What changes your deco, and why

What it is

When you tune a plan and watch a stop appear, lengthen, or vanish, this is what the engine is doing underneath. It is the cause and effect a deco-literate diver wants to be able to verify rather than take on faith, and every one of these is reproducible in the MultiDeco comparison.

How each setting moves your schedule

Lower GF-Low → a deeper first stop. A lower GF-Low makes you stop deeper and start decompressing sooner. On most dives it shifts time deeper without changing the total by much; on a deep or long dive it can add total time, because holding deeper keeps the slow tissues loading while the fast ones unload. The depth where supersaturation first begins (the decompression-zone start, or decozone) does not move; it sits on the ambient line, which no gradient factor changes.

Lower GF-High → longer shallow stops, later to the surface. The leading tissue must sit further from its limit before you can surface, so the shallow holds stretch out. This is the lever that most changes total runtime.

Switch to a richer gas → shorter stops. Your tissues unload nitrogen faster when there is less nitrogen in the gas you breathe. A richer mix (more oxygen, so less nitrogen) widens the gap between the nitrogen in your tissues and the nitrogen in your lungs, and the wider that gap, the faster you off-gas. That is why divers switch to EAN50 and then to oxygen in the shallows. The engine recomputes this every second, so the faster unloading starts the moment you switch, not at the next stop.

Add helium → far less narcosis, and a re-shaped deco profile. Helium carries no narcosis but loads and unloads about 2.65× faster than nitrogen. Because the engine tracks the two gases separately, this falls out of the physics: flip air → trimix at the same depth and the END (equivalent narcotic depth) drops while the stop shape changes (deeper first stop, brisker early off-gassing).

Go deeper, or stay longer → disproportionately more deco. The slow tissues load the whole time you are down, and loading is exponential toward equilibrium, so an extra few metres or minutes adds more decompression than a linear rule of thumb suggests.

A lean deco gas switches shallower than a single-cap model would. Capped at 1.4 rather than 1.6, a Tx21/35 switches at about 54 m instead of the ~66 m a flat cap would give. That keeps PPO₂ and the CNS (central-nervous-system oxygen) clock lower through the deep part. The effect on stop times is small.

Fresh water or altitude → a lighter or heavier obligation. Fresh water is slightly less absolute pressure (lighter deco); altitude lowers your surface pressure, making the ascent a bigger relative pressure drop (heavier). The water-type factor feeds both loading and the ceiling, so the effect is consistent.

A second dive soon after → more deco. You start with residual gas still dissolved, so you reach any given ceiling sooner and owe more time.

Set a gas-switch (purge) time → slightly more runtime. The Switch Time setting holds you at the switch depth for those seconds while you change regulators and check the gas, breathing the old gas the whole time. Your tissues keep loading during the pause, so the plan grows by roughly the time you entered. Open-circuit switches only; closed-circuit rebreather (CCR) setpoint changes on the loop are instant.

Turn on oxygen breaks → slightly more deco. When the Oxygen air breaks setting inserts a break, you come off the rich deco gas onto a leaner back gas for a few minutes. The fast tissues take a little inert gas back on, which the rest of the plan then has to clear, so total deco rises by more than the break itself. You spend that time to cut the central-nervous-system oxygen load on long, oxygen-rich stops.

Exclude a gas (or keep it as bailout) → the engine will not switch onto it. A cylinder marked Exclude from Deco stays out of the automatic ascent switching even when its depth limit would allow it, so the schedule uses only the gases you intend for deco.

Why a stop sometimes vanishes

Stops snap to the grid, so a small change that pulls a ceiling barely under a grid line can drop a whole listed stop while total decompression barely moves. In rounded mode a stop can also merge into its neighbour when both round to the same minute; in precise mode you would still see the brief touch as, say, 0'15".

Related

• Gradient factors explains GF-Low and GF-High in full.
• The decisions that shape your schedule covers the stop grid and rounding that make stops snap and merge.
• Deco Planner is where you change these settings and watch the effect.