the pairs delayed their ascent for an unnecessary period of time. In two cases no ascent was made for more than two minutes - a serious waste of air.

Most of the divers failed to form a good grip: It's vital that both divers get a secure grip of each other as quickly as possible. If they separate, the out-of-air diver will lose their only air source and may drown. Only three pairs carried this out successfully. Most of the divers, including some highly qualified instructors, appeared unsure of which grip they preferred to use, and even which hand to hold on with. We watched divers completely releasing their grip to dump air from the BCD or drysuit.

Some of the divers descended without noticing: The exercises were carried out in mid-water. Of the ten pairs, three descended several metres and seemed unaware of it. Incident reports detail divers accidentally descending 25 m to the sea-bed before realising.

It's important to choose the appropriate alternative air source for the diving you do. Divers usually use one type in training and often look no further when choosing their own alternative air source. If they become involved in more adventurous diving they run the risk of being poorly equipped to deal with an emergency. We took different types and makes of alternative air sources to the BSAC maximum recreational depth of 50 m. Performances were monitored at low tank-pressures and during ascent.

Octopus: The octopus is the most commonly-used alternative air source. This is basically any regulator second stage intended for use as an octopus, although most divers use a manufacturer's dedicated octopus second stage, which is brightly coloured and connected to a longer hose.

Choosing the same make of octopus as the first stage and primary second stage ensures the best performance. Divers who make penetrations into wrecks or caves must use an extra-long, two-metre hose to enable a buddy to breathe from the alternative air source while swimming directly behind them. These have been adopted by other recreational divers who prefer the system.

Of the alternative air sources we tested, the octopus system provided the highest performance. We would recommend you use the highest performance fist stage you can afford. The performance of your octopus is directly affected by the ability of the first stage to deliver enough air - this becomes very important when there are two divers breathing from one regulator. The octopus is a safe system to use at depth, but you must carry enough air in reserve for two divers; or use a pony cylinder.

Octo-inflators: These combine the octopus with the inflator of a BCD, either integrated into one unit, or as a specially designed octopus placed in line with the inflator. It's location dictates that in an emergency, the air donor must offer their primary regulator to the out-of-air diver, using the octo-inflator themselves. Photographer's models choose to use them to reduce the number of hoses that show in a picture, or travelers, who wish to carry the minimum amount of kit. We were surprised by the relatively good performance of these at 50 m. There was more breathing resistance than with the octopus regulators during both inhalation and exhalation, but they delivered good amounts of air. Their effectiveness in a real emergency is something that can't be tested. We recommend this system is used at shallower depths and with a buddy who is familiar with it. If you do use an octo-inflator, remember that dumping air through your inflator might be a problem when you're breathing from it (some new systems have overcome this problem). Fit an independent pull-dump on the opposite shoulder.

Pony Set-up: Redundant air is the choice of most deep divers and other more conscientious divers. While small redundant air supplies with built-in regulators do exist, they provide little air. To be of any real use in an emergency you need a two- or three-litre pony with a first and second stage regulator. Divers undertaking extended decompression stops will carry larger redundant air or nitrox, should it be required.

With a pony set-up there should only ever by one diver breathing from it, which created less demand on the first stage. So, if you can't afford a high performance regulator, go for a basic but robust system for use with a pony bottle. Remember, if you ever find yourself using it there's only one way you should be going: up!

Breathing rates vary dramatically between divers, a fact that must be taken into account when determining the correct amount of air to leave in reserve on a dive. You may think you've allowed enough, but what happens when your air-guzzling buddy wants some of yours as well? We ran some tests on air consumption, and got some revealing results.

Dives were made in pairs, following the same profile, with a maximum depth of 50 m - two sets of stops on ascent, one minute at 9 m followed by six minutes at 6 m. Total ascent time was 11 minutes. The test air was carried in redundant cylinders. The divers only switched to the test air as they began ascending.

One pair used more than half their air during the ascent: On reaching the surface one pair had breathed 1,300 litres of air. The second pair had used 1,000 litres. If this had been a real dive and the agreed air reserve was 100 bar (a realistic pressure), the first pair would have run out during ascent - forcing them to omit mandatory decompression stops.

A full three-litre pony bottle didn't get one diver to the surface: Before ascending, the diver switched to the full pony bottle. After just five minutes the cylinder was at 100 bar. Two minutes into the 6 m decompression stop it ran out. In just two breaths the regulator went from 'breathing fine' to failing to deliver air - an indication that failure to check your cylinder pressure could be fatal.

The pressure inside the pony bottle dropped significantly as it cooled at depth: The cylinder had been filled at least two days before it was used on the dive. At the surface it read 200 bar, but at 50 m the pressure was only 170 bar.

Blind reliance on an alternative air source may be misplaced and could cost your life and that of your buddy. There are many reasons why a diver may not be able to breathe underwater. These can be loosely categorised into problems of technique which reflect an individual's personal skills and training, and equipment failures for which the diver may or may not be wholly responsible. These examples either reflect incidents that we have personally witnessed or are drawn from documented accident and fatality reports.

Out of air - You would be amazed at the number of people who think an octopus will miraculously provide them with air despite their tank being empty. Alternative air sources connected to your tank obviously DO NOT WORK when the tank is empty.

Free-flows and lock-ups - A free-flow can leave you totally out of air unless you have another cylinder. A first stage lock-up will cut off the air supply to both primary and secondary mouthpieces.

Inadequate performance - Air sharing will at least double the demand on your regulator. A low performance, poorly-maintained first stage may fail to deliver.

Breathing from wrong gas source - Divers using ponies have been known to accidentally begin the dive breathing from the pony regulator. When the pony ran out at depth they spat out the mouthpiece believing that their primary had failed, and reinstalled the regulator from the empty pony with tragic results.

Inadequate gas supplies - An AAS is only good for as long as it lasts. If there is insufficient gas in it to get you, or you and your buddy to the surface, it's nearly useless: you'll simply run out again.

Inaccessible AAS - We've seen AASs zipped into BCD pockets and secured to D-rings with karabiners. Seconds count. Failure to quickly and cleanly deploy or obtain an AAS may render it useless in an emergency.

Poorly maintained secondary - We've seen one second stage with a paper clip in it, know of another that was home to a fireworm, and have breathed off regulators with leaky exhaust valves or diaphragms. Some we've tried have been almost impossible to draw air through even in a pool.

Deliberately disabled secondary - One tester had disconnected his combined octopus/inflator hose because a valve failure meant it kept inflating his BCD.

Problems with pressure gauge readings - Over time, many gauges lose accuracy. Fatalities have occurred, for example, when the pressure gauge read 40 bar but the tank was empty. In other incidents problems have occurred with air remaining. It's possible that while air remains in the tank the regulator can't supply a sufficient volume to support both divers.

Air turned off by accident underwater - Incidents have occurred when cylinder valves closed as they became caught on cave walls or wreckage. In cases where divers have used more than one cylinder, the wrong valve has been turned off by accident, leaving a diver without air.

Unfamiliar kit - Purge buttons are not always obvious and second stages can seem to face the 'wrong' way. This can increase the chances of the second stage flooding, or breaking your mask seal with the upturned exhaust tee. Anti-free-flow devices also make breathing more difficult and may cause additional stress. Task loading can become insurmountable.

At a recreational diving level, equipment configuration is discussed but never really highlighted as an important element of safe diving. For this reason it's something many divers never truly understand. If we're lucky, we may learn some basic tips, such as ensuring the octopus is the last hose to be attached, so it is unobstructed should it need to be released quickly. This is an attitude far removed from the technical diving community, for whom configuration is emphasised as a critical safety element. In fact, it is recognised as an area that demands hours and hours of fine-tuning in the water and on the surface.

Here are some basic ways of wearing an alternative air source. Find a method you are comfortable with and stick to it - and make sure you know how your buddy configures his or her alternative air source.

This is the most convenient way to carry an alternative air source.
For: The hose is kept under the arm, reducing the chances of it snagging.
Against: The diver must release it from under the arm when it's being used. It's difficult for an out-of-air diver to locate the second stage unless their buddy is facing them.
Recommendation: This system must be used with a coloured hose.
Hose under the left arm - This system is the same as under the right arm, but any diver that uses it must have the correct second stage. A dedicated left-hand regulator or one designed to be used from either side must be used. This has been adopted by some recreational diving instructors.
For: The hose is unrestricted. If a diver needs to use the alternative air source, they simply take it. When it's used with a coloured hose it is clearly visible to other divers.
Against: The hose is more likely to become snagged. There are various views on the most appropriate place to secure an octopus second stage. The torso triangle is taught by training agencies and serves as a rough guide for novices. Some would argue that the bottom right position is best for right-handed divers, and the reverse for left-handers. We found it was most visible towards the top half of the chest. The extra-long hose came from cave divers, who sometimes need to exit small spaces, so can't dive side-by-side.
For: The additional length is more comfortable for the diver breathing from the alternative air source.
Against: It's easy for divers to drift apart, as the out-of-air diver would feel less likely to lose the second stage.

This system is an excellent choice for divers involved in any advanced penetration diving, but the extra length of hose must be stowed correctly. For these set-ups your primary regulator is your octopus:
Set-up A: Simply tuck the excess length of hose into bungees around your cylinder. If you donate the octopus, the hose will come loose as your buddy pulls it.
Set-up B: Bring the second stage on the long hose under your right arm. Take it around the back of your neck and stick it your mouth. If your buddy needs it, they take it out of your mouth and you dip your head to help its release. You then breathe from your other second stage.  

See also:

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