Scuba Diving International (SDI) Computer Nitrox Practice Exam

What is the default FO2 setting for most dive computers?

1. 40%

2. 30%

3. 21%

4. 25%

The correct answer is: 21%

Most dive computers are designed with a default Fraction of Oxygen (FO2) setting that typically reflects the oxygen content of ambient air at sea level, which is 21%. This setting is essential as it allows the dive computer to calculate key dive parameters, such as no-decompression limits and safety stops, based on the assumption that the diver is using air. Using the correct FO2 is crucial for ensuring accurate readings and safety during dives. If a diver switches to a nitrox mixture with a higher oxygen content, they must manually adjust the FO2 setting on their computer to correspond with the new gas mixture. This practice ensures proper decompression planning and helps prevent oxygen toxicity. Thus, the default setting of 21% is both a safe and standard practice for dive computers operating in air mode, creating a reliable baseline for divers' calculations and dive profiles.