What are the two gases that most Nitrox diving computers consider when calculating allowable bottom time?

The correct answer is based on the understanding of how Nitrox, which is a breathing gas mix of nitrogen and oxygen, affects dive planning and safety. When diving with Nitrox, divers primarily consider the effects of nitrogen and oxygen under pressure, as these are the two main components of the gas mixture.

Nitrogen is significant because it is absorbed into the body during the dive and can lead to nitrogen narcosis or decompression sickness if not managed properly. By using a gas mixture with a higher proportion of oxygen, such as Nitrox, divers can reduce the amount of nitrogen absorbed and thereby extend their allowable dive time.

Oxygen is crucial because it has its own toxicity limits at depth, known as oxygen toxicity. Diving computers calculate allowable bottom time by monitoring both nitrogen absorption and the partial pressures of oxygen to ensure divers stay within safe limits for both gases.

The other gas choices presented do not directly relate to the primary considerations for Nitrox diving. Thus, focusing on nitrogen and oxygen gives the foundational understanding needed for safe and effective dive planning with Nitrox.