Decompression Theory: Half-Times 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 explain nitrogen absorption and half times using animated graphs that show tissue loading in real time — the foundation you need before the compartments, M-values, and Navy vs RDP videos make sense.

Watch the video first This topic uses animated graphs that show nitrogen loading in real time. Read these notes after watching the video, not instead of it.

The Haldanean Model — Where Dive Tables Come From

Virtually every dive table and dive computer in the world — including the RDP — is based on a Haldanean decompression model, named after John Scott Haldane who built the first mathematical decompression model and produced the first dive tables in 1906. Modern models use the same core ideas, refined with better data.

The core principle When you descend, nitrogen pressure in your breathing gas exceeds nitrogen pressure in your tissues — so nitrogen dissolves in. When you ascend, the reverse happens — tissue pressure exceeds breathing gas pressure and nitrogen is released. The difference between these two pressures at any moment is called the pressure gradient. On ascent, tissues can tolerate some gradient safely. If the gradient gets too large, nitrogen forms bubbles — that is decompression sickness.

What Is a Half Time?

Different tissues absorb and release nitrogen at different speeds. A half time describes that speed — specifically, the time in minutes for a tissue to go halfway between its starting nitrogen pressure and full saturation at the current depth.

Think of a teabag dropped into hot water. Tea infuses rapidly at first, then slower and slower as the water darkens — the rate of change decreases the closer it gets to equilibrium. Nitrogen loading in your tissues works exactly the same way.

Saturation Over Six Half Times

Each half time, the tissue goes halfway from its current level to full saturation. It never technically reaches 100% mathematically — so after six half times, the compartment is considered full (or empty on ascent).

Half times elapsed	Tissue saturation
1	50%
2	75%
3	87.5%
4	93.75%
5	96.875%
6	98.4375% — considered saturated

Exam trap — saturation takes six half times, not one After one half time a tissue is only 50% saturated. It takes six half times for a compartment to be considered full or empty.

Measuring Nitrogen — Feet or Metres of Seawater

To track dissolved nitrogen, decompression models use feet of seawater (fsw) or metres of seawater (msw) as the unit of measurement. Think of it like litres or kilograms — it describes an amount, not a physical depth.

If you have 20 fsw of nitrogen dissolved and another diver has 40 fsw, they have twice as much as you. That is all it means.

Worked example — dive to 100 feet, starting with zero nitrogen Full saturation at this depth = 100 fsw.

After 1 half time: 50 fsw
After 2 half times: 75 fsw
After 3 half times: 87.5 fsw
After 4 half times: 93.75 fsw
After 5 half times: 96.875 fsw
After 6 half times: ≈ 100 fsw — tissue considered saturated

Depth Determines How Much Nitrogen, Not How Fast

Two tissues with the same half time but at different depths will reach saturation in the same number of half times — but the shallower one will have far less nitrogen dissolved at the end. Depth sets the target; the half time sets the rate of approach.

Side-by-side comparison — same half time, different depths After six half times at 60 feet: tissue contains 60 fsw of nitrogen.
After six half times at 100 feet: tissue contains 100 fsw of nitrogen.
Same time. Same number of half times. Very different nitrogen loading.

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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