Dive Physics: Part 1 (Understanding Partial Pressures)

Understanding partial pressures is key to being a safe deep, technical or commercial diver. Around multiple dive shops I have seen poorly trained instructors say "You don't need that...", and if all you ever want to do is casual dives with your local dive shop, they may be right. So, if you are an Open Water Diver or Advanced Open Water diver with no aspirations to be a Deep Diver, Divemaster or Technical Diver, this is not for you.

Why understand partial pressures?

Partial pressures are perhaps the easiest way of educating people on gas contents and ensuring they understand that Maximum Operating Depths (MOD's) are not a 'reccomendation', they are the furthest you can reasonably go without killing yourself.

So... Air is Oxygen?

If you ask me to fill your tank with Oxygen, you had better not plan on going deeper than about 5msw. Oxygen (O2) makes up about 21% of the air on earth (in polluted countries like China you may have slightly less and in uninhabited parts of the world you may have slightly more, but we always approximate air as 21% oxygen).

There is also about 78% Nitrogen (N) in the air. The remaining 1% is a blend of gases which human bodies have become used to like Carbon Dioxide, Hydrogen, Helium etc.

What is air?

78% N (Nitrogen)

21% O2 (Oxygen)

1% OG (Other Gases)

Partial Pressure, give me an example

For partial pressure it is not often expressed in percentage but usually as a single number. For instance 100% Oxygen under atmospheric pressure would have a partial pressure of 1.00 O2.

If we break down the atmospheric gases it would be something like 0.78 N + 0.21 O2 + 0.01 OG = 1.00 Air. (0.78:0.21:0.01)

How does this affect diving?

When a cylinder is filled at a dive shop, in this case we will work with Air, whilst it is filled to around 230 bar, the pressure around the cylinder is still 1 bar (atmospheric pressure).

As you descend in salt water, every 10 metres is another atmosphere (bar) of pressure.

Surface = 1 Bar, 10 msw = 2 bar, 20 msw = 3 bar and so on. This also increases the partial pressure of the gases in the cylinder.

At 10 metres, you would be under 2 bar of pressure. The gas partial pressure would be multiplied by a factor of two (2). The new partial pressure would be 1.56 Nitrogen : 0.42 Oxygen : 0.02 Other gases.

In short, at the surface, your tank was 78% Nitrogen and 21% Oxygen, at 10 metres it is equivalent to 42% Oxygen.

PADI works off of a 1.4 pp and 1.6 pp rule for Oxygen. 1.4 pp Oxygen is the recommended maximum partial pressure for diving, 1.6 pp is the absolute maximum (for instance, if you had to go deeper than 1.4 for an emergency). Once the partial pressure of oxygen has reached 1.6, Oxygen toxicity is very likely and death is almost expected.

So, once you are breathing the equivalent of 160% oxygen, you are in very serious danger.

How deep would I have to go to get a partial pressure of 1.6?

If you are breathing normal air, 66 metres.

How do divers go really deep then?

They have seperate tanks that are 'hypoxic' (low on oxygen at surface). For instance, If I wanted to go down to 80 metres (without exceeding my Oxygen pp of 1.4), I would have to be breathing in no more than 16% oxygen (which would be equivalent to 140% oxygen at 80 metres). This is acheived through 'TriMix' (a blend of Nitrogen, Helium and Oxygen where the helium takes up the space that would otherwise be filled with oxygen).

Did I not explain this well? Let me know at

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