Scuba Diving International (SDI) Computer Nitrox Practice Exam

Which of the following is not a Nitrox mixture?

1. EAN24

2. EAN21

3. EAN40

4. EAN30

The correct answer is: EAN21

The composition of Nitrox mixtures is defined by the percentage of oxygen they contain, with the remainder being nitrogen. Each designation, such as EAN24, EAN21, EAN40, and EAN30, specifies a mixture enriched with oxygen relative to standard air, which is approximately 21% oxygen. In this context, EAN21 refers to a Nitrox mixture that contains 21% oxygen and 79% nitrogen. This mixture is effectively the same as normal air, as standard air consists of approximately 21% oxygen. Consequently, EAN21 cannot be classified as a Nitrox mixture in the context of scuba diving, which typically refers to mixtures that have a higher percentage of oxygen than air. In contrast, EAN24, EAN30, and EAN40 are all enriched with oxygen, indicating they contain more oxygen than the atmospheric average and are thus used in specific diving scenarios to optimize gas management and minimize the risk of nitrogen narcosis. These mixtures are designed to support divers at various depths while managing the effects of increased partial pressures of oxygen.