The single biggest constraint in all forms of scuba diving is time. Whether it’s decompression obligations or the amount of breathing gas, the clock is always ticking. For the average diver the normal signal that it’s time to end a dive is running low on air.
There are obvious and well-used solutions: carry more gas in multiple cylinders or use a rebreather to recycle and optimise what you have.
Another, far less popular, solution to extending your bottom time is to carry more gas in higher-pressure cylinders. A standard 12L steel cylinder is rated to 232 bar and an 11.1L aluminium to around 200 bar. However, cylinders rated to 300 bar are available and, using basic Open Water physics, should give up to a 50% increase in gas and, theoretically, dive time
On the surface, this looks like the easiest solution to give divers more time in the water. Why use more or larger cylinders when you can use the same sized cylinder with more gas in it? So the question is: why aren’t 300 bar cylinders popular?
Practicalities
The most obvious answer is that there aren’t that many places that offer fills up to 300 bar. The compressors themselves are available; it’s more a question of time. To get a good fill to a standard pressure of 230 bar takes time and some effort. If a cylinder is filled quickly, the gas inside heats and the pressure increases. It looks good initially, but when the cylinder cools after the fill is finished, the pressure will drop — easily losing 30–40 bar.
So to get the best fill — keeping the cylinder cool (like in water) and doing it slowly — is key. You can normally get a far better fill if you do it yourself rather than at a dive centre, where they’re often trying to fill multiple cylinders as fast as possible. This problem would be significantly increased when trying to routinely fill cylinders up to 300 bar and is one reason few dive centres offer the service.
There’s also the question of weight. 300 bar cylinders require thicker walls and are significantly heavier than their lower-pressure siblings. In fact, a 12L 300 bar cylinder is only fractionally lighter than a 15L 232 bar — negating another advantage.
However, the most significant reason for the unpopularity of 300 bar cylinders lies in the physics.
Ideal Gas Law
We now need to go back to some basic diving physics. All divers should have an instinctive understanding of a couple of key principles (though their ability to remember their names may vary…): Boyle’s Law: if you double the pressure, you halve the volume — and Charles’ Law: the volume of a gas is proportional to its temperature (i.e. gas heats up if compressed).
Boyle’s Law and Charles’ Law can then be combined with Avogadro’s Law and Gay-Lussac’s Law (both less relevant to diving) to form the Ideal Gas Law. This describes the overall relationship between pressure, temperature, volume, and the amount of substance (molecules of gas). It can be expressed as:
This turns out to be a very good approximation of how gases generally behave. You can calculate nearly all diving physics using it. However, it does have one big caveat: its accuracy significantly reduces at higher pressures — such as in a 300 bar cylinder.
Things Get Weird
The Ideal Gas Law makes two key assumptions that have to be considered when making calculations at higher pressures: it assumes gas molecules are dimensionless points and that they only have kinetic collisions.
Both of these are incorrect: gas molecules do occupy a measurable volume and have complex intermolecular interactions.
The two considerations are negligible at standard atmospheric pressures, where gas molecules are relatively far apart and moving slowly. When compressed, and the gas molecules are closer together, these two assumptions need to be accounted for. This is described as a non-ideal gas and can be shown with the van der Waals equation:
This looks far scarier than the Ideal Gas Law, unless you’re into physics. However, all that has been added are two correction factors to account for the size of the gas particle and the intermolecular forces. These are unique values for each different gas.
What this means in practice is that each gas compresses differently, so you need to know what you are compressing to accurately predict its behaviour at high pressure.
To illustrate the impact, I’ve done an example below showing the difference between calculating a cylinder of air filled as both an ideal and non-ideal gas. As a reminder, moles are the unit of measure for the quantity of molecules and the easiest measure of gas quantity that totally ignores pressure.
| Ideal Gas (moles) | Non-Ideal Gas (moles) | |
| 200 bar 12L Cylinder | 96.2 | 76.6 |
| 300 bar 12L Cylinder | 144.3 | 104.3 |
| Percentage Increase | 50% | 36% |
As expected, when the Ideal Gas Law is used and the cylinder pressure is increased by 50%, there is an equal rise in the quantity of gas. That increase is not matched when calculating its content as a non-ideal gas, only returning a 36% uplift.
Final Thoughts
It’s a simple question that has quite an interesting explanation. 300 bar cylinders aren’t popular because they don’t offer the benefits they appear to have at first glance. They’re not worth the effort to fill, nor the limited additional gas they actually deliver. Most people will opt for a larger cylinder.
There will always be niche applications where divers need every available piece of performance, and 300 bar cylinders no doubt have a role. Interestingly, the more I’ve looked at the physics, it’s clear the penalty of non-ideal gases is greatly reduced with helium. It’s a light, small, and non-polar element, so behaves very close to an ideal gas even at high pressures.
Volume is used as the standard measure of the quantity of gas in scuba diving. It makes things neat and easy — e.g. if I have a 12L cylinder at 200 bar, I know I should have 2400L. Combine this with your breathing rate, accounting for depth, and dive planning is relatively simple
The behaviour of non-ideal gases demonstrates this isn’t the case in the real world. I don’t think calculating gas quantity in moles is likely to catch on — 1 mole equals 6 × 10²³ molecules of a given substance, so it’s hardly intuitive. However, it does show that the basic diving physics we’ve all been taught is a little more complicated in practice.
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