A gas card carries three labels: the name, the role and the cylinder configuration. You type at most one of them.
21/35
PrimaryTMX 21/35O₂21%He35%N₂44%
Open circuit
ppO₂ ≤1.40 bar
The name
Mix Name, on the wizard’s Label & Customization step, is optional. Left empty, the planner names the gas from its composition:
- Any helium at all:
TMX 21/35, oxygen then helium. - 21 % O₂ and no helium:
Air. - 100 % O₂:
O₂. - Anything else:
Nitrox 32, the oxygen percentage alone.
A name you type replaces that at the top of the card, and the generated name moves to the start of the composition line so the mix stays readable. Typing the name the planner would have written counts as no name at all.
The role
On a single, backmount double or sidemount cylinder the role is fixed to Bottom Gas and labelled Back Gas. Mixes and roles gives each role’s ceiling.
Change Cylinder Configuration and the role follows it, so the two are never left disagreeing:
- A stage cylinder on a rebreather becomes Offboard Diluent, a drop cylinder Bailout.
- A back-gas configuration becomes Bottom Gas.
- Leaving a back-gas configuration falls back to Travel Gas, so a gas moved out of the doubles is not left claiming to be the bottom mix.
Set the cylinder first, then the role, or the cylinder will overwrite it.
The primary gas
A gas is the diver’s primary when its role is Bottom Gas, or when it sits in a single, backmount double or sidemount cylinder. The card chips it Primary. A primary gas has no depth ceiling of its own.
Next
Colour coding is the last of the labels a gas carries, and the one that identifies it on the profile.