Gas Fraction in Scuba Diving


Gas fraction, a fundamental concept in scuba diving, refers to the proportion of a particular gas in a gas mixture, as measured by molecular count, volume, or pressure. This entry will provide an in-depth understanding of gas fractions in scuba diving, their significance, and how they influence various aspects of the sport, including breathing gas mixtures, decompression planning, and equipment selection.

Breathing Gas Mixtures

Scuba divers use various breathing gas mixtures to optimize their dive experience and maintain safety at different depths. These mixtures consist primarily of nitrogen (N2), oxygen (O2), and occasionally helium (He), with each gas contributing to a specific fraction of the mixture.

  1. Air: The most common breathing gas, atmospheric air, comprises 21% O2 and 79% N2 by volume. This mixture is suitable for recreational dives up to 130 feet (40 meters) in depth.
  2. Nitrox: Nitrox, or Enriched Air Nitrox (EAN), contains an elevated concentration of O2 (22-40%) and a reduced proportion of N2. Nitrox allows for longer bottom times and shorter surface intervals, as it reduces the risk of decompression sickness (DCS) by minimizing nitrogen absorption. Nitrox mixtures are designated by their O2 fraction, such as EAN32 (32% O2 and 68% N2).
  3. Trimix: For technical diving at greater depths, trimix combines O2, N2, and He. The helium fraction replaces a portion of nitrogen, reducing narcotic effects and DCS risk. Trimix mixtures are labeled by their O2 and He fractions, like TMX18/45, which contains 18% O2, 45% He, and 37% N2.
  4. Heliox: Primarily used in deep commercial and military diving, heliox replaces all nitrogen with helium, reducing narcosis and DCS risk to a minimum. A typical heliox mixture might consist of 10% O2 and 90% He.

Decompression Planning

Gas fractions play a critical role in decompression planning, as the amount of dissolved inert gas (N2 or He) in a diver’s tissues determines the risk of DCS. By understanding gas fractions and their physiological effects, divers can calculate decompression schedules based on factors such as depth, bottom time, and breathing gas mixture.

  1. Partial Pressure: The partial pressure of a gas is the product of the gas fraction and the ambient pressure at a given depth. For example, at a depth of 66 feet (20 meters) in seawater, the ambient pressure is 3 atmospheres absolute (ATA). Breathing air, the partial pressures of O2 and N2 are 0.63 ATA (3 ATA x 0.21) and 2.37 ATA (3 ATA x 0.79), respectively.
  2. Maximum Operating Depth (MOD): The MOD is the maximum safe depth for a breathing gas, considering the risk of oxygen toxicity. For example, with a partial pressure of oxygen (PPO2) limit of 1.4 ATA, the MOD for EAN32 is calculated as (1.4 ATA / 0.32) – 1 ATA = 3.375 ATA, or approximately 79 feet (24 meters).
  3. Equivalent Air Depth (EAD) and Equivalent Narcotic Depth (END): EAD and END are used to simplify decompression planning for nitrox and trimix dives. They represent the depth at which air or a less narcotic mix would produce the same amount of nitrogen or total inert gas loading, respectively. EAD and END calculations consider the gas fractions of the breathing mixture and allow for the use of air-based decompression tables or dive computers.
  4. Gradient Factors: Technical divers use gradient factors to customize decompression profiles based on their risk tolerance. These factors, expressed as percentages, influence the permissible level of supersaturation (the amount of inert gas dissolved in tissues) during ascent. By adjusting gradient factors, divers can modify their decompression stops to balance the risk of DCS with the efficiency of their ascent.

Equipment Considerations

Gas fractions also impact the choice of scuba diving equipment, as different mixtures may require specialized regulators, cylinders, and dive computers.

  1. Regulators: While most regulators are compatible with air and nitrox mixtures up to 40% O2, mixtures with higher O2 fractions or helium content necessitate specialized regulators, designed to resist corrosion and perform efficiently at greater depths.
  2. Cylinders: Divers should use dedicated cylinders for different gas mixtures, clearly marked with the gas type and fraction. This ensures the correct mixture is used during a dive and helps avoid potential confusion or errors.
  3. Dive Computers: Advanced dive computers offer the capability to program and switch between multiple gas mixtures during a dive. These computers automatically adjust decompression schedules based on the breathing gas, allowing for greater flexibility and safety in multi-gas dives.
  4. Gas Analyzers: Divers should analyze their breathing gas mixtures before each dive using an oxygen or helium analyzer. This ensures the gas fractions are accurate, and the dive plan can be adjusted accordingly.


Gas fractions play a vital role in various aspects of scuba diving, from choosing the optimal breathing gas mixture to calculating decompression schedules and selecting appropriate equipment. A thorough understanding of gas fractions and their implications is essential for divers to ensure a safe and enjoyable diving experience. By mastering the concepts presented in this entry, divers can make informed decisions about their breathing gas mixtures, decompression planning, and equipment, ultimately enhancing their underwater adventures.