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Diving Physics and You

Charles' Law | Dalton's Law |
Henry's Law

BOYLE'S LAW

Boyle's law states:

At a constant temperature, the volume of a gas varies inversely with the pressure, while the density of a gas varies directly with pressure.

Simplified:

If the temperature is constant and air pressure increases, the density of the air increases also, while the volume decreases and vice versa.

As a diver, Boyles law affects you every time you enter the water. Air spaces in the body are subjected to pressure and volume change, in direct proportion to your depth.

Without doubt, understanding Boyle's Law is very important in scuba diving.

Note that Boyle's law also relates to gas density. This part of the law becomes particularly important on deep dives; inhaled air will become denser the deeper one goes. It follows that increased gas density increases gas absorption.

Some applications of Boyle's Law in action:

CHARLES' LAW

Charles' law states:

At a constant volume, the pressure of gas varies directly with absolute temperature.

Simplified:

Given a constant volume of gas, the higher the temperature the higher the gas pressure, and vice versa.

Suppose you have a 10lt steel scuba tank holding 200 bars of air this equates to 10 x 200= 2000 litres of available air filled when the air temperature was 20°c . Now you take the tank into water that is 10°c. Before you take your first breath of that tank's air, Charles's law predicts that the tank pressure will be lower than 200 bar.

Since the water temperature is less than the air temperature the law predicts that the pressure now will be less than at 20°c. Dive shop owners know about Charles's law, which is why they often fill tanks in water where the temperature is kept lower than the surrounding temperature.

So what problems can occur with Charles law?

 

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DALTON'S LAW

Dalton's law states:

The total pressure exerted by a mixture of gases is equal to the sum of the pressures that would be exerted by each of the gases if it alone were present and occupied the total volume.

Simplified:

The total pressure of any gas mixture is the sum of it's parts. For example, with air:

Gas

Pressure on surfacePressure at 50msw
Air1 bar absolute6 bar absolute
Air made up of:
Nitrogen, 79%0.79 bar absolute4.74 bar absolute
Oxygen, 21%0.21 bar absolute1.26 bar absolute

In the case above, you can see that on surface the partial pressure of nitrogen is 0.79 bar absolute and oxygen 0.21 bar absolute, which relates directly to the percentage. At 50msw the percentages stay the same, but the partial pressure increases.

The amount of gas absorbed by the diver at depth is directly proportional to the partial pressure of the gases breathed.

So what problems can occur with Daltons law?

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HENRY'S LAW

Henry's law states:

At a constant temperature, the amount of a given gas dissolved in a given type and volume of liquid is directly proportional to the partial pressure of that gas in equilibrium with that liquid.

Simplified:

When the ambient pressure is increased with depth, the partial pressure of oxygen and nitrogen in the body rises. There will be more molecules of each gas dissolved in the blood and tissues.

Dissolved gases will diffuse out via the lungs on ascent as ambient pressure decreases, until a new equilibrium is established. This will continue even after surfacing until all the dissolved gas is removed. A controlled ascent rate and the completion of decompression stops goes a long way to avoiding problems.

So what problems can occur with Henry's law?

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