To both experienced and beginning scuba divers, the partial pressure concept is often confusing. If you are currently wondering what is the partial pressure of oxygen or what is the partial pressure of nitrogen, this article will help you figure out the answers to these questions.
What is Partial Pressure in Scuba Diving?
When diving, scuba divers have a mixture of gases in their scuba diving tank. Simply defined, PP (Partial Pressure) refers to the pressure exerted by each gas, in the mixture.
You can think of the partial pressure as the concentration of a specific gas in the scuba diver’s breathing gas mixture. As the concentration of a specific gas increases, its psychological and physiological effects may change or increase. To give you an example, if the oxygen partial pressure is too high, the scuba diver may end up dealing with oxygen toxicity. If the nitrogen concentration is too high, it may cause narcosis.
Factors Affecting the Partial Pressure of a Gas
The partial pressure of any gas, in scuba diving, is affected by 2 main factors. The factors are:
- The fraction (or the percentage) of the gas in the diver’s breathing mixture.
- The dive depth (and hence the ambient pressure) at which the scuba diver is breathing the gas.
The partial pressure of a gas will increase as both the depth and the gas percentage increases.
How to Calculate the Partial Pressure of Gas
Calculating the partial pressure of a gas is extremely easy. You will only need to know the ambient dive pressure and the percentage of the gas. Next, you will need to find the product of the gas percentage and the ambient dive pressure.
Let’s assume that you intend to use your best rebreather to stay at a depth of 66 feet. If you are breathing air, the answer to what is the partial pressure of oxygen should be:
0.21 x 3 atm/bar = 0.63 atm or bar
Note: 0.21 is the oxygen percentage (21%) as a decimal fraction. 3 atm or bar is the ambient dive pressure at 66 feet. 0.63 atm or bar is the partial pressure of the oxygen gas at 66 feet.
The ideal units for the partial pressure are either the bar or the atmosphere. Although the units are different, they are close. They can be used interchangeably in all but very picky calculations.
Partial Pressure Abbreviations
Scuba divers will use either “pp” or “P” to refer to the partial pressure of a gas. For example, when referring to the Nitrogen (N2) partial pressure, a diver may use PN2, pp N2, or N2 pp.
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As a scuba diver, understanding what partial pressure means is crucial. Incredibly high partial pressures may affect you negatively. Ensuring that your partial pressure is within the right range can help you avoid issues like oxygen toxicity and narcosis. While controlling the gas percentage may be impossible, you can always regulate the partial pressure of a gas by adjusting the dive depth.
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