Partial Pressure in Diving Breathing Gases
In recreational diving and professional diving the richness of individual component gases of breathing gases is expressed by partial pressure.
Using diving terms, partial pressure is calculated as:
- partial pressure = (total absolute pressure) × (volume fraction of gas component)
For the component gas "i":
- ppi = P × Fi
For example, at 50 metres (165 feet), the total absolute pressure is 6 bar (600 kPa) (i.e., 1 bar of atmospheric pressure + 5 bar of water pressure) and the partial pressures of the main components of air, oxygen 21% by volume and nitrogen 79% by volume are:
- ppN2 = 6 bar × 0.79 = 4.7 bar absolute
- ppO2 = 6 bar × 0.21 = 1.3 bar absolute
| where: | |
| ppi | = partial pressure of gas component i = in the terms used in this article |
|---|---|
| P | = total pressure = in the terms used in this article |
| Fi | = volume fraction of gas component i = mole fraction, in the terms used in this article |
| ppN2 | = partial pressure of nitrogen = in the terms used in this article |
| ppO2 | = partial pressure of oxygen = in the terms used in this article |
The minimum safe lower limit for the partial pressures of oxygen in a gas mixture is 0.16 bar (16 kPa) absolute. Hypoxia and sudden unconsciousness becomes a problem with an oxygen partial pressure of less than 0.16 bar absolute. Oxygen toxicity, involving convulsions, becomes a problem when oxygen partial pressure is too high. The NOAA Diving Manual recommends a maximum single exposure of 45 minutes at 1.6 bar absolute, of 120 minutes at 1.5 bar absolute, of 150 minutes at 1.4 bar absolute, of 180 minutes at 1.3 bar absolute and of 210 minutes at 1.2 bar absolute. Oxygen toxicity becomes a risk when these oxygen partial pressures and exposures are exceeded. The partial pressure of oxygen determines the maximum operating depth of a gas mixture.
Nitrogen narcosis is a problem when breathing gases at high pressure. Typically, the maximum total partial pressure of narcotic gases used when planning for technical diving is 4.5 bar absolute, based on an equivalent narcotic depth of 35 metres (115 ft).
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