Decompression Theory: Surface Interval Credit and the WXYZ Rule

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 why nitrogen leaves your body unevenly across compartments during a surface interval, why the RDP uses a 60-minute gas washout tissue, and exactly why the WXYZ rule exists.

Watch the previous videos first Surface interval credit requires understanding half times, compartments, and M-values. Watch those three videos before this one.

How Nitrogen Leaves the Body

Nitrogen washes out of your tissues on the surface the same way it washed in underwater — exponentially. The rate of release is fastest immediately after surfacing and gets progressively slower as the tissue approaches equilibrium with surface pressure. This is called exponential release (the same EEE — exponential uptake, exponential release — principle that governs absorption).

Fast compartments off-gas quickly; slow compartments take much longer. The bigger the pressure gradient between the tissue and the surface, the faster the nitrogen leaves — which also means slow compartments on shallow dives (which accumulated significant nitrogen) can take a very long time to clear.

The 60-Minute Gas Washout Tissue

Rather than tracking each of the 14 compartments individually during a surface interval, the RDP simplifies things: for the purpose of calculating surface interval credit, it assumes all compartments release nitrogen at a 60-minute half time.

This is a deliberate approximation. It does not match reality perfectly — and that mismatch is intentional and important:

For fast compartments (e.g. 5- or 10-minute half times): the 60-minute washout assumes they retain more nitrogen after a surface interval than they actually do. This is conservative — it overestimates residual nitrogen in fast tissues.
For slow compartments (e.g. 90- or 120-minute half times): the 60-minute washout assumes they retain less nitrogen than they actually do. This could underestimate residual nitrogen in slow tissues.

The 6-hour proof — look at your RDP Open the Surface Interval Credit Table on the RDP. The bottom right-hand corner ends at exactly 6 hours. That is not arbitrary — it is 6 × 60 minutes, proving that the RDP uses a 60-minute gas washout tissue. After 6 hours, the RDP considers all residual nitrogen cleared and the diver is treated as starting fresh with no pressure group at all — falling below Pressure Group A.

Exam trap — there IS a level below Pressure Group A Many candidates assume Pressure Group A is the lowest possible group. It is not — it is simply the lowest group shown on the table. A diver who has waited 6 hours or more has no residual nitrogen loading whatsoever. They are below PG A. This question comes up regularly and is frequently answered incorrectly.

The WXYZ Rule — Why It Exists

Because the 60-minute washout underestimates residual nitrogen in slow compartments after shallow dives, the RDP includes a safety rule for situations where this matters most.

Long shallow dives (to 40 feet or less) load slow compartments significantly — often close to their M-values. After a surface interval, the 60-minute washout calculation may suggest those slow compartments have cleared more than they actually have. A repetitive dive in this situation could push a slow compartment over its M-value.

The WXYZ rule enforces a minimum surface interval to ensure slow tissues genuinely clear:

End pressure group	Minimum surface interval
W or X	60 minutes
Y or Z	3 hours

When does the WXYZ rule apply? To reach pressure group W, X, Y, or Z, you would typically have done a long, shallow dive — 40 feet or less for an extended period. These are precisely the dives that load slow compartments most significantly, and where the 60-minute washout underestimation is most dangerous on a repetitive dive.

Decompression Theory — Topics

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

← Deco Hub ← Theory Hub