This is a common question I hear from people who have experienced a distressing problem while diving and from (usually novice) divers who are anxious about how long their air will last. It can also be connected to concerns about buddies or letting the group down. If you are reading this, then you are probably already aware that there are a range of options for carrying extra air. This article is for those considering the small cylinders that you can fill from your main cylinder. About the size of a water bottle, the idea is they can be stored in a pocket for emergency use if you run out of air scuba diving.

In answer, there are a series of questions for you to consider in making this decision. First, the key question to ask yourself:

Do I know how long will the spare supply last if I run out of air scuba diving?

What size is the cylinder you are thinking of carrying? Right now, can you say approximately how long that air will last at the depth you plan to dive at? If you are considering carrying a spare cylinder due to a concern that you will run out of air scuba diving, then knowing how long the emergency supply will last is surely important to you. Here’s how:

Surface Air Consumption

To work out how much air you will use while diving you can follow a fairly simple process to calculate your Surface Air Consumption rate. (SAC rate for short). SAC rate is used to estimate the amount of air consumed by a diver at the surface. It is typically measured in litres or cubic feet per minute. Many people find the maths a bit off-putting but bear in mind this will not only let you estimate how long a spare supply would last, you can use it to estimate how long your total air supply will last on any given dive. To calculate your SAC rate, you would follow these steps:

• Start with a full scuba tank and record its pressure, (bar or psi). Dive to a specific depth and remain there for a predetermined time, while breathing normally and without exertion. After the dive, record the pressure in the scuba cylinder again. (Note this may be impractical, other alternatives are to measure the pressure change over a short window of time, e.g. 10 minutes at 10 metres).
• Calculate the difference in pressure between the beginning and end of the dive (or the beginning and end of the time that you were measuring), as well as the time spent at the specific depth.
• Divide the pressure difference by the time to get the average air consumption rate in psi or bar per minute (at that depth).
• Convert that to how much air breathed on the surface, by dividing by the atmospheric pressure. (Remember we breathe a greater volume of air at depth).
• When you are looking for a way to calculate how long a different sized cylinder will last, then you will need to convert the pressure per minute into litres or per minute. This is called respiratory minute volume (RMV).

For example, if you used 30 bar of air in 10 minutes at 10 metres in the sea, with a 10 litre cylinder. Thirty bar divided by 10 minutes is 3 bar per minute. To get the surface rate, divide by the atmospheric pressure at 10 metres below sea level (2 ATA/bar). Three bar divided by 2 is 1.5 bar per minute at the surface. To find out how many litres per minute you breathe at the surface multiply the pressure by the volume of the cylinder you were diving with: 1.5 bar multiplied by 10 litres is 15 litres.

Now, how long will a small pocket gas supply last? Lets say it’s a 0.5 litre cylinder, and it is filled to 200 bar. To work out how much air is in there, multiply the pressure by the volume: 0.5 times 200 equals 100 litres. If you breathe at a rate of 15 litres per minute, this air will last approximately 6.6 minutes on the surface. This sounds like quite a bit, but what if your dive is to 20 metres? At 20 metres, the atmospheric pressure is 3 bar. So you will be breathing more like 45 litres per minute, so now we are down to slightly over 2 minutes. At 30 metres? 4 ATA, so breathing 60 litres per minute, that’s about 100 seconds air supply.

Still sounds like it may be helpful in a pinch?

Note that the calculation was for a 0.5 litre cylinder at 200 bar. Some systems are smaller than that, and will not always be filled to 200 bar. More importantly, remember that this example is using a SAC rate of 1.5 bar per minute, which is a pretty efficient rate for a diver. Check out this poll showing a median rate around 16 litres (likely biased to experienced divers and reporting of lower rates).

Newer or nervous divers will tend to have a much higher rate of gas consumption, and surface rate could easily range 20 to 30 litres per minute. Some training agencies recommend planning dives based on 25 litres per minute at the surface. Just to be clear, that is the rate the national training agency gives is recommending as a starting point for the average diver. Someone with that sort of air consumption may use up a small spare cylinder in seconds!

And, bear in mind that all of these calculations are based on the rate of breathing of a comfortable, relaxed diver, swimming slowly. If your air runs out unexpectedly, how relaxed will you be? Loss of gas supply is a significant physical and psychological stressor. It can really push up breathing rate. To estimate how much air would be used in reality, we would also need to add in a “stress factor”, which will reduce the durations above considerably. Even for an experience diver, air consumption rates can increase dramatically due to distress.

If you are still unsure, what all this means, get in touch with a diving instructor who is familiar with SAC rates to teach you how to make these calculations. Depending on your current level and comfort, you may want to consider a a course that goes more in depth with gas planning. You can also check out this blog article and these instructions from Divers Alert Network.

In any case, it would be recommended to have a basic grasp of gas planning, before making decisions about the size of any cylinders you want to buy. Making the wrong decision with such purchases can be costly. You may even find that awareness of your gas consumption helps you to feel more in control and reassured that you are diving within your capacity. Which brings us to the next big question:

What problem do you desire to solve?

Before jumping to a solution, it can be useful to take a step back and reflect on the problem. Often when divers are considering carrying an emergency supply of gas, they are responding to a concern that they will run out of air or otherwise lose access to their gas supply.

Just a few ways to run out of air or lose access to air supply
Failure to notice depletion of air supply, possibly due to distraction or ineffective habits for checking.
Equipment malfunction: cylinder valve o-ring worn and leaking or bursts suddenly; failure of the regulator leading to freeflow, water ingress on inhalation or (more rarely) cut-off supply; hose damage leading to leaks or sudden burst and lead to loss of air/gas; disconnection of mouthpiece making it difficult to use the regulator; cold temperature causing freeflow leading to loss of supply …
Issues with dive planning or failure to plan may lead to divers running out of air before the end of the dive.
Diving deeper than planned means air/gas will be consumed faster than accounted for
Becoming trapped or entangled may prevent or delay surfacing before the depletion of air supply.
Exertion or stress during the dive may lead to divers consuming air more quickly than expected.
Desire to keep up with a buddy or group may drive social pressure that can lead to divers not reporting remaining supply accurately.
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These are just a few ways divers end up with insufficient gas to breathe. There are lots of factors that can drive the above situations, and various other ways to run out of air when scuba diving. Can you think of more?

Deciding and acting on an appropriate solution depends on first understanding the problem you want to solve. There are lots of options for addressing the above concerns that do not involve taking a small spare supply of air. We will com back to those below.

But what if the person is not really looking for a logical solution to a practical problem, what if instead they are more focused on getting away from fear or anxiety? It is a subtle difference, and both can be achieved with the same action. Neither is wrong, it’s just useful to be aware. Take a moment on your desire for the tiny cylinder equipment purchase. Look at what is motivating your decision. Being honest with yourself, is there an anxiety or fear driving you? Or do you notice a relatively calm state with an intention to effectively manage risk?

If you think about what has caused you to consider carrying emergency air, especially if something distressing happened, do you notice a tightness or resistance in your mind or body? Because if there is a trauma or block there, all the equipment in the world won’t address it without a level of acceptance or willingness to let go of the block. Consider addressing the psychological block, and buying the equipment or doing the training after letting go of the fear. It does not always need to be in that order, but actions that come out of avoidance have a tendency to increase anxiety, so you may find the problem just carries over to something else.

This is a significant issue, because using breathing devices as a form of avoidance may prevent learning, healing, and increase actual risk. This is a valid concern. Relying solely on an emergency backup system instead of addressing the underlying causes of anxiety or improving one’s skills and comfort in the water can lead to a false sense of security. It is essential for divers to address anxiety through appropriate means such as education, training, experience, and possibly seeking guidance from mental health professionals.

Ultimately, diving safety relies on a combination of factors, including proper training, adherence to established diving protocols, equipment redundancy, and psychological well-being. Rather than relying solely on emergency breathing devices, it is important to engage in appropriate preparation, training and your own practices for safe diving.

Some additional issues to consider with spare air

This is a bit of a sidenote, to mention that there are costs to consider in carrying a spare air supply. Firstly, there is the task load of ensuring that extra cylinder is ready for diving, and storing it during the dive. Secondly, there are servicing issues to consider. Depending on location and the technical specifications of the cylinder, there may or may not be legal obligation to service. And because these cylinders are filled by the user from their main cylinder, there may be no barriers to continuing to use an out of service cylinder. Added to this, it’s easy for these cylinders to be emptied, risking water (saltwater) ingress. Therefore, issues such as corrosion can be a concern. A corroded or otherwise damaged cylinder filled to high pressure, typically 200 bar, has a risk of explosion.

Alternative / additional solutions to your concern you will run out of air scuba diving

The first suggestion is to go back to the basics of scuba diving training. If you are a recreational diver using a single cylinder, the standard practice would be to follow the buddy system. Here is a reminder of the key points of the buddy system. Quite often divers drift away from these practices or (for whatever reason) did not fully absorb the concepts in training) so it is worth a reminder.

• Dive with a buddy.
• Stay within reach of your buddy: you should be able to reach your buddy within 2 seconds (that’s a couple of fin kicks).
• Each diver has an alternate regulator to donate and therefore share air in an emergency low or out of gas situation. This could be an additional regulator on a slightly longer hose (“octopus”) or a long-hose set up.
• Plan the dive with your buddy and include gas planning as appropriate for your skill level and breathing rate. For shallower dives, this is often a simple plan with awareness of roughly how long your cylinder lasts and agreement to turn the dive at a certain cylinder pressure, often 100 bar, a reserve and the intention to be back on the surface with 50 bar remaining. Or you can build in more accuracy by applying your personal SAC rate. For more on gas planning, and discussion of turn pressure, minimum gas and reserves.
• As part the buddy check, before every dive, check yours and your buddy’s alternate air supply. The regulator should be: working; checked by breathing (because pressing the purge alone fails to pick up on some faults); have a mouthpiece (!); and be accessible, visible and easy to deploy.
• Practice sharing air regularly to get more comfortable and competent with those skills. The more smoothly you can do it in practice, the easier it will come to you in an emergency.
• Practice effective habits of communication with your buddy and be aware of each other throughout the dive.
• Develop effective habits of checking air/gas supply and reviewing the dive plan. This could be as simple as a practice of checking SPG and computer at regular interval, depending on your level of diving.

If the above is not making sense to you or sounds too difficult, then a skills review / refresher with an instructor will probably be a good idea.

If you are doing all of this, and your air consumption is simply fast would like, there are lots of options to help you use less air when scuba diving:

• Dive shallow to use air at a slower rate (though this should be at the same depth as your buddy / the group. This may also lead to a more relaxed dive, less stress and exertion.
• Scale back expectations and dive in conditions that are appropriate for current skill and experience level. Consumption is likely to then improve due to manageable stress and task loads (which can then be increased as experience is gained).
• Consider physical fitness level and changes that lead to lower breathing rate such as losing weight or improving cardiovascular fitness.
• Improve breathing habits through breathwork training, pranayama or learning diaphragmatic breathing.
• Train relaxation response and parasympathetic activation via meditation or yoga.
• Review your equipment, particularly amount of lead and trim. Diving with optimal weighting for neutral buoyancy can have a significant impact on breathing: too much lead makes finning harder work and can mean lots of dumping and adding air to stay neutral; conversely, too little lead means continually having to empty lungs and the stress of a potential rapid ascent. Issues with trim can mean that finning and hovering takes significantly more effort than it needs to. There are lots of other issues with kit that will affect air consumption.
• Manage your temperature with appropriate exposure suit.
• Discuss concerns and dive with people who are supportive. The stress of feeling you have to eek out your gas supply or worries about what others think can affect breathing habits.
• Let go of expectation. Clinging to a pre-conceived idea of how much air you “should” breath is likely to create stress; and stress increases breathing rate! Rather than attempting to breathe to last a certain dive time, plan the dive to fit your current rate of consumption.

Are you thinking (/ranting at the screen) “WELL THIS IS ALL JUST COMMON SENSE!!!” ?
Please check out this article.

Many divers, particularly in the shallower ranges will find these practices are sufficient to address the concern you may run out of air diving. However, if you prefer to be more skilled as a diver, or are keen to be self-sufficient (perhaps if not diving with regular buddies) then you may want to look into available upgrades to your diving!

Sufficient and redundant spare gas/air

There are several options to dive with more air, and so manage the risk that you would run out of air scuba diving. All involve: carrying more than one cylinder; sufficient volume of air; more than one set of regulators/gauges. A redundant system is one where a whole cylinder and its regulators can fail, and you still have another option for returning to the surface slowly, completing any required stops. This may be the addition of a pony or stage cylinder (but remember to do those calculations above, because even a 5 litre cylinder may not last as long as you need at depth). You could consider moving into twinset diving, using two separate cylinder and regulator systems mounted on your back. Or, similarly, carrying the two cylinders on under your arms as a sidemount diver.

If these options interest you, then look out for courses such as: deep diving, solo/self-sufficient diving, sidemount, twinset or intro to technical diving. Advancing into these areas of scuba diving, with the additional equipment, weight, expense and training is not for everyone. However, even it it is not for you, doing a bit of reading around these areas, or introductory courses, will help build your awareness and ability to manage the risk that you will run out of air scuba diving.