Diving Glossary
Essential acronyms and concepts every diver should know
Bühlmann ZH-L16C
- A decompression algorithm developed by Professor Albert A. Bühlmann at the University of Zurich that models 16 theoretical tissue compartments with half-times from 4 to 635 minutes. The 'C' variant uses the most conservative published M-values and is the basis for most modern dive computers, including Shearwater, Suunto, Garmin, and the DiveToolbox planner.
CNS (Central Nervous System oxygen toxicity)
- A form of acute oxygen toxicity that can cause underwater seizures — nearly always fatal in scuba. Risk accumulates with time spent above various ppO₂ thresholds. CNS percentage tracks lifetime exposure for a single dive and is reset between dives. The standard limit is 1.4 ata for working bottom gas and 1.6 ata for decompression gas breathed at rest.
EAD (Equivalent Air Depth)
- The depth at which air would have the same nitrogen partial pressure as your nitrox mix. Used when consulting air-only decompression tables on a nitrox dive — a shallower EAD means less nitrogen loading and less decompression obligation. The DiveToolbox MOD calculator returns EAD for any nitrox mix and depth.
END (Equivalent Narcotic Depth)
- The depth at which air would produce the same narcotic load as your trimix mix. The standard convention treats helium as non-narcotic and includes oxygen in the narcotic fraction, so adding helium dramatically reduces END. Tech divers typically aim for END under 30 m for clear cognitive function on deeper dives.
GF (Gradient Factors)
- Tunable conservatism parameters introduced by Erik C. Baker in 1998 to modify Bühlmann M-values. GF Low is the fraction of the gradient accepted at the first stop (lower = deeper first stop); GF High is the fraction accepted at the surface (lower = more shallow stops). Common presets: 30/70 conservative, 40/85 default, 50/85 aggressive.
ICD (Isobaric Counter Diffusion)
- A form of decompression sickness affecting the inner ear (vestibular DCS) that occurs at a gas switch when helium leaves the body faster than nitrogen enters. The Doolette 1:5 rule (modern standard) and Hamilton 1:3 rule (older, more permissive) set safe limits on the helium-to-nitrogen swing at any switch.
M-value
- The maximum tolerated inert-gas tension per tissue compartment in the Bühlmann model — the surface-equivalent partial pressure at which a compartment would just begin to release decompression bubbles. Gradient factors scale M-values from 0% to 100% to adjust conservatism without changing the underlying algorithm.
MOD (Maximum Operating Depth)
- The deepest depth at which a gas mix can be breathed without exceeding a chosen ppO₂ limit, typically 1.4 ata for working bottom gas and 1.6 ata for deco gas at rest. Exceeding MOD risks acute CNS oxygen toxicity. Every nitrox and trimix dive requires an MOD calculation before descent — use the DiveToolbox MOD calculator.
NDL (No-Decompression Limit)
- The maximum bottom time at a given depth that allows direct ascent to the surface without mandatory decompression stops beyond the standard safety stop. NDL depends on depth, gas mix, and personal conservatism (gradient factors). The DiveToolbox planner returns NDL for any depth and mix on the chosen Bühlmann ZH-L16C settings.
Nitrox / EAN
- A breathing gas mix of nitrogen and oxygen with oxygen content above the 21% of air. EAN32 (32% O₂) and EAN36 (36% O₂) are the most common recreational blends, extending no-decompression limits at the cost of a shallower MOD. Standard nitrox certification covers blends up to 40% O₂; higher mixes (above 40%) typically require deco-procedures or advanced training.
OTU (Oxygen Toxicity Unit)
- A cumulative measure of whole-body (pulmonary) oxygen toxicity, distinct from CNS oxygen toxicity. OTU accumulates with time at elevated ppO₂ across multiple dives and days, mostly relevant for technical and saturation diving with high exposure. A typical daily limit is 300 OTU; recovery between days reduces accumulated load.
ppO₂ (partial pressure of oxygen)
- The fraction of total breathing-gas pressure contributed by oxygen alone, expressed in atmospheres absolute (ata). ppO₂ = O₂ fraction × absolute pressure at depth. Working ppO₂ limit is 1.4 ata, decompression ppO₂ limit is 1.6 ata at rest. ppO₂ drives MOD calculations and CNS toxicity accumulation.
RGBM (Reduced Gradient Bubble Model)
- An alternative decompression model developed by Bruce Wienke that explicitly tracks bubble formation and growth, in contrast to Bühlmann's dissolved-gas approach. Used by Suunto computers (RGBM variants) and Mares. DiveToolbox uses Bühlmann ZH-L16C with gradient factors because it remains the industry standard and is more transparent for educational planning.
RMV (Respiratory Minute Volume)
- The actual minute volume of breath at surface pressure, in litres per minute, independent of cylinder geometry. Equivalent to SAC in physiological meaning but expressed independently of gauge readings. RMV transfers across cylinder sizes and gas-pressure systems — useful when comparing your consumption between regions or gear setups. Use the DiveToolbox SAC calculator to derive both.
SAC (Surface Air Consumption)
- Your gas consumption rate normalised to surface pressure, in litres per minute. Calculated by dividing real consumption at depth by the absolute pressure at average depth. SAC is the foundation of gas planning — it drives Rock Bottom, turn pressure, bottom-time budget, and cylinder selection. Track your personal SAC across many dives, not a textbook average.
Rock Bottom
- The minimum cylinder pressure at which you can still complete a safe emergency ascent for two divers — including stressed buddy share at depth, controlled ascent with stops, and a safety stop — without running out of gas. The floor under every other gas-planning number. Use the DiveToolbox Rock Bottom calculator for any depth, cylinder, SAC, and stress factor.
Trimix
- A breathing gas mix of oxygen, helium, and nitrogen used for deep technical dives, typically beyond 40 metres. Adding helium reduces narcosis (low END) and gas density (lower work of breathing) while displacing nitrogen. Common blends are written as O₂/He percentages (21/35 = 21% O₂, 35% He, 44% N₂). Requires dedicated technical training (TDI, IANTD, GUE).
Helitrox / Heliox
- Helitrox is a nitrogen-bearing trimix (oxygen + helium + nitrogen). Heliox is a pure binary mix of oxygen and helium with no nitrogen — used for the deepest commercial and military dives where minimising nitrogen narcosis and gas density is critical. Pure heliox eliminates ICD on gas switches (no nitrogen to counter-diffuse) but is expensive and rarely used recreationally.
DCS (Decompression Sickness)
- Also called "the bends". A range of injuries caused by inert-gas bubbles forming in blood and tissues when ascent is too fast or decompression obligation is skipped. Symptoms range from joint pain and skin mottling to neurological deficits, paralysis, or death. Type I covers limb/skin; Type II covers neurological and pulmonary. Recompression in a chamber is the treatment — call DAN's emergency line if suspected.
Half-time
- The time required for a theoretical tissue compartment to reach 50% saturation toward the ambient inert-gas pressure during on-gassing (or release 50% during off-gassing). Bühlmann ZH-L16C models 16 compartments with half-times from 4 minutes (fastest, blood-perfused) to 635 minutes (slowest, dense tissues). Fast compartments drive shallow no-stop dives; slow compartments drive deep or repetitive deco.
VPM (Varying Permeability Model)
- A bubble-mechanics decompression model developed by Yount, Hoffman and Maiken, refined by Erik C. Baker (VPM-B). Unlike Bühlmann's dissolved-gas approach, VPM tracks micronuclei that grow into bubbles during ascent and limits stops to prevent critical bubble volume. Tends to produce deeper first stops than Bühlmann with GF 30/70. Used by Multi-Deco, ratio-deco planners, and some Shearwater modes.
NEDU (Navy Experimental Diving Unit)
- The US Navy research unit in Panama City, Florida, that conducts diving-physiology trials. Their 2011 deep-stops study (Doolette, Gerth & Gault) showed that deep-stop profiles produced more DCS than equivalent shallow-stop profiles for the same total time. This finding pushed the technical-diving community toward higher GF Low values (40-50) and the modern Bühlmann GF consensus.
Air break
- A scheduled switch from a high-ppO₂ deco gas (typically pure O₂ at 6 m / 20 ft) back to a lower-O₂ mix or bottom gas for several minutes, to reset CNS clock ticks and reduce pulmonary OTU load. The conventional protocol breathes O₂ for 20-25 minutes, then air or bottom mix for 5 minutes, then resumes O₂. Standard practice on long pure-O₂ decompression stops.
CCR (Closed-Circuit Rebreather)
- A breathing apparatus that recirculates exhaled gas through a CO₂ scrubber and injects oxygen as the diver metabolises it, holding the loop at a target ppO₂ setpoint. CCRs dramatically extend bottom time and reduce gas logistics for deep technical dives, but add failure modes (hypoxia, hyperoxia, hypercapnia). The DiveToolbox planner supports CCR mode with diluent and setpoint inputs.
OC (Open Circuit)
- Traditional scuba: every exhaled breath vents to the water as bubbles. Simpler, more failure-tolerant and cheaper than CCR, but gas-consumption is proportional to depth, so deep technical dives need multiple bailout cylinders. OC is the baseline assumption for all recreational diving and for most of the DiveToolbox calculators (SAC, Rock Bottom, gas density, blending) unless CCR mode is selected.
Setpoint
- The target oxygen partial pressure a CCR maintains in the breathing loop, typically 0.7 ata on the surface, switched to 1.2 - 1.3 ata at depth. The CCR's solenoid injects O₂ whenever ppO₂ drops below setpoint. Higher setpoint reduces inert-gas loading (shorter deco) but increases CNS and OTU accumulation. Choice of setpoint shapes the entire decompression profile.
Diluent
- On a CCR, the inert-gas mix (air, trimix, or heliox) injected into the loop to dilute the oxygen and balance the volume as the diver descends. Diluent choice determines END and gas density at depth — divers select trimix diluent below 40 m to keep END manageable and density under the 5.2 g/L DAN ceiling. The diluent is also the diver's primary bailout option for a flooded or failed loop.
Bailout gas
- Emergency open-circuit gas a CCR diver carries to abort the dive if the loop fails (flood, hypercapnia, electronics failure, hyperoxia). Bailout volume must cover the worst-case ascent from maximum depth on OC, including decompression and Rock Bottom. Trimix and CCR planners typically require bailout calculations as rigorous as the primary plan — failures occur at the worst possible depths.
Best Mix
- The richest nitrox blend whose MOD matches a target depth at a chosen ppO₂. Calculated as FO₂ = ppO₂ / (depth/10 + 1) in metres. For a 30 m max dive at 1.4 ata, Best Mix is EAN35. Using Best Mix maximises NDL and reduces nitrogen loading at depth, but leaves no MOD margin if depth is exceeded. The DiveToolbox MOD calculator returns Best Mix for any depth and ppO₂ limit.
Surface interval
- The time between surfacing from one dive and starting the next, during which tissues off-gas residual inert gas. A longer surface interval reduces nitrogen and helium load on the next dive's tissues, extending NDL and shortening required decompression. The DiveToolbox successive-dives calculator tracks Bühlmann tissue pressures across multiple dives and surface intervals to compute realistic residual loading.
Hypoxia / Hyperoxia
- Hypoxia: ppO₂ too low (under ~0.18 ata) — causes loss of consciousness without warning. Classic risk on CCR surface descent if diluent flushes loop too low, or on hypoxic trimix at shallow depth. Hyperoxia: ppO₂ too high (over 1.6 ata) — risks acute CNS oxygen toxicity and convulsion. Both extremes are why every gas needs a verified MOD and minimum operating depth before the dive.
Narcosis (Nitrogen narcosis)
- Cognitive impairment caused by elevated nitrogen partial pressure at depth, often noticeable below 30 m on air and severe below 50 m. Symptoms: euphoria, tunnel vision, slowed reaction time, poor judgement. Aliases: "rapture of the deep", "Martini effect". Treated by ascending to shallower depth or switching to trimix. END (Equivalent Narcotic Depth) quantifies narcotic load — see END entry.
DAN (Divers Alert Network)
- The non-profit dive safety organisation that runs the 24/7 diving emergency hotline, dive-accident insurance, research and medical-advisory programs. DAN's 2018 Anthony & Mitchell consensus set the 5.2 g/L gas-density ceiling adopted across technical diving. The DiveToolbox Gas Density calculator references this limit. DAN regional chapters: DAN America, DAN Europe, DAN Asia-Pacific, DAN Brasil, DAN Southern Africa.