PADI Enriched Air Nitrox Diving 3 — Questions and Answers
Question 1: A diver's nitrox computer shows a CNS clock at 75% after the first dive. What should concern the diver most before a second dive?
- The remaining battery life of the computer
- Accumulated CNS oxygen toxicity exposure nearing its limit (Correct answer)
- The surface interval effect on tank pressure
- Nitrogen narcosis buildup from the first dive
Correct answer: Accumulated CNS oxygen toxicity exposure nearing its limit
The CNS clock tracks cumulative oxygen toxicity exposure; diving again without adequate surface interval risks reaching 100% and potential toxicity symptoms.
Question 2: Why does enriched air nitrox reduce a diver's no-decompression limit (NDL) compared to air at the same depth?
- It doesn't — nitrox extends NDLs by reducing nitrogen percentage (Correct answer)
- Because the higher oxygen accelerates tissue saturation
- Because nitrox divers must ascend faster to avoid oxygen toxicity
- Because nitrox requires mandatory safety stops at every dive
Correct answer: It doesn't — nitrox extends NDLs by reducing nitrogen percentage
Nitrox contains less nitrogen than air, so tissues absorb nitrogen more slowly, resulting in longer NDLs for the same depth.
Question 3: What is the recommended PADI pO₂ limit for normal recreational enriched air diving?
- 1.2 ata
- 1.4 ata (Correct answer)
- 1.6 ata
- 1.8 ata
Correct answer: 1.4 ata
PADI recommends a maximum pO₂ of 1.4 ata for recreational diving, with 1.6 ata only acceptable in exceptional circumstances with close monitoring.
Question 4: A buddy team discovers one diver's cylinder is labeled EANx40 but was only analyzed at EANx36. What is the correct action?
- Dive to the MOD for EANx38, splitting the difference
- Use the EANx36 MOD since that was the measured value
- Dive to the air NDL as a conservative measure
- Skip the dive and have the cylinder re-analyzed by a qualified technician (Correct answer)
Correct answer: Skip the dive and have the cylinder re-analyzed by a qualified technician
Any discrepancy between the label and analysis requires the cylinder to be re-checked by a qualified technician before it is safe to dive.
Question 5: Which factor does NOT affect the rate at which a diver accumulates CNS oxygen toxicity?
- Depth of the dive
- Time at depth
- Water temperature (Correct answer)
- Oxygen fraction of the breathing mix
Correct answer: Water temperature
CNS toxicity accumulation is determined by the partial pressure of oxygen (a function of depth and mix fraction) and exposure time, not water temperature.
Question 6: A diver using EANx32 plans a dive to 60 feet. Using the EAD formula, what is the approximate equivalent air depth?
- 40 feet
- 47 feet (Correct answer)
- 55 feet
- 60 feet
Correct answer: 47 feet
EAD = [(1 − FO₂) / 0.79 × (D + 33)] − 33; EAD = [(0.68/0.79) × 93] − 33 ≈ 47 feet.
Question 7: During a surface interval after an enriched air dive, what happens to dissolved nitrogen in the diver's body?
- It is converted to oxygen by the lungs
- It remains constant until the next dive begins
- It is gradually eliminated through normal respiration (Correct answer)
- It is absorbed into bone tissue permanently
Correct answer: It is gradually eliminated through normal respiration
During the surface interval, excess dissolved nitrogen off-gasses through the lungs as the body returns toward equilibrium with surface pressure.
PADI Enriched Air Nitrox Diving 3
A diver's nitrox computer shows a CNS clock at 75% after the first dive.
What should concern the diver most before a second dive?