Gradient factors

What they are

Plain Bühlmann lets you ascend right up to the M-value line, the hard ceiling of supersaturation each tissue can tolerate. Diving to that absolute limit leaves no margin for the things a model cannot see: cold, exertion, dehydration, your own physiology on the day. Most technical divers want a buffer.

Gradient factors are that buffer, expressed as a dial. Dive Kit uses Erik Baker’s gradient factor method, the de-facto standard across modern technical diving.

To picture it, recall the two lines from how the engine works. The ambient-pressure line is where supersaturation begins. The M-value line is the hard ceiling. The band between them is the decompression zone. A gradient factor is how far across that band you allow yourself to go:

• 0% means stay on the ambient line. No supersaturation at all, which means an impractically slow ascent.
• 100% means go right up to the M-value line. This is plain, unmodified Bühlmann.

A gradient factor of 70% means you allow yourself 70% of the way from the ambient line up to the ceiling, keeping a 30% margin below the limit.

GF Low and GF High

You do not set one number. You set two, written as a pair like 50/80.

• GF Low is the margin you keep at your first (deepest) stop. It controls where your first stop begins. A lower GF Low pulls the deep ceiling down, so the engine starts holding you deeper.
• GF High is the margin you keep at the surface. It controls how much supersaturation you carry to the surface. A lower GF High means your leading tissue must sit further from its limit before you can surface, which stretches out the shallow stops.

Between the first stop and the surface the gradient factor changes in a straight line, interpolated linearly with depth, exactly as Baker’s method specifies. So a 50/80 profile holds you 50% across the band at the deepest stop and gradually relaxes to 80% by the time you surface.

You can set the two numbers independently, including an inverted pair where GF Low is higher than GF High (say 90/70). An inverted pair is unconventional: it means you accept more supersaturation at depth than near the surface, so the shallow stops grow and the deep ones shrink. No mainstream training agency teaches it, but the math is well defined and the engine computes it faithfully. If you meant 70/90 and typed 90/70, the GF inputs show an inverted-pair hint the moment the values cross, and the warning described below catches the cases that matter.

Two practical takeaways:

• GF Low mostly redistributes deco deeper. Lowering it moves time deeper rather than adding much total time. (It does not change where the decompression zone starts; gradient factors move the M-value ceiling, not the ambient line.)
• GF High mostly changes total runtime. Lowering it lengthens the shallow holds where the slow tissues are still unloading. This is the bigger lever on how long your deco takes overall.

The default: GF 50/80

Dive Kit defaults to GF 50/80. It is a widely used, moderately conservative setting: a reasonably deep first stop and a sensible margin at the surface. It is the same default many divers run on their computers.

Lower numbers are more conservative. You stop deeper, hold longer, and stay further from the ceiling. Higher numbers are more aggressive, with shorter, shallower deco and less margin.

Common presets

Open the Settings card and expand the Gradient Factors section to find the preset pills, along with the GF Low and GF High inputs. The app ships three preset pills. You can also type your own pair, with each input accepting 10 to 100.

Preset	Character	What it does
30/70	Conservative	Deep first stop and long shallow holds. A common choice for divers who want extra margin.
50/80	Balanced (default)	Moderate first-stop depth and surfacing margin.
80/80	Aggressive	Shallower first stop and a thinner surfacing margin. Shorter total deco.

These are starting points, not prescriptions.

Typing 100/100 gives plain, unmodified Bühlmann right up to the M-value line. It is not one of the preset pills, and it is not recommended.

When a GF High cannot surface the plan

Some combinations ask the model for the impossible. The clearest example: air as your only gas, a 6 m last stop, and a low GF High. Holding at 6 m on air, your tissues keep absorbing nitrogen toward about 1.22 bar of tissue tension, and a GF High of 50 never tolerates that much at the surface. Past a certain amount of bottom time, the 6 m stop would simply never end. No schedule exists.

Dive Kit detects this exactly and answers with the nearest plan that does exist: it raises GF High to the lowest value that can complete the decompression and computes that schedule. When this happens you cannot miss it:

• A red critical banner sits directly above the dive profile graph, naming the GF High you asked for and the one the schedule uses.
• The same warning appears in the Plan Issues sheet, and the graph card carries a red border.
• Every export (slate text, PDF, image, and the share link) records the GF the schedule was actually computed with, never the one you typed. A printed slate will not claim a conservatism its runtimes do not deliver.

If you would rather keep your requested conservatism, the fix is one of: a 3 m last stop (at 3 m on air you always off-gas relative to the surface), a richer deco gas (EAN50 or oxygen at the last stop), or simply more GF High. If even GF High 100 cannot complete the plan, the planner shows a clear error instead of a schedule.

The full story, including the guarantee threshold for each last-stop depth and how the engine proves impossibility without false alarms, lives on Impossible plans.

How to choose

There is no single correct setting. The right gradient factors depend on the dive, the conditions, and you.

• Match what you trained on and what your computer runs. Diving a plan more aggressive than your wrist computer means you may surface owing deco it still wants you to hold. Keep them consistent.
• Go more conservative (lower numbers) when conditions or your state are worse. Cold, hard work, current, fatigue, dehydration, or a long surface interval since your last meal all argue for more margin.
• Lower GF High first if you want more overall margin. It is the lever that most increases your surfacing safety, by lengthening the shallow stops where slow tissues clear.
• Lower GF Low if you specifically want deeper stops. Understand that this mostly shifts time deeper rather than adding a large margin overall.

Related

• How the engine works explains the tissue model and the two lines gradient factors sit between.
• Deco Planner is where you set GF Low and GF High, per plan or as your default.