Diluents

The diluent supplies the inert fraction of the loop: the setpoint fixes the oxygen, the diluent the ratio of nitrogen to helium in what is left. The role is chosen in the gas wizard’s Tank step, from the Gas Role dropdown; an onboard diluent has none, because the role follows the cylinder.

The four CCR roles

• Onboard Diluent supplies the loop from the unit’s own cylinder. Mandatory: the plan will not run without one.
• Offboard Diluent is a second loop diluent in a stage tank, with a switch depth.
• Bailout is open circuit only, enumerated in bailout scenarios.
• Diluent + Bailout is one stage cylinder doing both jobs. Because it may be breathed directly, a ppO₂ ceiling always applies in both roles.

The 1.1 bar loop limit

Both diluent roles default their ppO₂ Limit to 1.1 bar, and from it the planner derives a maximum operating depth. For the 21/35 diluent that is 42.3 m, the depth at which 21 % oxygen reaches 1.1 bar. The MOD on the gas card is that figure, not the 1.4 bar one a bottom gas is held to.

The dive goes to 55 m and the planner does not object: a pure diluent is never inhaled directly, so an onboard diluent has no ppO₂ gate unless you set a CCR limit on it.

A switch depth is a ceiling: the diluent is usable at or shallower than it, never deeper (Switch depths). Leave an offboard diluent’s empty and the planner uses that maximum operating depth, 42.3 m here; enter a deeper one and the MOD is planned instead, silently. An onboard diluent with no switch depth stays available at every depth.

Which diluent the loop is on

When more than one diluent is eligible, the planner picks by direction of travel: descending and on the bottom, the most helium; ascending, the least; ties on the most oxygen. Because switch depth and depth limit are ceilings, the eligible set only grows on the way up, so switches are one-way.

Next

• Bailout scenarios covers the open-circuit-capable gases; Switch depths gives the ceiling convention.