About Decompression Schedule
Generates a full ascent profile with deco stops using the selected algorithm. Decompression gases switch automatically at the specified depths during ascent and are factored into tissue off-gassing. Configure bottom gas, optional travel gas, and up to two deco gases below.
Dive Parameters
Depth: Maximum depth in metres. All 16 tissue compartments are loaded based on bottom time at this depth.
BT Time: Total bottom time in minutes, measured from the surface. Includes descent — descent duration is subtracted automatically for tissue loading.
Bottom Gas: Breathing mix for descent and bottom phase. Determines N₂ fraction for tissue loading and ppO₂ throughout the dive.
Ascent Rates
Descent Rate: Speed of descent in m/min. Affects how quickly tissues load N₂. Standard technical rate is 10–22 m/min.
Ascent Rate: Speed from the bottom to the first deco stop. Faster = less off-gassing during transit = more conservative stops. Standard: 9–10 m/min.
Deco Ascent Rate: Speed of travel between deco stops. Slower means more off-gassing between stops, which can shorten stop times. Standard: 3–9 m/min.
Surf Ascent Rate: Speed from the last deco stop to the surface. Usually the same as deco ascent rate.
Deco Stop Settings
Deco Step Size: Depth interval between stops in metres. Standard is 3 m. Most dive computers use 3 m.
Last Deco Stop: Shallowest mandatory stop depth. Common: 3 m (standard) or 6 m (conservative). The diver holds here until tissues are safe to surface at GF High.
Min Stop Time: Minimum time at any deco stop regardless of ceiling clearance. Standard is 1 min. Smaller values give more precise plans but are harder to execute.
Transit Mode: Controls how ascent segments between deco stops are modelled in Bühlmann GF mode.
Schreiner (accurate) — tissues off-gas normally via the Schreiner equation during each ascent segment between stops. This is the most physically correct method. Stop durations shown reflect the actual hold time at that depth only. Run time includes transit segments separately.
MultiDeco compatible — ascent between deco stops is treated as instantaneous for tissue loading, matching MultiDeco 4.x behaviour. Transit time is added to the displayed stop duration, so stop times and total run time match MultiDeco output exactly. This mode is slightly more conservative because transit off-gassing is not credited. Deco-zone ascent rows are hidden from the table (folded into the stop time, as MultiDeco displays them).
To fully replicate MultiDeco output: set Transit Mode: MultiDeco compatible + Stop Rounding: Yes + Water Vapor: 0.0577 bar.
Stop Rounding: Whether deco stop times are rounded to whole minutes.
• No (Fractional) — transit time between stops is absorbed, giving precise values like 0:33 or 1:40. Matches ApexDeco output.
• Yes (Whole min) — every stop rounds up to the nearest whole minute. Simpler to execute underwater. Closer to MultiDeco output.
Water Vapor: Alveolar water vapor pressure used in tissue loading calculations.
• 0.0627 bar (B — Bühlmann) — Bühlmann's original standard at 37°C body temperature.
• 0.0577 bar (M — MultiDeco) — value used by MultiDeco and some other implementations. Produces slightly more tissue loading, shifting stop times by a few seconds at boundaries.
Pair Stop Rounding: Yes + 0.0577 bar (M) to most closely match MultiDeco output. Pair No + 0.0627 bar (B) for the Bühlmann standard.
🟠 Travel Gas
Definition: A travel gas (also called a back-gas or transit gas) is a breathing mix used from the surface to the depth where the bottom gas MOD is reached on descent, and again on ascent when you rise above the bottom gas MOD until the first deco gas switch. It prevents O₂ toxicity at shallower depths when the bottom mix has a high O₂ fraction.
When you need it: Any time your bottom gas ppO₂ exceeds the configured limit in shallow water. Common scenario: EAN32 as bottom gas on a 40 m dive — EAN32 hits 1.4 bar ppO₂ at ~33 m, so you need a travel gas (e.g. AIR or a custom mix) for the surface-to-33 m segment.
Switch depth — Auto: Calculated as the MOD of the travel gas at the configured ppO₂ Bottom limit. This is where the travel gas itself reaches its limit — the planner switches you to bottom gas exactly there. Formula: depth = (ppO₂_limit / fO₂_travel − P_surface) / 0.099
Switch depth — Manual: Diver-specified. Useful when coordinating with a buddy, aligning with a site landmark, or when using a hypoxic travel gas that has a minimum safe depth.
Table output: When a travel gas is active, the descent in the deco table splits into two rows: travel gas (orange) surface → switch depth, then bottom gas (blue) switch depth → bottom. On ascent, a travel gas segment also appears above the bottom gas MOD. Gas consumption is tracked separately per gas.
Cylinder fields: Travel gas cylinder size (L) and pressure (bar) — included in the gas consumption summary alongside bottom and deco gases.
Gas & SAC
BT SAC: Surface Air Consumption during the bottom phase (L/min). Used to calculate cylinder gas consumption. Typical technical diver: 18–22 L/min. Stress and exertion increase SAC significantly.
Deco SAC: SAC rate during deco stops. Lower than bottom SAC since the diver is stationary. Typical: 12–18 L/min.
Narcosis
N₂ / O₂ Narcotic: Which gases count toward the Equivalent Narcotic Depth (END).
• Both Yes — conservative, used by GUE/DIR. END equals actual depth on air.
• N₂ only — more optimistic END for high-O₂ mixes.
END above 30 m warrants caution. Above 40 m is a serious narcosis risk shown in red.
ppO₂ Limits
Max Bottom ppO₂: Maximum O₂ partial pressure for the bottom gas. Standard: 1.4 bar. Sets the maximum operating depth (MOD) for the bottom mix.
Max Deco ppO₂: Maximum ppO₂ for deco gases (EAN50, O₂). Standard: 1.6 bar. The planner calculates each gas switch depth as the deepest depth where ppO₂ stays within this limit.
O₂ @ MOD Setting
Allow at MOD (default) — Pure O₂ (100%) is always selectable as a deco gas at its Maximum Operating Depth (6 m). At 6 m, ppO₂ of pure O₂ = 1.608 bar — marginally above 1.6 bar but accepted in real-world practice. Matches Shearwater, V-Planner and most dive computers.
Strict ppO₂ — Pure O₂ still uses a 6 m floor (same as Allow at MOD). The Strict flag has no effect on other deco gases — only pure O₂ has an override that Strict mode bypasses.
Gradient Factors (GF Low / GF High) — Evidence Base
GF Low ≥ 55: Based on the US Navy NEDU deep-stop study (Doolette et al. 2011). Divers with first stop at 40 fsw had significantly lower DCS incidence than those stopping first at 70 fsw. Iterative ZHL-16C modelling places GF Low ~55 at the conservative bound, ~70 at the midpoint. Lower GF Low (deeper stops) is not proven safer and may increase shallow tissue loading.
GF High ≤ 80: Based on Howie et al. (2017, PLoS ONE) — at the 0.1% serious DCS criterion for no-stop dives, ZHL-16C iterative matching gives GF High = 80. For decompression dives, GF High must decrease as dive severity increases (see PrT below). GF High 85–95 exceeds the validated safety margin for any non-trivial dive.
Belgian military study (PMC10735712): Bühlmann ZHL-16C + GF replicated in Python and optimised against USN and DCIEM tables (both considered acceptably safe). Result: GF Low = 100, GF High ≥ 75 best matches validated tables. Conclusion: lower GF Low (deep stops) is not necessarily safer — shallower-biased profiles with GF Lo=100 matched real-world validated tables most closely.
PrT — Pressure×Root-Time severity index:
PrT = P_rel (bar) × √T (min)
where P_rel = gauge pressure at depth (bar) = depth × BAR_PER_METRE (10.000 bar/m salt, 10.330 fresh, 10.080 EN13319), T = bottom time (min).
• PrT < 15: recreational — GF High 80–90 is appropriate
• PrT 15–25: moderate deco — GF High 75–80
• PrT 25–40: serious deco — GF High 60–75 (VVAL-79/Van Liew)
• PrT > 40: outside validated range — extreme conservatism required
References: Fraedrich DS (2018/2019), Diving Hyperbaric Med. · Doolette DJ et al. NEDU TR 11-06 (2011) · Howie LE et al. PLoS ONE 12(3):e0172665 (2017) · Van Liew & Flynn, Undersea Hyperb Med 32:199 (2005) · Belgian Defence study PMC10735712 (2023)
