To find the partial pressure of a gas at depth, multiply the absolute pressure at that depth by the fraction (or percentage) of that gas in the breathing mixture.
Understanding the Concept
Partial pressure refers to the pressure exerted by an individual gas within a mixture of gases. In diving, it's crucial to understand the partial pressure of gases like oxygen and nitrogen, as they affect physiology at depth. For example, high partial pressures of oxygen can lead to oxygen toxicity, while high partial pressures of nitrogen can cause nitrogen narcosis.
Calculating Partial Pressure
The formula for calculating partial pressure is:
Partial Pressure (Pp) = Total Pressure (P) x Fraction of Gas (F)
Where:
- Pp is the partial pressure of the gas.
- P is the absolute pressure at the given depth. This includes atmospheric pressure (1 bar or 1 atmosphere at sea level). For every 10 meters (approximately 33 feet) of depth in seawater, the pressure increases by approximately 1 bar (or 1 atmosphere). So, at 10 meters, the absolute pressure is 2 bar, at 20 meters it is 3 bar, and so on.
- F is the fraction of the gas in the breathing mixture. For example, if you are breathing air, the fraction of oxygen is approximately 0.21 (21%). If you are breathing Nitrox 32, the fraction of oxygen is 0.32 (32%).
Examples
Here are a couple of examples illustrating how to calculate partial pressure:
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Example 1: Breathing Air at 30 meters (99 feet)
- Depth: 30 meters
- Absolute Pressure (P): 4 bar (1 bar atmospheric pressure + 3 bar from the depth)
- Fraction of Oxygen in Air (F): 0.21
- Partial Pressure of Oxygen (PpO2): 4 bar x 0.21 = 0.84 bar
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Example 2: Breathing Nitrox 32 at 40 meters (132 feet)
- Depth: 40 meters
- Absolute Pressure (P): 5 bar (1 bar atmospheric pressure + 4 bar from the depth)
- Fraction of Oxygen in Nitrox 32 (F): 0.32
- Partial Pressure of Oxygen (PpO2): 5 bar x 0.32 = 1.6 bar
Importance in Diving
Calculating and monitoring partial pressures, especially PpO2, is vital in dive planning and execution. Divers must stay within safe partial pressure limits to avoid oxygen toxicity and other gas-related issues. Many dive computers automatically calculate and display partial pressures based on depth and gas mixture settings.
