A closed-circuit gas role decides two things: which circuit the planner may breathe the gas on, and whether it belongs to the dive you are planning or only to a contingency. The role is on the gas wizard’s Tank step, in the Gas Role dropdown. A stage cylinder chooses among the stage roles, the Onboard cylinder chooses Onboard Diluent or Onboard Oxygen, and a drop tank implies its own.
The five roles
- Onboard Diluent: the unit’s own cylinder. It supplies the loop from the surface down, and a CCR plan will not run without one.
- Onboard Oxygen: the unit’s oxygen cylinder. Never breathed and never a diluent; its size and pressure set the oxygen supply the plan is graded against. Its ppO₂ is fixed at 1.6 bar and it takes no switch depth.
- Offboard Diluent: a stage diluent the loop switches onto during the planned dive, at or shallower than its switch depth.
- Bailout: open circuit only. The loop never sees it.
- Diluent + Bailout: a stage cylinder plumbed to both. The planner may fly the loop on it and carries it in every bailout scenario.
What each role changes
- Bailout changes nothing about the planned dive. The schedule, the runtime and the time to surface are unchanged; the gas is used only when you ask what happens if you leave the loop.
- Diluent + Bailout changes the loop. On a 45 m dive with a 21/35 diluent, an 80 % in this role becomes the loop diluent at 9 m on the way up: a switch step in the schedule, and a slightly different time to surface.
Descending and on the bottom a dedicated diluent always outranks it, so the deep part of the dive stays on your back diluent either way.
Deciding where the loop may go
The cylinder’s ppO₂ Limit sets how deep the loop may run on it. Left at the bailout default, that 80 % is loop-eligible from about 10 m up. Give it the 1.1 bar limit a diluent carries and it never becomes eligible on a 45 m dive, and the plan matches Bailout exactly.
Choose Bailout for a cylinder you carry only to leave the loop on, and Diluent + Bailout for one you would also plug into the loop.
Next
- Diluents covers the loop limit and how the planner chooses between eligible diluents.
- Bailout scenarios covers what the bailout roles are for.