How is the maximum operating depth of a gas mix determined?

The maximum operating depth of a gas mix is primarily determined by the maximum acceptable partial pressure of oxygen. This concept is critical in the use of enriched air nitrox, where the composition of the gas includes a higher percentage of oxygen than regular air.

Oxygen becomes toxic at higher partial pressures, which poses a risk to divers as they descend further underwater. The maximum partial pressure of oxygen for recreational diving is typically accepted to be 1.4 ATA (atmospheres absolute) for the duration of the dive, ensuring that divers minimize the risk of oxygen toxicity. This means that as divers go deeper, the partial pressure of oxygen in the gas mix increases, and thus the maximum depth is calculated to ensure that the partial pressure does not exceed this safe limit.

Other factors, while important in diving safety and planning, do not directly affect the definition of maximum operating depth as significantly as the partial pressure of oxygen. For example, the maximum allowable exposure time does relate to decompression limits but does not govern the depth at which a diver can safely operate with a specific gas mix. Likewise, water temperature and a diver's experience level may influence safety and comfort but do not set the operational limits of the gas composition itself.