Decompression theory?

One of the most common things that aspiring scuba divers as well as people that are simply interested in the sport are wondering about is the idea of decompression and the effects that it can have on a diver’s body. It is true that every responsible diver should understand the basics of decompression theory and how to avoid decompression sickness.

A lot of literature has been written and research conducted on this matter dating back to the early 1900’s when the first famous scientific work on decompression theory was published by J.S. Haldane, a Scottish physiologist, and his colleague. Today, most dive computers and dive tables use the decompression model developed by Haldane to help divers plan safe and enjoyable dives.

What is decompression theory?

One can write an essay on decompression theory, but let us focus on a few basic ideas. Decompression theory essentially deals with the rate at which a diver’s body absorbs and eliminates nitrogen. Nitrogen, an inert gas, constitutes 79% of the air that we breathe. The deeper a diver descends to a particular depth and the longer a diver spends on that depth, the more nitrogen his/her body will absorb – Contolled Ascentnitrogen gas dissolves into a diver’s blood and is carried, in solution, to different parts of the body. This process is set into motion by the increase in surrounding i.e. ambient pressure at depth (atmospheric pressure and hydrostatic pressure). On ascent, ambient pressure decreases and nitrogen elimination process ensues. There are two important laws that are at work here.

Dalton’s law

One of them is Dalton’s law (John Dalton was an English chemist, meteorologist and physicist). The law stipulates that the total pressure of a gas mixture is equal to the sum of the partial pressures of gases in that mixture. This means that with increase in ambient pressure, partial pressures of gases in a gas mixture increase proportionately. For example, ambient pressure on the surface is 1 ATM or 1 BAR. If you have a scuba cylinder filled with air (for simplicity’s sake we will assume that air is 21% oxygen and 79% nitrogen), then 1 BAR (total pressure) = 0.21 BAR (partial pressure of oxygen) + 0.79 BAR (partial pressure of nitrogen). At 10 metres of depth, ambient pressure is 2 BAR i.e. 2 BAR (total pressure) = 0.42 BAR (oxygen) + 1.58 BAR (nitrogen).

Henry’s law

William Henry was an English physician and chemist. Henry’s law states that the amount of gas which dissolves in a liquid is proportional to the partial pressure of that gas. In other words, the higher the partial pressure, the more gas will dissolve in a liquid. This is exactly what happens with nitrogen – the deeper the diver descends, the higher the partial pressure of nitrogen in the air that he/she inhales, the more nitrogen will dissolve in that diver’s body. Naturally, the more time this diver spends at depth, the more time there is for nitrogen to get absorbed by the body.

Decompression sickness

Nitrogen itself has absolutely no effect on our body, our body does not use it for anything – it is, indeed, an inert gas. However, as a diver ascends, ambient pressure decreases, as does partial pressure of nitrogen, and his/her body starts to off-gas i.e. give off/get rid of nitrogen – back from tissues to blood to lungs to exhalation. However, if a diver is not careful enough and ascends to the surface too quickly, nitrogen, instead of staying in solution, starts forming into bubbles and blocking the diver’s blood vessels. Imagine shaking a can of Coke and then opening the lid – this is equivalent to what happens in a diver’s body if he/she ascends too fast. Effects and symptoms are different depending on where the blockages occur – in the capillaries near the surface of the skin, in the chest, in joints or muscles, in the spinal cord or in the brain. This is called decompression sickness.

Prevention of decompression sickness

Safety StopPrevention is all about proper dive planning and proper ascent procedures. This is the reason why divers make safety stops at 5 metres for 3 to 5 minutes on every dive – just hanging at that depth to allow ample time for their bodies to off-gas or, in other words, get rid of excess nitrogen. This is also why divers take care to ascend no faster than 9 metres per minute, ensuring that nitrogen does not come out of solution and form bubbles. Divers also use dive computers and dive tables so they can plan and execute their dives safely and avoid decompression sickness. Technical divers, in turn, are specially trained in decompression diving, which is beyond the limits of recreational diving, and make decompression stops at various depths, sometimes for hours at a time.

There is much more that can be said about decompression theory, decompression sickness, its prevention and treatment. These are just the basics. Hopefully, you found this article interesting and would like to find out more. One option is to take the SSI Science of Diving Course.

Happy diving!

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