Switch depths

A switch depth is the deepest depth at which a gas may be breathed, a ceiling on availability rather than a command to change gas there. Set 21 m on a 50 % mix and the gas exists from 21 m upwards; where you reach for it depends on what else is in the water.

The Switch row is that ceiling, and MOD, ppO₂, EAD and density are all evaluated at it. A switch depth deeper than the gas’s MOD produces a schedule with an oxygen warning attached, not a refusal.

How the planner picks a gas

At every depth of an open-circuit ascent, in this order:

1. A gas you gave a switch depth is available at or shallower than it. A manual switch depth is authoritative: if the switch is hot, the planner warns you and schedules it anyway.
2. A gas with no switch depth turns on at its own MOD, judged against its ppO₂ limit or the plan’s decompression ppO₂ limit, whichever is lower.
3. Among the qualifying gases, the richest wins.
4. The gas in use is a floor. The planner never hands back a leaner gas, so a switch is one-way.
5. A candidate carrying more helium than the gas in use is rejected.

With nothing qualifying, the planner falls back to the first gas with no switch depth.

Rules 4 and 5 mean a leaner-but-heliox gas configured shallow will never be selected on the way up if a helium-free gas of equal or greater oxygen is in use. Nothing warns you; it simply never appears in the schedule.

The limits one gas can carry

Optional overrides at the wizard’s Gas Limits stage; leave one blank and the plan’s limit applies.

• ppO₂ limit: this gas’s ceiling in place of the role default, itself capped by the plan’s decompression ppO₂ limit.
• OC ppO₂ limit and OC depth limit: open-circuit ceilings, deciding whether the gas can carry a bailout.
• CCR ppO₂ limit and CCR depth limit: the loop-side pair, deciding diluent eligibility.

On a bailout ascent the gas becomes available at the shallowest of its open-circuit MOD, its open-circuit depth limit, and the switch depth you set.

Diluents are chosen differently

A diluent’s switch depth is a ceiling in the same sense, but the ranking flips with direction of travel: descending and on the bottom the planner takes the deepest MOD, breaking ties on least helium; ascending it takes least helium first, then most oxygen. Before the schedule runs, each diluent’s switch depth is reduced to its MOD at the loop ppO₂ limit.

Next

• Mixes and roles gives each role’s default ceiling.
• Diluents covers the rebreather side.