Minimum Gas for Deep Diving: The Risk of Single-Tank Bounce Dives
Updated: Sep 30
We have probably all heard stories of divers making deep bounce dives beyond their training limits: descend, touch a depth, take a photo of the computer and come straight back up.
Recently I was on a boat where an experienced instructor made a bounce dive to around 60 metres on a single cylinder and later shared the depth on social media. I am not writing this to criticise that individual, and a photo of a dive computer certainly does not tell us everything about how a dive was planned, but it did make me think.
These calculations show how quickly minimum gas increases with depth. They are not a recommendation to dive deep on a single cylinder. Beyond normal recreational depths, gas volume is only part of the picture. Proper training, experience, appropriate equipment, redundancy, gas density, narcosis and decompression all matter too.
Earlier this year, the diving community was also reminded in the saddest possible way how unforgiving diving can become when people venture beyond their training or use equipment that is not appropriate for the dive. Rather than discussing any particular diver or accident, I thought it would be more useful to look at one specific part of the problem: gas. The question is not simply whether you have enough gas to get yourself down and back, but whether, if something goes wrong at the deepest point of the dive, you still have enough gas to get both yourself and your buddy safely home.
One of the most common justifications for a bounce dive is that the diver will only be deep for a few minutes. That certainly reduces the gas used during the planned part of the dive, but it does not significantly reduce the gas needed to deal with an emergency at maximum depth. If your buddy loses access to their gas at 40 metres, you are still at 40 metres. They now need your gas, both of you may be stressed, you need to establish gas sharing and control the situation, and then you still have to get two people all the way back to the surface. Whether you planned to spend three minutes or twenty minutes at depth does not change where that emergency starts.
That is where minimum gas — or rock bottom — becomes important.
What Is Minimum Gas (Rock Bottom)?
Minimum gas is the amount of gas you deliberately reserve so that two divers can deal with a serious problem at the worst point of the dive and still reach the surface, or the next available breathable gas supply, safely. You may also hear the term rock bottom used for essentially the same concept. Reserve gas is a broader term and may simply mean gas someone has decided not to use, such as a fixed 50 bar reserve, but that is not necessarily the same thing as a calculated minimum gas.
The important distinction is that minimum gas is not the pressure you hope to have left when you reach the boat. It is gas that has already been allocated to an emergency, which means that once you reach minimum gas the normal part of the dive is over. This also changes how we think about usable gas. If you start an AL80 with 200 bar and calculate that your minimum gas is 130 bar, you do not really have 200 bar available for exploring; you have about 70 bar of usable gas, with the remaining 130 bar reserved for the emergency you hope never happens.
Breathing Rate Matters
To calculate minimum gas we need to make an assumption about how much gas two divers might use during an emergency. Divers often refer to this as SAC or RMV. A relaxed recreational diver might have an RMV somewhere around 12–20 litres per minute, although this varies considerably with experience, fitness, workload, current, temperature and stress.
During an emergency, breathing rates can increase very quickly, which is why minimum-gas calculations normally use a higher figure. For the examples in this article I am using 30 litres per minute per diver, so two divers sharing one gas supply have a combined surface-equivalent breathing rate of 60 litres per minute.
At depth, gas use increases with pressure. At 40 metres the surrounding pressure is about five times surface pressure, so those same two divers would initially be consuming around 300 litres per minute from the cylinder. That is why depth changes the gas equation so quickly.
What Does That Look Like in an AL80?
Here in Mahahual we commonly use aluminium 80 cylinders. An AL80 has a water capacity of approximately 11.1 litres and is typically filled to around 200–207 bar. For the examples below I am assuming two divers sharing one cylinder, a breathing rate of 30 L/min per diver, one minute at maximum depth to recognise the problem and establish gas sharing, an ascent rate of approximately 9 metres per minute, a three-minute stop at 6 metres, and then a controlled final ascent to the surface.
These are planning assumptions rather than universal rules, but they give us a useful way of seeing what happens as depth increases.
The trend is the important part. At 20 metres we still have a substantial portion of the cylinder available for the dive, but by 30 metres close to half the cylinder may already be spoken for. At 40 metres our example requires around 127 bar purely as emergency gas, at 50 metres around 166 bar, and by 60 metres the calculated minimum gas is around 210 bar, effectively the entire contents of a normally filled AL80 before the diver has even descended.
That does not mean the solution is to keep adjusting the assumptions until the numbers fit the cylinder. It means the gas plan is telling us that, beyond a certain point, the equipment and the dive no longer match.
A Worked Example at 40 Metres
Let’s look at 40 metres in more detail. We have two divers, one of whom has lost access to their gas and is now breathing from the other diver’s AL80. With our chosen emergency breathing rate, the two divers together are using the equivalent of 60 litres per minute at the surface.
At 40 metres the ambient pressure is approximately 5 ATA, so during the first minute while the problem is recognised, gas is donated and control is established, the pair will use approximately:
60 × 5 × 1 = 300 litres
That first minute can disappear quickly because the divers need to recognise the problem, establish gas sharing, control buoyancy, communicate and begin the ascent.
From 40 metres to 6 metres is 34 metres. At an ascent rate of around 9 metres per minute, that takes approximately 3.8 minutes. Because pressure is decreasing throughout the ascent, we can use an average pressure of around 3.3 ATA, which gives:
60 × 3.3 × 3.8 ≈ 750 litres
By the time the divers reach 6 metres, they have used roughly 1,050 litres.
A three-minute stop at 6 metres adds:
60 × 1.6 × 3 = 288 litres
Allowing for the final controlled ascent to the surface brings the total to around 1,400 litres. In an 11.1-litre AL80 that is approximately:
1,400 ÷ 11.1 ≈ 126 bar
For practical planning, that gives us a rock-bottom/minimum-gas reserve of about 130 bar minimum gas at 40 metres.
Now consider what that actually means. If you are sitting at 40 metres with 100 bar and thinking you still have half a tank, under this scenario you are already below your calculated minimum gas — your rock bottom — before anything has gone wrong.
That is the problem with treating cylinder pressure as a simple fraction. Two hundred bar feels full, 100 bar feels like half a cylinder and 50 bar feels like reserve, but those numbers do not mean the same thing at every depth. In an AL80, 100 bar is around 1,110 litres of gas. At a relatively shallow depth that may be a very useful reserve. At 40 metres, two stressed divers may initially be using gas at around 300 litres per minute between them.
Suddenly 100 bar looks very different.
Your NDL Does Not Answer the Same Question
Another common justification for a deep bounce is that the diver will remain within their no-decompression limit. That may be true, but NDL and gas supply are completely different limits.
A dive computer is estimating decompression exposure. It is not calculating whether one remaining cylinder contains enough gas for two people to deal with a serious failure and reach the surface. It is entirely possible to have plenty of NDL remaining while already being below an appropriate minimum gas reserve. If the computer says you can stay another ten minutes but your gas plan says you should already be ascending, the gas plan wins.
The same applies to the three-minute stop used in this example. If two divers were genuinely running short of gas after an emergency, no sensible diver would deliberately run out of gas simply to complete a safety stop. The stop is included because I do not want a planned emergency reserve to depend on everything going perfectly followed by the fastest possible ascent directly to the surface. It builds some margin into the calculation.
Different divers may choose slightly different assumptions, and that is fine. The important thing is that those assumptions are made before the dive rather than being invented at depth to justify the gas remaining in the cylinder.
Gas Is Only Part of the Problem
As depth increases, gas quantity is not the only thing that changes. Narcosis increases, no-decompression limits get shorter, equipment failures carry greater consequences, and redundancy matters more.
Gas density is also a major issue. The deeper we go, the denser the gas becomes, so every breath takes more effort. That can make it easier to retain CO₂, especially when working hard or stressed. High CO₂ can cause breathlessness, anxiety and poor thinking, and can make narcosis worse. We often call this nitrogen narcosis, but gas narcosis is a better term because nitrogen is not the only gas involved. At greater depths, helium is often used in technical diving to reduce both narcosis and gas density.
A bigger cylinder, or simply carrying more cylinders, does not solve all of those problems. As dives get deeper, the equipment, training and gas plan need to change too.
The Point Is Not to Avoid Going Deep
I love deep diving, and some of the most interesting places underwater are beyond normal recreational limits. The issue is not depth itself, but what we are trying to achieve there. A dive computer showing 50 or 60 metres might look impressive, but if the whole exercise consists of dropping down, touching the depth and immediately beginning the ascent, then the number has become the achievement rather than a means of getting somewhere worth exploring. What matters is having the training, gas, redundancy and equipment to make the dive worthwhile.
Technical diving changes the question from “Can I get to 50 metres?” to “How can we safely plan a dive to 50 metres, spend meaningful time there, deal with foreseeable problems and get the team home?” That is a much more interesting goal.
A properly planned technical dive gives us something a bounce dive doesn't: meaningful time at depth. Time to explore the reef, photograph something properly, follow a wall or simply enjoy being somewhere that made going deeper worthwhile in the first place.
If you want to understand more about how technical diving changes the way we plan and execute deeper dives, read “What Is Technical Diving – and Why Learn in Mahahual?” or explore our PADI TecRec training options.
Go deeper because there is something worth seeing, and plan the dive well enough that when you get there, you can stay awhile.
Want to see what's down there?
Whether that means taking your first step into technical diving, exploring more of Mahahual’s reefs with a DPV, or simply talking about where you want your diving to take you next — let’s go diving.
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