Boyles Law & The Martini Effect

If you are planning on taking part in recreational diving, it is important to understand some basic laws of physics, so that you can apply them to your new underwater environment. Understanding Boyles law will help ensure a safer dive. As our bodies require oxygen to survive, this is supplied as a gas during diving. These gases interact with the fluids in our bodies, according to specific rules of pressure and temperature.

Photo by ClifB

Boyle’s law was named after Robert Boyle, a physicist and chemist, .although two amateur scientists, were actually the first to make note of this relationship between pressure and volume. Boyle’s law came about after several experiments with air. However, a French physicist, Edme Mariotte had also discovered this law by himself in 1676. By this time Boyle had already published it and so, Boyle’s law is also referred to as Marietta’s or the Boyle-Marietta’s law.

The original law was first published by Boyle in 1662 and deals with the relationship between the absolute pressure and volume of a gas in a closed system, where the temperature is kept constant.

Boyle’s law explains how the volume of a gas varies with surrounding pressure and that the product of the pressure and volume for a fixed amount of an ideal gas will always be the same, if kept at a constant temperature.

pV = k, is the mathematical formula relating to Boyle's law.
p represents the pressure of the system
V represents the volume of the gas
k represents a fixed (or constant) number

Although this may seem a bit daunting, in simple terms it basically means that when you multiply pressure surrounding a gas, by the volume of the gas, the number will always be the same. As the constant cannot change, if the pressure surrounding a gas is increased, the volume has to decrease. In the same way, if the pressure is decreased, the volume will increase. Remember that this only applies when the temperature is constant and not if the temperature of the gas in increased or decreased.

Boyle's law is relevant in most areas of scuba diving, due to the underwater pressure. Water pressure increases when a diver descends, causing the air in the BCD to compress. When he ascends, the water pressure decreases and the air will expand. That is why divers are required to release excess air during the ascent.

Another Law which comes into effect during scuba diving is Martini's Law or the Martini Effect. This is also known as nitrogen narcosis and has altering effects on the consciousness of the diver, at various depths. Narcosis is said to be similar to alcohol intoxication and divers associate every 10 metres of depth with consuming one martini. Although the martini effect can occur during shallow dives, Nitrogen narcosis usually becomes apparent at a depth of about 30 metres where the effect is compared to having drunk 3 martinis. Therefore 40 metres would be 4 martinis, 50 metres 5 martinis and so on. This is only a rough guide used to give new divers an idea of the effect by referring to something with which they may be more familiar.

There are some factors which can increase the chances of narcosis. These include rapid descent, stress, cold temperatures, alcohol intake and an excess of carbon dioxide.

During the early stages, the martini effect, or narcosis, gives the diver a feeling of peace and lack of anxiety. However , narcosis can be dangerous due to the fact that the diver’s judgment becomes impaired, resulting in poor decision making, loss of co-ordination, hallucinations and ultimately coma. Nitrogen narcosis can be fatal.