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 roles the loop can use

• 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.
• Diluent + Bailout is one stage cylinder doing both jobs: the loop may be flown on it, and it is carried in bailout scenarios too.

Bailout is the one role the loop never sees. See Gas roles.

The 1.1 bar loop limit

Onboard and Offboard Diluent 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 breathed 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 switch depth 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.

What each diluent is budgeted

Every diluent you carry gets its own card in the Gas Consumption view, offboard ones included, and each is charged the loop’s jobs at the deepest point that diluent is in use: the fill down to that depth, the mask clears, the diluent flushes, and the wing or drysuit where they run off diluent. An offboard diluent switched to at 21 m is charged at 21 m, so its share is smaller than the bottom diluent’s.

Each card is a stock check on one cylinder. The plan cannot know which diluent is connected when your mask floods, so it asks whether each one on its own holds enough to do the job - which means the cards do not add up to a total, and should not be added.

A Diluent + Bailout cylinder counts as a diluent whenever the loop is actually flown on it - commonly as the ascent diluent, once you are shallower than its 1.1 bar limit - and is budgeted at the depth it is switched to, like any other. On the loop plan it is charged that and nothing else: the dive as planned is one you do not bail on. What it would cost you as bailout is asked in bailout scenarios.

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, a dedicated diluent outranks a Diluent + Bailout cylinder, then the gas usable deepest on the loop wins; ascending, the least helium, 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.