Gradient Factor Presets Compared: 30/70 vs 40/85 vs 50/85

What GF Low and GF High mean

Gradient Factors are conservatism modifiers applied to the Bühlmann ZH-L16C M-values. GF Low limits how close the leading tissue compartment can come to its M-value at the first deep stop — a lower number forces a deeper first stop. GF High limits the supersaturation allowed at the surface — a lower number extends shallow-stop time. The two numbers together shape the entire ascent profile, but they don't change the bottom phase. Baker 1998 introduced the concept to give divers a single, defensible knob to tune Bühlmann's known weak point: pure ZH-L16 (GF 100/100) tolerates more supersaturation than most divers find comfortable on deep dives. See the gradient-factors and dive-planner tools to test each preset on your profile.

GF 30/70 — Deep stops, long shallow stops

30/70 is the conservative end of the practical range. The low 30 forces the first stop deep — typically 60-70% of bottom depth — and the 70 ceiling at the surface mandates significant shallow time. A 40 m/20 min air dive jumps from roughly 14 minutes total runtime at 50/85 to about 24 minutes at 30/70. Users: GUE-style hard dives, cold water, repetitive multi-day diving, divers recovering from a minor DCS event, and crews who want margin for an unplanned workload spike. Pros: best protection of slow tissues, comforting on dives close to NDL. Cons: substantial extra runtime and gas, deep stops are now controversial after NEDU 2011 showed shallow-bias profiles outperformed deep-stop profiles on a 170 ft mission.

GF 40/85 — The de-facto rec-tech default

40/85 is the balanced preset most agencies and modern dive computers ship with by default (Shearwater, Garmin Descent). It still adds a deep stop relative to pure Bühlmann but leaves the shallow stops at a duration most divers tolerate without significant gas penalty. On the same 40 m/20 min air profile, runtime sits around 17-18 minutes — roughly 4 minutes more than 50/85 and 6 minutes less than 30/70. Users: technical divers on planned profiles, recreational divers wanting margin without over-stopping, instructors building consistent class plans. Pros: matches what your dive buddy's computer is likely running, well-documented field record, sensible default if you're unsure. Cons: still carries deep-stop bias some modern researchers argue against.

GF 50/85 — Aggressive, surface-biased

50/85 pushes the first stop shallower and rewards shallow-stop discipline. The 50 means deep tissues are allowed closer to their M-value before the first stop, which mimics the surface-bias philosophy NEDU 2011 found favorable. On the 40 m/20 min profile runtime drops back near 13-14 minutes. Users: warm-water rec divers extending bottom time on a single tank, fit divers in good cardiovascular shape, simple square profiles where no decompression obligation accrues. Pros: minimum gas overhead, less surface-interval debt. Cons: less margin if a problem extends bottom time, not recommended for cold water, fatigued divers, dehydration, or repetitive multi-day profiles. Never use 50/85 as a one-size-fits-all default.

How to choose

Pick a preset that matches the day, not the dive plan on paper. Cold water, recent flights, repetitive diving, fatigue, dehydration, or PFO history all push you toward 30/70. Warm water, well-rested, single dive, fit team, light workload, near-NDL profile push you toward 50/85. 40/85 is the sane default when in doubt. Test each preset on your specific profile in the dive-planner to see the runtime cost — the difference between 30/70 and 50/85 is rarely more than 10-12 minutes on rec dives but can exceed 30 minutes on deep tech dives. Never change GF mid-dive based on feel; commit to a number before splashing.