IDC Dive Equipment & Technology 2 — Questions and Answers
Question 1: What is a dive computer's 'no-decompression limit' (NDL) display and why is it critical for divers?
- The battery remaining in the computer
- The remaining time a diver can stay at current depth without incurring a decompression obligation (Correct answer)
- The total dive time elapsed
- The depth limit for recreational diving
Correct answer: The remaining time a diver can stay at current depth without incurring a decompression obligation
The NDL display shows the remaining time at current depth before mandatory decompression stops would be required — when it reaches zero, the diver must begin ascending.
The no-decompression limit (NDL) display on a dive computer continuously calculates the remaining time the diver can stay at their current depth without accumulating a decompression obligation. As the diver descends deeper or stays at depth longer, the NDL decreases. Dive computers use algorithms such as the Bühlmann ZHL model to calculate NDL based on the diver's actual dive profile. When the NDL reaches zero, the computer will show a decompression stop requirement.
Question 2: What is the purpose of the 'dump valve' on a BCD?
- To add air to the BCD quickly during an ascent
- To rapidly release air from the BCD, used during ascent to prevent uncontrolled rise (Correct answer)
- To drain water from the BCD after diving
- To connect the inflator hose to the tank
Correct answer: To rapidly release air from the BCD, used during ascent to prevent uncontrolled rise
Dump valves allow divers to rapidly release air from the BCD to slow an ascent or vent expanding gas during ascent — they are essential for buoyancy control.
BCDs typically have multiple dump valves: the shoulder dump (operated by pulling the inflator hose or a cord) and the hip/lower dump. These valves allow divers to release air from the BCD rapidly when needed. During ascent, as the surrounding pressure decreases, air in the BCD expands, increasing buoyancy and potentially accelerating the ascent. Divers must vent this expanding air through the dump valves to maintain a controlled ascent rate.
Question 3: What is enriched air nitrox and what are its key advantages and limitations compared to standard air?
- Air mixed with pure oxygen for unlimited bottom time
- A breathing mixture with higher oxygen content than air, offering longer NDLs but with an oxygen toxicity risk and maximum depth limit (Correct answer)
- A gas mixture that eliminates the risk of decompression sickness entirely
- A specialty gas used only in technical diving
Correct answer: A breathing mixture with higher oxygen content than air, offering longer NDLs but with an oxygen toxicity risk and maximum depth limit
Enriched air nitrox contains more oxygen than standard air (typically 32% or 36% O2), which reduces nitrogen content and extends no-decompression limits, but imposes a maximum operating depth due to oxygen toxicity risk.
Enriched Air Nitrox (EAN) is a breathing mixture with an oxygen content higher than air's 21%. Common recreational mixes are EAN32 (32% O2) and EAN36 (36% O2). The higher oxygen percentage means lower nitrogen content, which extends no-decompression limits and reduces residual nitrogen for repetitive dives. However, the higher PO2 creates a maximum operating depth (MOD) based on oxygen toxicity limits. Divers must analyze their gas mixture before every dive, set their computer to the correct mix, and never exceed their MOD.
Question 4: What regular maintenance is required for a scuba regulator to ensure safe performance?
- Rinse with fresh water after use only — no other maintenance required
- Annual or manufacturer-specified service by a qualified technician, plus fresh water rinsing after every use (Correct answer)
- Replacement every 5 years regardless of condition
- Daily disassembly and lubrication by the diver
Correct answer: Annual or manufacturer-specified service by a qualified technician, plus fresh water rinsing after every use
Regulators require professional servicing at manufacturer-specified intervals (typically annually or every 100-200 dives) and fresh water rinsing after each use to prevent salt and corrosion damage.
Scuba regulators are life-support equipment that must be maintained to manufacturer specifications. Typical maintenance requirements include: annual professional service involving disassembly, cleaning, replacement of wear parts (O-rings, seats, springs), and testing; and rinsing with fresh water after every use in salt water to prevent corrosion. Divers should also perform basic function checks before every dive: check for free-flow, verify second stage delivers air on demand, inspect hoses and connections for damage.
Question 5: What is the purpose of a surface marker buoy (SMB) and when should it be deployed?
- To mark the dive site location before diving
- To signal the diver's position at the surface during ascent or after surfacing, especially in areas with boat traffic (Correct answer)
- To attach the guideline to the dive site structure
- To measure water depth from the surface
Correct answer: To signal the diver's position at the surface during ascent or after surfacing, especially in areas with boat traffic
An SMB (safety sausage) is an inflatable signal device deployed during ascent or at the surface to alert boat traffic and surface support of the diver's location.
A surface marker buoy (SMB), also called a safety sausage or DSMB (Delayed SMB), is an inflatable tube that divers deploy to signal their position. It is critically important when surfacing in areas with boat traffic, making a blue-water ascent, or diving in currents where surface support needs to track the diver's position. A DSMB is deployed underwater and inflated via the diver's second stage exhaust or a small spool; it rises to the surface trailing a line to the diver below.
Question 6: What is the difference between a 'piston' first stage and a 'diaphragm' first stage regulator?
- They are identical in function — only the brand name differs
- Piston stages use a moving piston to reduce pressure; diaphragm stages use a flexible membrane isolating internal parts from water, making them preferable in cold or contaminated water (Correct answer)
- Piston stages are for warm water only; diaphragm stages are for recreational use
- Diaphragm stages are lower quality than piston stages
Correct answer: Piston stages use a moving piston to reduce pressure; diaphragm stages use a flexible membrane isolating internal parts from water, making them preferable in cold or contaminated water
Piston first stages use a moving piston directly exposed to water; diaphragm types use a flexible membrane that isolates the internal mechanism from water, making them better in cold or contaminated conditions.
Both piston and diaphragm first stages reduce tank pressure to an intermediate pressure, but through different mechanisms. Piston first stages are mechanically simpler, generally less expensive, and perform excellently in warm, clean water. Diaphragm first stages use a flexible membrane to separate the regulator's internal mechanism from surrounding water — preferred in cold water (where freezing can cause a piston to free-flow) or in contaminated water environments because the internal mechanism is protected.
IDC Dive Equipment & Technology 2
What is a dive computer's 'no-decompression limit' (NDL) display and why is it critical for divers?