Give the planner a mix, a depth and an oxygen ceiling and it answers with four numbers and a list of warnings. That answer is the block on every gas card, and the block the gas wizard shows at its limits and confirm stages.
21/35
PrimaryBottom Gas · Backmount Double
12 L × 2232 bar
TMX 21/35
Open circuit
ppO₂ ≤1.40 bar
The four numbers
- MOD, the maximum operating depth, is where the mix’s ppO₂ reaches the ceiling the gas is held to. It rounds down to the whole metre, never deeper than the real figure. A mix with no oxygen shows nothing here.
- ppO₂ is the oxygen partial pressure at the depth being judged, in bar, and what the two oxygen warnings are measured against.
- EAD is the depth at which the mix’s nitrogen load feels like air: helium and oxygen count as non-narcotic. It rounds up to 0.1 m, never less than the real figure. The wizard labels it END.
- Density is grams per litre at that depth, a work-of-breathing figure rather than a decompression one. See Gas density.
The planner picks the depth rather than asking: a back gas at the plan’s maximum depth, a staged gas at its switch or drop depth, anything else at its own MOD. That is why the same 50 % mix can show different numbers on different cards.
The warnings
Six conditions, in this order. The two density conditions are mutually exclusive, as are the two oxygen conditions.
| Condition | Severity | What you see |
|---|---|---|
| Density above 6.2 g/L | Error | High gas density - exceeds 6.2 g/L |
| Density above 5.2 g/L, at or below 6.2 | Warning | Elevated gas density - exceeds 5.2 g/L |
| Narcotic depth above 30 m | Warning | High EAD - exceeds 30m |
| ppO₂ above 1.6 bar | Error | ppO₂ too high - exceeds 1.6 at depth |
| ppO₂ above your own limit, at or below 1.6 | Warning | ppO₂ exceeds target 1.4 |
| MOD shallower than the depth being judged | Error | MOD (22m) is shallower than dive depth |
The 1.6 bar error is absolute. It fires whatever ceiling you set on the gas, so raising your own limit past 1.6 does not silence it. The 5.2 and 6.2 g/L figures are the SPUMS advisory and the NEDU limit.
The severity column is what the wizard shows. On a gas card in the planner every condition renders as an amber advisory, because a mix unbreathable at one depth can still serve shallower. Whether the gases you carry cover the dive is judged for the plan as a whole, in the Gas Mixes section. See Entering a gas.
The card’s numbers use the conventional gas-planning pressure approximation rather than the schedule’s, so on an altitude or fresh-water plan a card’s MOD and the schedule’s depths will not agree exactly.
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Switch depths sets the depth at which each of these numbers is judged, and the depth the gas becomes available at.