Ratio Deco Explained: A Decompression Strategy, Not an Algorithm

Twenty-five minutes into the dive, the plan changes.

The wreck is better than anyone expected. The team stays five minutes longer. The average depth comes out shallower than you planned. So what's your decompression now?

If your answer is "I'll check my computer," here's the follow-up. Which computer? Yours, or your teammate's? The one set to 30/70, or the one set to 50/80?

Ratio deco is the answer to that problem. Not a better computer. A better plan, carried in every diver's head.

What is ratio deco?

Ratio deco is a decompression strategy. UTD describes it as a methodology that lets a diver apply existing decompression models as one cohesive strategy the whole team can run during the dive. Decompression on the fly.

The core idea is simple enough to say in one sentence. At a given depth, your decompression time is a ratio of your bottom time. The deeper you go, the more decompression each minute of bottom time costs you.

Around that idea sit a few simple rules about the shape of the ascent and where the gas switches happen. Simple enough to run in your head at depth. Simple enough that every diver on the team gets the same answer.

That's the point.

What is it based on?

Not on a hunch. On a century of decompression research.

UTD's Ratio Deco course starts with the history: Haldane, the US Navy tables, Bühlmann's dissolved-gas model, and the bubble models such as VPM and RGBM. Ratio deco takes the profiles those models produce and simplifies them into rules a person can apply without a laptop.

So it isn't a new theory competing with Bühlmann. It sits on top of the models, the same way a pilot's rules of thumb sit on top of aerodynamics.

Strategy vs. algorithm: what's the difference?

Here's the thing most people miss.

An algorithm is math a computer runs continuously. It tracks one diver's depth, second by second, through a model of how tissues load and unload gas. It's precise. It's also private. Your computer knows your profile and your settings. It knows nothing about your team.

A strategy is a set of rules people apply together. It's built from what the algorithms produce, but it lives in the team's heads, not in one diver's wrist. Everyone knows the plan before the dive. Everyone can recalculate it when the dive changes. Nobody's waiting for a screen to tell them what happens next.

UTD puts the goal plainly: plan decompression without primary reliance on tables or computers, and build a thinking decompression diver who can adjust when the profile changes. That's also why I teach with computers in gauge mode.

What does ratio deco need to work?

It isn't a shortcut, and it isn't for self-study. UTD is explicit that it depends on:

• Standard gases. Bottom gas and deco gas are always the same mixes, so the rules always apply. See Standard Gases.
• A real team. Everyone runs the same plan and checks each other.
• Situational awareness. You track time, depth, average depth and gas the whole dive.
• Discipline. You stay inside its limits, every time.

Take any of those away and the strategy stops working.

Is ratio deco proven?

Let's be honest about this. Ratio deco has a long record of successful dives inside the DIR community, but most training agencies haven't adopted it, and it hasn't been formally validated the way some algorithms have (Wikipedia). It has also changed over time. UTD's current version, Ratio Deco 3.0, reflects how the research has moved, including on deep stops.

That's not a reason to dismiss it. It's a reason to learn it properly, from the people who maintain it, and to respect its limits.

How do you learn it?

Not from me in a blog post. And not from a forum thread.

UTD teaches the full method in its Ratio Deco 3.0 course: the goals of decompression, strategy vs. algorithm, computers vs. tables vs. planning software, and how to put it into practice. Take it there, from the source.

Then come put it in the water. The theory is the easy part. Running it at the end of a real dive, with your team, while the plan changes, is the skill.

Ready to run it for real?

So, could you recalculate your decompression right now, at depth, if the dive changed? Would your teammate get the same number?

If not, start with the course. Then bring it to Techreational, where trimix 25/25 and an oxygen bottle turn ratio deco from a theory into a habit.