Block 1 · Fundamentals of Decompression

Introduction

This block lays the foundation for everything that follows: how inert gas gets into the body, how it leaves again, and why we use mathematical models (e.g. Bühlmann) to plan ascent profiles. This is not about formulas, but about relationships — and what they mean in practice. We’ll work with the core ideas behind Bühlmann-style models (compartments, half-times, M-values) and how they are modified by Gradient Factors. You’ll understand how to apply them — and where the limits and uncertainties are.

Before we get into decompression, a quick reminder of the most basic gas laws.

On-gassing and off-gassing

As soon as you descend, ambient pressure increases — and with it the partial pressure of inert gases in your breathing gas. In the lungs, inert gas diffuses into the blood and is carried throughout the body. If the inert-gas pressure in the blood is higher than in the surrounding tissues, gas diffuses into those tissues. If the inert-gas pressure in the blood is lower, gas leaves the tissues again and can be eliminated via the lungs.

On-gassing: what changes under pressure?

The basic principle behind on-gassing and off-gassing is Henry’s Law. Whenever a gas borders a liquid — or different tissues border each other — partial pressures will tend to equalise over time. During descent, ambient pressure increases, and with it the nitrogen partial pressure in the breathing gas. That drives increased inert gas uptake. During ascent, pressure drops again — and the gas has to leave the body. You can watch that process more closely in the animation.

Tissue compartments and half-times

In reality, different tissues take up and release inert gas at very different speeds. We can’t calculate that for every organ individually. That’s why models use artificial “compartments”: each one is assigned a half-time, and together they cover the full spectrum of human tissues.
The half-time describes how long a compartment needs to adapt halfway to a new pressure situation. Example: a 5-minute half-time → after 5 minutes it’s at 50%, after 10 minutes at 75%, after 15 minutes at 87.5%.
The most commonly used model today, the Bühlmann ZHL-16C algorithm, uses 16 model tissues with half-times between 5 and 635 minutes. You can explore how saturation develops in these tissues over time in the next tool.

Saturation in a single model tissue

We now have the building blocks: tissues on-gas, they do so at different speeds, and as pressure decreases, they off-gas again.
To talk about off-gassing — and about limits of supersaturation — this plot is a very helpful mental model. It makes saturation and, especially, off-gassing in a model tissue easy to see.

Part 1: How is the plot constructed?

Part 2: How can we track saturation and desaturation – and what limits are we talking about?

With this basic picture in mind, you’re well prepared for the next section: the limits of supersaturation.

Limits of supersaturation: M-values and Gradient Factors

Because we can’t directly look inside the body, decompression models exist. They aim to represent the processes as plausibly as possible and provide a workable estimate of risk. The most widely used model is the Bühlmann ZHL-16C algorithm, which we’ll use here to explain the concepts.
We’ve already seen the central question behind all decompression theory: how supersaturated can a tissue become before the risk of DCS becomes unacceptably high? That’s exactly what M-values — and their modification via Gradient Factors — are about.

M-values

What does this look like in practice for a single model tissue — in terms of M-values, no-decompression limits, and decompression? Let’s go back to our off-gassing plot.

M-values and the no-decompression limit

“No-decompression” means: a direct ascent would not exceed any M-value. That doesn’t eliminate risk, but it keeps it within a range the model defines as acceptable.
At different depths, different tissues are the first to reach their maximal tolerated load and therefore end the no-deco time. In this tool you can see, for each depth, which tissue is limiting.

Gradient Factors

Gradient Factors are a way to modify M-values and build an additional safety buffer.
To understand how they work, we’ll return once more to our basic plot of off-gassing in a model tissue.

Keeping an eye on all tissues: dive planning

Up to this point we’ve mostly looked at what happens in a single model tissue, and used that to define limits of supersaturation.
But the model works with 16 tissues with very different properties — so during ascent we have to keep all of them in view.
The no-deco finder has already shown that different tissues become limiting at different depths. What applies to no-deco time also matters in decompression: the tissue that comes closest to its M-value can change during ascent. We need a way to watch all model tissues at the same time.
Heatmaps offer exactly that kind of visualisation. To keep the entry as simple as possible, here’s a very basic deco planner that shows the profile and a heatmap.
You can enter one or more depths and times, choose your breathing gas and your GFs. When you hit calculate, you’ll get the dive profile, the heatmap, and a runtime.
The heatmap shows saturation of each tissue in shades of blue: the darker it gets, the closer the tissue is to being saturated. During ascent the tissues move into the supersaturation range, shown here in traffic-light colours. Green means a very low gradient; red means 100% of the M-value is reached.
For real dive planning, you should use a properly validated planner. But here you can clearly see what changes when you shift your Gradient Factors.
Full dive-planning practice is covered in Block 6.

Finished playing with the planner? Then try to see if you can recognise which profiles and heatmaps belong together.