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Gases Under Pressure


Much has been said about how the 300bar, higher pressure diving cylinders actually have less gas in them than the gauge suggests, and there's a need to understand why this is.

Any cylinder with a pressure of 232 bar or more is effectively holding less gas per bar of pressure than a lower pressurised cylinder.


The gases we use in diving cylinders work as 'ideal gases' under lower working pressures, but start to react differently under pressures over a certain point. (we'll ignore temperature for this explanation).

A layman's explanation.

The best way is to explain each term as if the reader knows nothing, and build upon that.


Of stable shape, usually rigid at normal temperatures.


Having a consistency like that of water or oil, incompressible, but not resistant to change of shape, neither solid or gaseous.


Any air-like or completely elastic fluid, especially not liquid or solid at ordinary temperatures. A gas will expand to fill the container it is enclosed in.


Consisting of particles that move freely among themselves and yield to slightest pressure. Not solid or rigid or stable. Gases and liquids can both be described as 'fluid', or being in a fluid state.

For the ease of explanation we will describe all diving gases, regardless of the mixture, as simply 'Gas'.

Ideal gases:

As far as divers are concerned, an ideal gas behaves in this fashion. When you compress it into a limited space, ie: a diving cylinder, for the most part it just gets 'thicker'.

Up to pressures of around 200 bars this is fine, and when the gauge reads 200 bars, then the cylinder actually contains 200 times the amount of gas that cylinder would hold at normal atmospheric pressure.

If the capacity of the cylinder is 10 litres, then this cylinder at 200 bars pressure contains 2000 litres (10 x 200 = 2000).


When a gas is placed under greater pressures than say, 232 bars, then the ideal gases theory starts to become a little hazy.

The differences aren't really that noticeable until the pressure starts to approach 300 bars, when the gauge is really quite inaccurate.

To put it into the simplest terms, the gas now starts to behave more like a liquid than a gas because it's so 'thick', and as we've already said, liquids are virtually incompressible, so you just can't pack any more gas into the cylinder, because it has got so 'thick' that there's hardly any room left.

However, the gauge climbs more quickly than it would normally, because even a small amount added squeezes the pressure far higher than it normally would.

Imagine trying to get 2000, or even 3000 litres of water into a 10 litre cylinder. No one would try, because it's obvious that it wouldn't work.


Webmaster 5/6/2001


Want to know more?

Want to see how to calculate how much gas you really have in your cylinder at 300 bars pressure?

Then click on these links below, for the really clever advanced stuff, or more precisely, the scientific way of explaining it, Don't say we didn't warn you:


Paul Thomas (Dive Instruct member)

Paul's work on comparison tables for different gases:

Johannes Diderik van der Waals laws


Nigel Hewitt:

Nigel's Home page

Nigel's Partial pressure page

Nigel's ' Johannes van der Waals law' page


Alasdair Allan

Alasdair's The Gas Laws and SCUBA Diving page


Johannes Diderik van der Waals:

Nobel prize winner 1910 - presentation speech

Biography of Johannes van der Waals



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