Gases in the Deco Planner

What this covers

The Gases card in the Deco Planner is your gas list for the dive: what you carry, in what cylinder, at what pressure. The planner picks from that list on the ascent, prices every litre against it, and warns you when a cylinder cannot do the job. This page is the reference for every field and every badge on it.

The rebreather’s own diluent and oxygen cylinders are not here. They live in the CCR rig, covered in CCR diving. On a rebreather plan this card is retitled OC / Bailout Gases, because that is the job those cylinders do.

The Gases card

The header reads Gases with the count beside it, Gases (3). Collapsed, it shows one pill per cylinder in your own order. The card starts collapsed on a phone and expanded on a tablet, and the i in the header explains how the planner uses the list.

With no gases, the card shows a warning: an open-circuit plan cannot be computed at all, and a rebreather plan computes the loop but has no bailout schedule to offer.

Two buttons sit at the bottom of the card:

• Add from Preset opens your saved gas presets. Picking one adds it straight to the plan with its mix, volume, working pressure and twinset setting, without opening the editor. With no presets saved, the same button offers the cylinder database instead.
• Add Gas opens the gas sheet on a new gas. It starts as air (21/0) and inherits the volume and fill pressure of the last gas in your list, so a second deco bottle does not start from scratch. The very first gas in an empty list defaults to a twinset, because the first gas is usually your back gas.

Reading a gas row

What you see	What it means
21/0, 50/0, 18/45	The mix, always written as O₂/He. The name is derived from the mix, so editing the mix renames the gas. There is no free-text name field
11.1L x2 @ 200bar	Metric: the cylinder’s water volume, the twinset multiplier, and the fill pressure
77.4ft³ x2 @ 3000psi	Imperial: the cylinder’s rated capacity rather than its water volume, because that is the figure a US cylinder is named for
6m–30m or ≤ 57m	The depth band this gas is usable in: from its minimum operating depth to its maximum, computed at the plan’s water density. A gas with no hypoxic floor shows only the ceiling
Excluded from deco	You marked this gas to be skipped by the automatic ascent switching
A violet left border	The mix is hypoxic (under 21 % oxygen) or carries helium
A faded row	The gas is excluded from deco and no level breathes it, so it does nothing in this plan

Tap a row to edit it. Long-press and drag to reorder the list: the order is cosmetic for the schedule, because the planner ranks gases by what fits the depth, not by position, but it does break ties when two cylinders are equally good. The trash icon removes a cylinder with no confirmation, and any level that was breathing it moves to the first remaining cylinder, so check your levels after deleting a gas a level was pinned to.

Add a gas

The gas sheet saves when you dismiss it. Swipe it down, tap the backdrop, or use the header tick, and the gas is written. There are no Save or Cancel buttons. A gas you added by accident is one tap on the trash icon to remove.

The mix

Field	Range	Notes
O₂	5 % to (100 minus He %)	21 % is air. Drives the maximum operating depth, the PPO₂ at every depth, and where the ascent switches onto this gas
He	0 % to (100 minus O₂ %)	0 % for nitrox or air. Helium cuts narcosis and speeds both on-gassing and off-gassing

The two fields cap each other, so the mix can never add up past 100 %. Under them the sheet prints the live usable range, MOD 6m–57m, so you can sanity-check a blend before you leave the sheet. The row and the sheet run the same arithmetic the gas picker does, at your plan’s water density, so a gas the band calls usable at a depth is a gas Auto will actually pick there.

Cylinder size

How you enter a cylinder follows your volume unit, because the two systems name cylinders differently.

Metric asks for Volume: the cylinder’s water volume in litres, the number stamped on the neck. A 12 L cylinder is 12. The range is 1 L to 25.5 L. Under the fields, a hint prints the free gas that volume holds at the fill pressure you entered. The field is one cylinder, the way the neck is stamped. The hint is what you are carrying, so on a twinset it counts both and says “(both cylinders)”.

Imperial asks for Capacity in cubic feet plus the Working pressure it is rated at. An aluminium 80 is 80 ft³ at 3000 psi. Dive Kit inverts that through its real-gas model to get the water volume it works in, and the hint prints the water volume so the stored figure is never invisible. Capacity runs from 1 ft³ to 400 ft³, and working pressure from 725 psi to 7252 psi.

Field	Range	Notes
Pressure (metric) or Fill pressure (imperial)	0 to 500 bar, 0 to 7252 psi	What is actually in the cylinder at the start of the dive, not what it is rated for. Overfills are allowed on purpose
Single cylinder / Twinset (×2 volume)	Two chips	Twinset doubles the gas at the same gauge pressure. Use it for back-mounted doubles and for a sidemount pair you treat as one supply

Both pressure fields carry an i explaining the difference: the working pressure is the rating behind the cylinder’s name, the fill pressure is today’s gauge reading.

MOD, MinOD and the PPO₂ bands

The maximum operating depth of a gas is the depth at which its oxygen partial pressure reaches the cap you allow. The cap is not one number: it depends on how rich the gas is, following the MultiDeco convention.

Band	O₂ content	Default max PPO₂
Lean	under 28 %	1.4 bar
Mid	28 % to under 45 %	1.5 bar
Rich	45 % and above	1.6 bar

So air and standard trimix are capped at 1.4 bar, EANx32 at 1.5 bar, and EANx50 or oxygen at 1.6 bar. All three caps are settings, under PPO₂ caps. The same cap sets the MOD and fires that gas’s hyperoxia warning, so the two can never disagree.

At the other end, a hypoxic mix has a minimum operating depth: the shallowest depth where its PPO₂ still reaches 0.16 bar, the hard hypoxia floor. Tx10/70 cannot be breathed at the surface, and the depth band on its row says where it becomes breathable. The separate Minimum PPO₂ setting (default 0.18 bar) is the warning threshold above that hard floor.

Advanced: a reserve for one cylinder

The Advanced section at the bottom of the gas sheet is collapsed until you need it. It holds two controls.

Reserve starts on Default, which means this cylinder follows the gas reserve rule you picked in Settings (thirds, fourths, fifths, sixths, or a fixed pressure). The row prints what Default is worth right under its label, “Thirds: 67 bar”, so you can read it without switching anything. Switch it to Custom and a pressure field appears, pre-filled with that same number so you are editing a real value. Type what this cylinder should still hold when you surface. The field accepts 5 bar to 300 bar (50 psi to 4351 psi).

This is worth doing where one rule does not fit every bottle. A stage you plan to drop at 30 m and a deco bottle you breathe on the way up do not need the same margin, and a rule of thirds on a 40 ft³ deco bottle reserves far more than the dive calls for.

A custom reserve never changes the schedule or the gas the planner picks. It moves the yellow low-gas line on that cylinder’s card and in the warnings list, it is the target the bottom time finder solves to, and on a rebreather it is what the “bailout leaves this cylinder below its reserve” warning fires on. One limit still applies whatever you type: under 50 bar (725 psi) the planner always flags a critical alert. What happens when a cylinder actually runs out is up to you, under When a cylinder runs out.

Exclude a gas from the ascent

Exclude from Deco, the second control under Advanced, keeps a cylinder out of the automatic ascent switching even when its depth band would allow it. The gas stays in the plan, is still priced, and can still be pinned to a dive level by hand, but the planner will never schedule a stop on it. Its ⓘ says when you would want that: a pony, a buddy’s bottle, or a mix you are carrying but do not mean to breathe.

Use it for a bottle you carry but do not intend to decompress on. It is hidden on rebreather plans, where the whole open-circuit list is bailout by definition.

Automatic gas switching

You do not place gas switches by hand. You give the planner the list and it picks.

On the way down and at a fixed level, the picker takes the richest gas that is legal at that depth. On the way up it walks the list in the smallest oxygen step that is legal at each stop, so you switch from back gas to EANx50 to oxygen rather than jumping straight to the richest bottle you own. Every switch honours that gas’s own PPO₂ cap, and the planner never breathes a gas past it.

Three settings shape the switching, all under Gas Switch in the plan’s settings:

• Gas Switch Point decides whether a switch happens at the gas’s MOD (the default and the modern standard) or only once you reach a deco stop.
• Switch Time holds you at the switch depth for the seconds you set while you change regulators, breathing the old gas the whole time. Default 0.
• When a cylinder runs out is below.

When a cylinder runs out

Two honest answers to one problem, and you pick which one you want. The setting sits with the other Gas Switch controls in a plan’s settings, and in your Deco Planner defaults for new plans. It travels in share links, so a plan you send computes the same way for the person who opens it.

Show shortfall is the default and is what every version before 2.9.0 did. The planner keeps breathing the gas it picked. A cylinder that runs dry reads 0 on its card with a red “Short by 34 bar” line underneath, and that figure is the answer to “how much more gas do I need?”. The same number appears in the warnings list. Size your cylinders with this.

Switch gas answers a different question: “can I get out with what I carry?”. When a cylinder empties, the planner does what you would do at the stop. It moves to the richest gas you still have that is safe at that depth, marks that row in the table with a red switch icon and a red gas name, and runs the rest of the deco on the new gas, which usually makes the deco longer. Every export marks the same row, each in the way it can: the printed and PDF slates print in one ink, so they draw a warning triangle where the switch arrows would go, and the text export adds a ⚠ plus one line under the table. That cylinder’s card goes red and carries a line naming where the plan left it and the gas that took over, “Handed over at 12 m · then 32/0”. It may still read a few bar: the handover comes when the cylinder cannot cover the next segment, not when it reads zero, so up to one stop step of gas can be left in it. A warning names both gases and how much more of the first one would have finished the deco, so the sizing answer is still one line away.

The switch is only ever made to a gas that is safe at that depth. If nothing you carry qualifies, the plan stays on the empty cylinder, its card goes solid red, and a critical warning says so. On a rebreather this all applies to the bailout views.

A handover also has to survive the rest of the ascent, not just the stop it happens at. Before the planner keeps one, it checks every later stop on the new gas against your Minimum PPO₂. If that gas would go too lean higher up and nothing else you carry is safe there, the handover is a dead end, so the planner does not make it: that cylinder is planned the way Show shortfall plans it, you get the same schedule the setting off would give you, and a critical warning names the shortfall. You are never handed an ascent that switches you onto a gas you cannot finish the dive on.

Every cylinder that ends at zero is red, whichever of these happened to it: empty is empty. What tells a handover apart is the border on its card, which is dashed instead of solid. Read the dash as “the plan carried on without this bottle” and a solid red card as “the plan ran short on this bottle”. A clean handover, where the emptied cylinder finishes at zero and the gas that took over carries you to the surface, is still only a caution in the warnings list; a handover that leaves that cylinder short is a critical one there, because the plan is asking for more gas than you packed.

A cylinder can also empty on the bottom, before the ascent even starts. That is not a handover and it is not a plan you can dive: the planner never re-works the bottom phase, so a cylinder that runs out down there was asked for more gas than it holds. The deco is worked out without it, a critical warning names it, and its card is solid red and reads “Empty at 45 m” with no gas after it. Fix the fill or the profile rather than reading the schedule as an answer.

A forced switch costs no gas switch time, because there is no old gas left to purge on, and it lands on your stop-rounding grid so a whole-minute table stays whole-minute. The bottom time finder ignores this setting entirely and always solves as though you stay on the cylinder, because “the longest stay with every cylinder above its reserve” only means something while each cylinder is judged on its own.

Duplicate mixes and twinsets

The gas list allows two cylinders of the same blend on purpose. Sidemount pairs, independent doubles, and two identical deco bottles are real kit, and the planner tracks each one separately so each drains on its own. From 2.9.0 that attribution is exact rather than estimated: the engine holds each cylinder as its own entry instead of pooling gases by composition, which is also what lets one bottle empty and hand over to the other. Totals do not move, but if you carry duplicate mixes the split between those two cards can differ slightly from 2.8.7, and saved plans recompute on first open.

A twinset is the opposite case: one gas supply in two bottles you breathe as one. Use the Twinset chip rather than adding the cylinder twice, so the plan does not think you can drain one half and switch to the other.

Mix the gases a plan assumes with the Gas Blender, and analyse the real cylinder before you dive it.

Behaviour and limits

• Everything on this card is what you typed. The planner has no way to know the real fill or the real analysis, so confirm both before the dive.
• Remaining gas uses Dive Kit’s real-gas model, so a full cylinder reads slightly less than “pressure times volume” would suggest. That is the safe direction: it never tells you that you have more gas than you do.
• Reordering the gas list never changes the schedule except as a tie-break between two equally good cylinders.
• Deleting a cylinder re-points any level that breathed it at the first remaining cylinder, which may not be the gas you would have chosen. Check your levels afterwards.

Related

• Deco Planner is the tour of the whole tool.
• Deco Planner settings holds the PPO₂ caps and the Gas Switch section.
• Reading a deco plan explains the Gas Consumption card these cylinders feed.
• Gas Presets saves a cylinder once and fills it everywhere.
• CCR diving covers the diluent and oxygen cylinders.