Latest News

Weather and Tides

Gas Laws

Central to all diving theory are the Gas laws.

Introduction

The behaviour of all gases is affected by three factors:  the temperature, pressure and volume of the gas. The relationships among these three factors have been defined in what are called the Gas Laws. The following:

• Boyle's Law, - Pressure & Volume;
• Charles' Law, - Temperature & Volume;
• Henry's Law, - Solubility of Gases;
• Dalton's Law, - Partial Pressures;

are of special importance to the diver.

Boyle's Law

Boyle's Law states:

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

For any gas at a constant temperature, Boyle's Law is:

	PV		= K
where
	P		= absolute pressure
	V		= volume
	K		= constant.

Boyle's Law is important to divers because it relates changes in pressure i.e., depth, to changes in the volume of a gas and defines the relationship between pressure and volume in breathing gas supplies.

Along with the volume of air in the balloon, the surrounding pressure will affect the density of the air as well. Density, simply stated, is how close the air molecules are packed together. The air in the balloon or container at the surface is at its standard density, but when we descend to the 10 Metres where its volume is reduced to one half, the density has doubled. At 20 Metres, the density has tripled. This is because the pressure has pushed the air molecules closer together.

The reverse also happens, suppose we inflate a balloon at 30 Metres  We know the air at this depth is 4 times denser than at the surface. As the balloon ascends, the external pressure lessens and  the balloon will expand, eventually bursting. 

In these examples of Boyle's Law, the temperature of the gas was considered a constant value. However, temperature significantly affects the pressure and volume of a gas; it is therefore essential to have a method of including this effect in calculations of pressure and volume. To a diver, knowing the effect of temperature is essential, because the temperature of deep water is often significantly different from the temperature of the air at the surface. The gas law that describes the physical effects of temperature on pressure and volume is Charles' Law.

Charles' Law

Charles' Law states:

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

Stated mathematically:

	V1	=	V2	(volume constant)
	¯¯¯¯ T1		¯¯¯¯ T2
where
	V1 =	Initial volume (absolute)
	T1 =	Initiall pressure (absolute)
	V2 =	final pressure (absolute)
	T2 =	final pressure (absolute)

Note temperatures must be in Kelvin. To convert Centigrade to Kelvin just add 273.15 degrees.

In practice for every change in temperature of one degree Celsius, the pressure in a scuba tank changes about 0.62 of a bar.

Henry's Law

Henry's Law states:

The amount of any given gas that will dissolve in a liquid at a given temperature
is a function of the partial pressure of the gas that is in contact with the liquid
and the solubility coefficient of the gas in the particular liquid.

This law simply states that, because a large percentage of the human body is water, more gas will dissolve into the blood and body tissues as depth increases, until the point of saturation is reached. Depending on the gas, saturation takes from 8 to 24 hours or longer. As long as the pressure is maintained, and regardless of the quantity of gas that has dissolved into the diver's tissues, the gas will remain in solution.

A simple example of the way in which Henry's Law works can be seen when a bottle of carbonated water is opened. Opening the container releases the pressure suddenly, causing the gases in solution to come out of the solution and to form bubbles. This is similar to what happens in a diver's tissues if the prescribed ascent rate is exceeded.

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.

In a gas mixture, the portion of the total pressure contributed by a single gas is called the partial pressure of that gas. Stated mathematically:

	PTotal	= Ppl + Pp2 + Ppn
where
	PTotal	= total pressure of that gas
	Pp1	= partial pressure of gas component 1
	Pp2	= partial pressure of gas component 2
	Ppn	= partial pressure of other gas components.

If a container (at 1 Bar) were filled with oxygen alone, the partial pressure of the oxygen would be 1 Bar. If the same container were filled with air, the partial pressures of each of the gases comprising air would contribute to the total pressure, as shown in the following table:

Percent of Component
x Total Pressure (Absolute) = Partial Pressure

When diving at a depth of 40 metres (5 Bar) you multiply the partial pressures by 5 and calculate the partial pressures at that depth