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What is Partial Pressure and Why Is It Important in Scuba Diving?

By

two scuba divers swim though the blue ocean

A nitrox diver must monitor the partial pressure of oxygen in his breathing gas to avoid oxygen toxicity.

istockphoto.com, Jman78

Layman's Definition:

Partial pressure refers to the pressure exerted by an individual gas in a mixture of gasses.

How Does Partial Pressure Apply 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.

What Determines the Partial Pressure of a Gas in Scuba Diving?:

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.

How Can a Diver Calculate 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% 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 gasses 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.
* If you don't understand why the diver is at 3 ata of ambient pressure, it is time to review the basics of Pressure and Scuba Diving.

• More scuba diving terms and definitions: A - J . . . K - T . . . U - Z

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