Decompression Theory: Tissue Compartments Explained

Decompression Theory — Topics

1 — Half Times 2 — Compartments 3 — M-Values 4 — Surface Interval Credit 5 — Navy Tables vs RDP Dive Computers

Watch Will Welbourn show how different tissue compartments load nitrogen at different speeds on the same dive — with side-by-side animations that make the relationship between half time, depth, and nitrogen loading visually clear.

Watch the half times video first Compartments only make sense once you understand half times. If you haven't watched that video yet, start there.

What Is a Compartment?

The human body is made up of many different tissues — muscle, fat, blood, bone, spinal fluid, organs. Each absorbs and releases nitrogen at a different speed. The primary factor is blood supply: well-supplied tissues absorb gas quickly; poorly supplied tissues absorb it slowly.

It would be impossible to model every tissue individually, so Haldane grouped tissues into theoretical compartments based on their absorption speed. These compartments do not directly correspond to any specific body tissue — they are mathematical constructs. Each compartment has its own half time.

Key fact Haldane's original model used 5 compartments with half times ranging from 5 to 75 minutes. The RDP uses 14 compartments with half times ranging from 5 to 480 minutes.

The RDP's 14 Compartments

RDP compartment half times (minutes)
5	10	20	30	40	60	80
100	120	200	240	300	360	480

How Different Compartments Load Differently

Faster compartments reach higher nitrogen levels sooner. Slower compartments take much longer to load — but they also take much longer to clear. This asymmetry is what makes repetitive dive planning necessary.

Example — 5-minute vs 10-minute compartment on a dive to 100 feet

After 5 minutes: 5-min compartment is at 50 fsw (one half time). The 10-min compartment is below 50 fsw — it has not yet completed one half time.
After 20 minutes: 5-min compartment is at 93.75 fsw (four half times). 10-min compartment is at 75 fsw (two half times).
After 30 minutes: 5-min compartment has completed six half times — it is saturated at 100 fsw and stops absorbing nitrogen. The 10-min compartment continues loading.
After 60 minutes: 10-min compartment has now completed six half times — saturated at 100 fsw.

Why Multiple Compartments Matter

On a short deep dive, the fast compartments load quickly and reach their limits first — they control how long you can stay. On a long shallow dive, the fast compartments may never reach their limits at that depth. Instead, the slow compartments gradually accumulate nitrogen and become the controlling factor.

This is why no-decompression limits are not simply proportional to depth — the controlling compartment changes depending on the dive profile.

These are mathematical models, not anatomy Compartments are theoretical constructs. There is no direct correspondence between a compartment and any specific body tissue. This is why there is always some statistical risk of DCS even when staying within table limits — the model is an approximation of a complex biological reality.

Decompression Theory — Topics

1 — Half Times 2 — Compartments 3 — M-Values 4 — Surface Interval Credit 5 — Navy Tables vs RDP Dive Computers

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