Scuba Diving International (SDI) Computer Nitrox Practice Exam

What can happen if you do not set your dive computer's FO2 prior to every dive?

1. The computer may notify you of oxygen levels

2. You risk the computer defaulting to 21 percent

3. You may decrease your dive time

4. Your computer will automatically adjust for depth

The correct answer is: You risk the computer defaulting to 21 percent

Setting the FO2 (Fraction of Oxygen) in your dive computer prior to every dive is crucial for accurate calculations related to oxygen exposure. If the FO2 is not set, the dive computer may default to an assumed percentage of oxygen in the breathing gas, which is typically 21 percent (the oxygen concentration in regular air). If your actual dive gas contains a higher percentage of oxygen (such as in Nitrox mixtures), then this default setting can lead to incorrect calculations regarding your no-decompression limits and potential exposure to oxygen toxicity. Thus, failing to set the FO2 correctly could expose you to levels of oxygen that are unsafe for the planned depth and duration of your dive. By ensuring that the FO2 is accurately set to match your dive gas, you can avoid those risks and utilize your dive computer to its full capabilities, ensuring safer diving practices. This is why it's so important to set the FO2 of your dive computer each time before diving, particularly when using mixtures with different oxygen levels.