PADI Enriched Air Nitrox Diving 5 — Questions and Answers
Question 1: What information must be recorded on a nitrox cylinder before it is approved for diving?
- Diver's certification number and dive date only
- Analyzed oxygen percentage, MOD, and diver's initials or signature (Correct answer)
- Fill station name, fill date, and cylinder serial number
- Tank pressure, water temperature, and planned dive profile
Correct answer: Analyzed oxygen percentage, MOD, and diver's initials or signature
The cylinder must be tagged with the verified FO₂, the calculated MOD, and the analyzing diver's initials to confirm the analysis was performed.
Question 2: Why do enriched air divers generally require less weighting than when diving air at the same depth?
- Nitrox cylinders are lighter than air cylinders
- They don't — weighting requirements are identical
- The reduced nitrogen reduces tissue density
- This is a misconception; weight requirements depend on exposure suit and diver physiology, not the gas mix (Correct answer)
Correct answer: This is a misconception; weight requirements depend on exposure suit and diver physiology, not the gas mix
Weighting is determined by the diver's buoyancy characteristics and exposure suit, not by whether they are breathing nitrox or air.
Question 3: Which type of oxygen toxicity is most relevant to breath-hold (free) divers rather than scuba divers?
- CNS (central nervous system) toxicity
- Pulmonary (lung) toxicity (Correct answer)
- Hyperoxic syndrome
- Hyperbaric oxygen disorder
Correct answer: Pulmonary (lung) toxicity
Pulmonary toxicity develops from prolonged lower-level oxygen exposure and is more of a concern in technical/surface-supply diving; CNS toxicity is the primary scuba concern.
Question 4: A student diver asks why they cannot dive a rented EANx36 cylinder without an Enriched Air certification. What is the most accurate explanation?
- Insurance regulations prohibit it in all US dive locations
- Without training, the diver doesn't know the correct MOD, CNS limits, or analysis requirements, creating serious safety risks (Correct answer)
- Uncertified divers absorb oxygen differently due to incomplete lung development
- EANx36 is classified as a technical gas requiring rebreather training
Correct answer: Without training, the diver doesn't know the correct MOD, CNS limits, or analysis requirements, creating serious safety risks
Enriched air certification teaches oxygen partial pressure limits, MOD calculation, CNS toxicity recognition, and proper cylinder analysis — all critical safety knowledge.
Question 5: What happens to the no-decompression limit (NDL) as the oxygen percentage in a nitrox mix increases (at a constant dive depth)?
- NDL decreases because oxygen toxicity limits time at depth
- NDL remains unchanged regardless of mix
- NDL increases because less nitrogen means slower tissue saturation (Correct answer)
- NDL varies randomly based on the diver's fitness
Correct answer: NDL increases because less nitrogen means slower tissue saturation
Higher oxygen fraction means lower nitrogen fraction, so tissues absorb nitrogen more slowly and the diver can stay longer before reaching decompression obligation.
Question 6: A dive shop offers a 'continuous flow' or 'membrane separation' nitrox system. What process does this describe?
- Liquefying air and boiling off nitrogen to concentrate oxygen
- Passing air through hollow fiber membranes that selectively allow oxygen to permeate, enriching the mix (Correct answer)
- Mixing bottled oxygen with filtered compressed air in a high-pressure manifold
- Chemically reacting air with a catalyst to break down nitrogen molecules
Correct answer: Passing air through hollow fiber membranes that selectively allow oxygen to permeate, enriching the mix
Membrane separation systems use semi-permeable hollow fibers through which oxygen molecules pass more readily than nitrogen, producing an oxygen-enriched breathing gas.
Question 7: When logging an enriched air dive, what additional information should a nitrox diver record compared to a standard air dive log?
- Tank manufacturer and hydrostatic test date
- Oxygen fraction used, MOD, and CNS % accumulated (Correct answer)
- Water salinity and visibility estimate
- Regulator make and model for manufacturer warranty records
Correct answer: Oxygen fraction used, MOD, and CNS % accumulated
Nitrox dive logs should include the FO₂, the MOD for that mix, and the CNS clock percentage to track cumulative oxygen exposure across multiple dives.
PADI Enriched Air Nitrox Diving 5
What information must be recorded on a nitrox cylinder before it is approved for diving?