CCR dive planning — setpoint, diluent, bailout

A closed-circuit rebreather turns gas planning inside out: instead of fixed gas fractions with a ppO₂ that varies by depth, you hold ppO₂ constant and let the breathed mix vary. That changes how decompression is computed, how you pick a diluent — and it makes the bailout plan a complete second dive plan you carry on your back. DecoLog plans all three.

The setpoint: one number, held everywhere

The rebreather's job is to keep the oxygen partial pressure in the loop at your chosen setpoint, injecting O₂ as your body consumes it. Typical practice runs a low setpoint around 0.7 bar near the surface and during descent (where a high setpoint would demand an impossibly high O₂ fraction), then a high setpoint of commonly 1.2–1.3 bar for the bottom phase and ascent. The trade is the classic oxygen trade: a higher setpoint dilutes inert gas harder and shortens deco, but spends your CNS clock faster.

Constant ppO₂ means a variable equivalent mix

Hold ppO₂ at 1.3 bar and the oxygen fraction of the loop changes continuously with depth: at 40 m (~5.1 ata) the loop is effectively 25% O₂; at 10 m it's about 64%; at 6 m, about 80%. The rest of the loop is diluent-proportioned inert gas. For decompression this is the CCR superpower: at every depth of the ascent you are breathing the richest mix that setpoint allows — as if an open-circuit diver were switching to a slightly better deco gas every metre. A deco engine models this by computing the inspired inert-gas pressure from setpoint and diluent at each depth, rather than from fixed fractions. DecoLog runs constant-ppO₂ profiles on both ZHL-16C with gradient factors and VPM-B.

Choosing the diluent

The diluent is the gas that makes up the rest of the loop, and three budgets constrain it — the same three that govern any bottom mix:

• Breathability: the diluent alone should be sane at target depth — a diluent flush or an open-circuit breath off it must not put ppO₂ dangerously high or hypoxically low.
• Narcosis: the loop's equivalent narcotic depth is set by the diluent's inert gas; deep dives need helium in the diluent just as they would in an OC bottom mix.
• Density: the 5.2 g/L guidance applies to the loop gas — and CO₂ is doubly unforgiving on a rebreather, where the scrubber is already part of the breathing resistance.

Air diluent is fine shallow; past the low-30s of metres the same arithmetic that pushes OC divers to trimix pushes CCR divers to trimix diluent.

Bailout: the dive plan you hope not to use

If the loop fails — flooded scrubber, cell failure, CO₂ break-through — you come off it and finish the dive on open circuit. Bailout planning answers: from the worst point of the dive, how much OC gas do two lungs actually need to reach the surface with all stops? That means a full OC decompression schedule computed from the moment of failure, volumes from a realistic stressed breathing rate (your logged RMV, elevated — a bailout is exactly when consumption spikes), and gas switches at each bailout cylinder's MOD. DecoLog builds the open-circuit bailout schedule alongside the loop plan, so you can size cylinders against the actual requirement instead of folklore.

Plan a CCR dive now

Set your setpoint and diluent, pick the algorithm and conservatism, add bailout gases — DecoLog returns the loop schedule, the bailout schedule, oxygen-exposure totals and gas requirements. Free, offline, no signup.

⚠ DecoLog is a planning aid, not a substitute for a dive computer, unit-specific training or your rebreather's own procedures. CCR diving requires certification on the specific unit — verify every plan independently.