Partial pressure is the pressure exerted by a specific gas within a group of gases that found in a mixture. Let’s discuss the science of partial pressure in scuba diving in this article.

Understanding Partial Pressure 

At core of “partial pressure” is the concept that gases in a mixture, such as air (21% oxygen and 79% nitrogen), act independently. As a result, When we expose these gases to a liquid, they dissolve into the liquid based on the pressure of each gas separately.

For example, When water is in contact with atmospheric air, the amount of oxygen that dissolves into the liquid is equivalent to the amount that would dissolve if the water were placed in a container with only 0.21 ATA of pure oygen.

Upon descending, the gas you breathe becomes denser, leading to more molecules of each gas in a given volume. This is like placing a gas under higher pressure in contact with a liquid. At 33 feet, the oxygen concentration remains 21%, but the total pressure increases to 2 ATA.

So, the oxygen pressure is 0.42 ATA, and the amount of oxygen that dissolves into your blood is similar to that of pure oxygen at 0.42 ATA. Each gas behaves independently without affecting other gasses.

Partial Pressure in Scuba Diving

A simple way to understand partial pressure in scuba diving is by dealing with it as the concentration of a specific gas in the diver’s breathing mixture.

When the concentration of a specific gas in a diver’s breathing mixture increases, the effects of that gas—physiological and psychological— may change. For instance, very high partial pressures of oxygen can lead to oxygen toxicity, while high levels of gases like nitrogen may result in nitrogen narcosis.

There are two factors to consider in order to determine the partial pressure of a specific gas in scuba diving:

• The percentage of that gas in the breathing mixture.
• The depth of the diver when breathing the gas.

The higher the gas percentage and the deeper your dive, the greater the partial pressure of the gas.

Calculating the Partial Pressure of a Gas

It’s simple! All you need to do is multiplying the gas percentage in the breathing mixture by the ambient pressure at the depth of that dive. For instance, if a diver is breathing normal air (containing 21% oxygen) at 66 feet of seawater, the partial pressure of oxygen would be:

0.21 (the oxygen percentage as a decimal fraction) x 3 ATA/bar (ambient pressure at the depth of the dive) = 0.63 ATA/bar (partial pressure of oxygen in air at 66 feet of seawater).

Partial pressures of a gas are measured in atmospheres or bars. Although it is considered different in the techniques, these units are used interchangeably in most calculations.

Abbreviations of Partial pressure in Scuba Diving

Divers use the abbreviations “P” and “pp” for the partial pressure of a gas. For example, for the partial pressure of oxygen (O2), a diver might see abbreviations like PO2, O2 pp, and pp O2.

If you don’t understand why the diver is at ambient pressure of 3 ATA? Then you should review the basics of partial pressure in scuba diving.