Every gas carries a composition, a role, a cylinder and a set of limits. The role matters most: it decides the ppO₂ ceiling the gas is held to and whether the planner may breathe it at all. The gas builder asks for them in five stages.
Tank Configuration. Tank size, quantity, working pressure and the cylinder configuration. See Cylinder configurations. A drop tank also asks for a Drop Depth (m).
Gas Composition. Manual, where you type O₂ % and He %, or
Calculator. See Best mix.
Gas Limits. The ppO₂ ceiling and, optionally, a switch depth. See Switch depths.
Label & Customization. The gas’s name and its colour. See Gas names and labels and Colour coding.
Confirm Gas Mix. A read-back of the cylinder and composition, with any warnings against the mix.
You enter oxygen and helium only. Nitrogen is the remainder, a read-only
N₂ % (auto) field, and a mix whose oxygen and helium exceed 100 together is
rejected.
The six roles
| Role | Default ceiling | When the planner may use it |
|---|---|---|
| Bottom gas | 1.4 bar | The primary gas, breathed from the surface on descent, when it has no switch depth. Selectable on the ascent. |
| Travel gas | 1.5 bar | Also a candidate primary gas. Selectable on the ascent. |
| Deco gas | 1.6 bar | Selectable on the ascent, but never primary unless nothing else qualifies. |
| OC bailout | 1.6 bar | Breathable open circuit, and counted as bailout-capable. |
| Onboard / offboard diluent | 1.1 bar | Loop only. Never breathed directly. |
| Diluent + bailout | 1.6 bar | Both: a loop diluent, and breathable open circuit. |
The default applies when you leave the gas’s own ppO₂ limit blank. The wizard pre-fills that field at 1.4 bar for the two bailout roles rather than 1.6, so a bailout gas built here carries 1.4 bar unless you change it.
The cylinder
- Tank size (L): internal volume. Starts at 11.1 L.
- Working pressure (bar): the fill pressure the plan assumes. Starts at 200 bar.
- Quantity: how many cylinders the gas is carried in. See Cylinder configurations.
Next
Consumption turns the cylinder you gave each role into litres, bar and a turn pressure.