Best Mix
Dalton's Law of Partial Pressures is a fundamental physical principle that describes how the total pressure of a gas mixture is distributed among its individual components. For divers, this law is crucial for ensuring safety when selecting the best possible gas mixture for different depths. Correct application of Dalton's Law helps minimize risks such as oxygen toxicity and decompression sickness. To calculate the best mix, you need the maximum depth of the dive and the maximum partial pressure of oxygen you want to allow.
Dalton's Law states that the total pressure of a mixture of gases is the sum of the individual partial pressures of all the gases present, expressed mathematically as:
Ptotal = P1 + P2 + … + Pn
In diving, this law is applied to ensure that partial pressures remain within safe limits. The partial pressure of oxygen must be monitored to avoid oxygen toxicity, while the partial pressure of nitrogen is regulated to reduce the risk of decompression sickness.
The best mix is the highest oxygen content that stays within the chosen ppO2 at the maximum depth:
Best mix = ppO2 / (Depth / 10 + 1) × 100
- ppO2
- maximum partial pressure of oxygen in bar, e.g. 1.4
- Depth
- maximum depth in meters
- Depth / 10 + 1
- absolute pressure at that depth in bar
This calculator only looks at oxygen. It does not consider nitrogen narcosis or the dive time; for deeper dives a trimix with helium may be needed.