TheCuriousDiver / Diver FAQ
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Diver FAQ

Common diver questions answered with cited limits and the formulas used by our tools. Math-honest, citation-first.

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TL;DR

These answers are sourced — PADI/DSAT for recreational PpO2 limits, NOAA for the CNS clock, public-domain physics for Boyle / Dalton / EAD. Where standards differ, the more conservative limit is shown. None of this replaces certified instruction or a verified dive computer.

01 / Physics

Gas physics basics

What is Boyle's law and why does it matter to divers?
Boyle's law states that at constant temperature, gas volume varies inversely with pressure (V1 × P1 = V2 × P2). Underwater, ambient pressure rises by about 1 bar every 10 metres of seawater, so a sealed gas volume halves between the surface and 10 m, drops to a third by 20 m, and to a quarter by 30 m. This is why a BCD needs venting on descent and inflating on ascent — and why holding your breath while ascending can rupture lung tissue. See the Boyle's Law Visualizer.
What is absolute pressure and why is it different from depth?
Absolute pressure is the total pressure on a diver: atmospheric pressure plus water pressure. At the surface it's 1 bar (atmosphere alone). At 10 m it's 2 bar (1 bar atmosphere + 1 bar water). The formula Pabs = depth/10 + 1 (in bar, sea water) is what every gas calculation uses, because gases only respond to total pressure — they don't care whether it comes from air or water.
What is partial pressure?
In a gas mixture, each gas contributes pressure proportional to its share of the mix (Dalton's law). For 21% oxygen in air at 2 bar absolute, the partial pressure of oxygen is 0.42 bar. Partial pressures matter because the body responds to the partial pressure of a gas, not its percentage — for example, EAN36 at 30 m delivers PpO2 = 0.36 × 4 = 1.44 bar, past the 1.4 bar working limit, even though 36% oxygen is harmless to breathe at the surface.

02 / Oxygen

Oxygen exposure

What is MOD and how do I calculate it?
MOD (Maximum Operating Depth) is the deepest a gas mix can be breathed before its oxygen partial pressure exceeds a safe limit. The formula is MOD (m) = 10 × (PpO2max / FO2 − 1), where PpO2max is the chosen oxygen partial-pressure ceiling and FO2 is the oxygen fraction in the mix. For EAN32 at 1.4 bar, MOD = 10 × (1.4 / 0.32 − 1) ≈ 33 m. Calculate any mix on the Nitrox Planner.
Why do recreational divers use 1.4 bar as a PpO₂ limit?
PADI/DSAT specifies 1.4 bar as the working partial-pressure ceiling for recreational diving, with 1.6 bar as an absolute maximum reserved for short exposures such as deep deco stops. The conservative working limit accounts for individual susceptibility variation, exercise, CO2 retention, and cold — all of which raise CNS oxygen-toxicity risk. Above 1.6 bar, central-nervous-system toxicity can manifest as convulsion, which is fatal underwater.
What is the CNS oxygen clock?
The CNS clock tracks cumulative oxygen exposure as a percentage of NOAA's single-exposure time limits at each PpO2. NOAA caps single exposures at 45 min at 1.6 bar, 120 min at 1.5, 150 min at 1.4, 180 min at 1.3, 210 min at 1.2, 240 min at 1.1, and 300 min at 1.0. A diver should plan to stay well under 100% on any single dive and track 24-hour cumulative exposure for repetitive dives.
Why does nitrox have a maximum depth?
Nitrox is enriched with oxygen, which means oxygen reaches its safe partial-pressure ceiling at a shallower depth than air does. EAN32 hits 1.4 bar PpO2 at about 33 m; EAN36 at about 29 m. Going deeper risks CNS oxygen toxicity. Nitrox extends bottom time at shallower depths, not deeper ones — it is not a deep-diving gas.

03 / Inert gas

Inert gas and narcosis

What is NDL?
NDL (No-Decompression Limit) is the maximum time a diver can spend at a given depth before required decompression stops would be incurred on ascent. It is depth-dependent and tabulated by certifying agencies (e.g. PADI's Recreational Dive Planner / DSAT tables). NDLs shorten as depth increases and as repetitive-dive nitrogen builds up.
What is EAD and why does it matter for nitrox?
EAD (Equivalent Air Depth) is the depth at which breathing air would deliver the same nitrogen partial pressure as your nitrox mix at your actual depth. Formula: EAD (m) = ((1 − FO2) × Pamb / 0.79) × 10 − 10. Because nitrogen drives no-decompression limits, looking up your NDL at the EAD (rather than your actual depth) gives nitrox its bottom-time advantage.
What is nitrogen narcosis and does nitrox help?
Nitrogen narcosis is a reversible impairment of judgement and reaction time caused by elevated partial pressure of nitrogen at depth. Onset is typically noticeable from about 30 m and intensifies with depth — by 40 m most divers feel some effect. It clears completely on ascent. Nitrox does not reduce narcosis significantly: substituting oxygen for nitrogen only marginally lowers nitrogen partial pressure, and oxygen is itself narcotic at elevated PpO2.

04 / Practicalities

Using these tools

Can I use these tools instead of a dive computer?
No. These tools are for planning, study, and verification — not for in-water decision-making. A dive computer monitors your actual profile in real time, tracks ascent rate, and adjusts NDLs based on what you actually do; a planning calculator cannot. Always carry a verified computer (two on technical dives) and follow its display in-water.
What is SAC and how is it different from RMV?
SAC (Surface Air Consumption) is the rate at which a diver depletes tank pressure, expressed in bar per minute at the surface. RMV (Respiratory Minute Volume) is the same idea in litres per minute, independent of tank size: RMV = SAC × tank_volume_L. RMV is the more useful number for planning gas needs across different tank sizes. Calculate yours from a logged dive on the SAC / RMV Calculator.
Why do agencies sometimes disagree on limits?
Decompression and oxygen-toxicity models are based on statistical risk, not hard physiological boundaries. PADI/DSAT, BSAC, GUE, and NOAA make slightly different conservatism choices (gradient factors, PpO2 ceilings, ascent rates). When standards differ, this site defaults to the more conservative limit and labels the source. Always follow the standards of your own certifying agency.
Why is everything metric by default?
Decompression and gas math are easier in metric — pressure adds at 1 bar per 10 metres, and metric formulas avoid most conversion errors. Every calculator has an imperial toggle for feet and psi. Whichever system you use, always show your work and double-check before going underwater.

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