# "Partial Pressure" or "PP" in Scuba Diving

Partial pressure refers to the pressure exerted by an individual gas in a mixture of gasses. Here's how it works!

## Partial Pressure Applied to Scuba Diving

An easy way to think of partial pressure in scuba diving is to consider it a measurement of the concentration of a particular gas in a diver's mixture of breathing gasses. As the concentration of a particular gas in a diver's breathing gas mixture increases, the physiological and psychological effects of that gas may increase or change. For example, extremely high partial pressures of oxygen may be toxic (oxygen toxicity) and very high concentrations of some gases, such as nitrogen, may cause narcosis.

Two factors determine the partial pressure of a gas in scuba diving—the percentage (or fraction) of the gas in the breathing mixture and the depth (and therefore the ambient pressure) at which a diver breathes the gas. The higher the percentage of a gas and the deeper a diver descends, the greater the partial pressure of the gas.

## Calculating the Partial Pressure of a Gas

It's easy! Simply multiply the percentage of the gas in the breathing gas mixture by the ambient pressure of the dive. For example, if a diver is breathing air (21 percent oxygen) at a depth of 66 feet of sea water, the partial pressure of oxygen is:

0.21 the percentage of oxygen as a decimal fraction
x 3 ata/bar*
the ambient pressure of the dive in units of either atmospheres or bar
= 0.63 ata/bar
the partial pressure of oxygen in air at 66 feet of sea water

Partial pressures of a gas are given in units of either atmospheres or bar. While these units are technically different, they are close enough to be used interchangeably in all but the most picky of calculations.

## Abbreviations

Divers use the abbreviations "P" and "pp" when referring to the partial pressure of a gas. For example, in reference to the partial pressure of oxygen (O2), a diver may encounter the following abbreviations: PO2, pp O2, and O2 pp.

Don't understand why the diver is at 3 ATA of ambient pressure? Then it is time to review the basics of pressure and scuba diving.