Gradient Factors Explained Without the Math Headache

If you've opened your dive computer's settings and found "GF 30/70" or "GF 50/80," you've met gradient factors. They're one of the most discussed settings in diving, and one of the most misunderstood. Here's what they actually do, without the equations.

What problem do gradient factors solve?

Most technical dive computers are based on the Bühlmann decompression model. For each of several theoretical tissue compartments, the model sets a limit, called an M-value, on how much dissolved gas pressure it treats as tolerable during an ascent.

Run the model at its raw limits and it lets you ascend right up to those M-values. Gradient factors, developed by Erik Baker, let you choose how close to those limits you go, as a percentage.

What do GF low and GF high do?

A gradient factor setting has two numbers, like 30/70.

• GF low (the first number) controls where your first stop is. A lower GF low keeps you further from the limit at depth, so the computer starts your stops deeper.
• GF high (the second number) controls the margin when you surface. A lower GF high keeps you further from the limit at the end of the dive, so you spend longer on the shallow stops.

Between the first stop and the surface, the computer draws a straight line from GF low to GF high and keeps your ascent under it.

What do the numbers mean in practice?

Changes to the profile affect gas, exposure and time in the water. A lower GF low can add time at depths where slower compartments continue taking up inert gas; longer or deeper stops are not automatically safer.

So what are the "best" gradient factors?

There isn't a single answer, and anyone who gives you one number for every diver and every dive is oversimplifying.

DAN's Alert Diver article on gradient factors makes a few points worth keeping in mind:

• The model is a model. Decompression models almost certainly don't capture the full truth. Decompression sickness can occur within any model's limits.
• The computer can't see everything. Cold, exertion and how you feel aren't meaningfully accounted for by today's computers.
• Settings aren't comparable across brands. Some manufacturers describe their conservatism settings poorly, which makes them hard to evaluate.
• Fiddling has side effects. Changing inputs to "fool" the algorithm can cause problems, such as missed oxygen warnings.

The practical advice: choose a setting with your instructor that fits your training and the diving you do, keep it consistent, and make sure your team knows what everyone is using.

How does UTD approach decompression?

UTD doesn't build its training around gradient-factor settings. It teaches ascent strategies that the team plans and runs together:

• Min Deco for no-stop recreational diving: no-decompression limits by depth and gas, and a standard ascent with stops. See the Min Deco table in Gas Management for Recreational Divers.
• Ratio Deco for decompression diving: a method for planning and adjusting a decompression ascent during the dive, using standard gases, applied by a trained team.

Ratio Deco isn't a new decompression model; UTD describes it as an application of existing models. Its training covers the history of decompression theory, including Bühlmann and bubble models, so divers understand what their computers are doing.

In that approach, a computer is a tool and a backup, not the plan. The team knows its ascent before it starts it.

Why it matters for your training

Gradient-factor tinkering is a common pain point for divers moving into technical diving. Changing numbers can feel like doing something about risk. The things that actually move risk, like gas planning, standard gases, a practiced ascent and staying within your training, happen outside the settings menu.

If you're heading toward decompression diving, Techreational is UTD's first step, with trimix 25/25 and oxygen decompression. For the gases themselves, see Standard Gases.