Why does the RDP surface interval credit table end at 6 hours?

The table ends at 6 hours because 6 × 60 minutes equals 6 half times for the 60-minute gas washout tissue. After 6 half times, a tissue is considered fully cleared. This proves the RDP uses a 60-minute gas washout tissue — and means a diver who has waited 6 hours or more has no residual nitrogen loading and falls below Pressure Group A.

What is the WXYZ rule and why does it exist?

The WXYZ rule requires a minimum surface interval for divers who end a dive in pressure groups W, X, Y, or Z. Pressure groups W and X require a minimum 1-hour surface interval; groups Y and Z require 3 hours. These groups result from long shallow dives that load slow compartments significantly. The 60-minute washout underestimates residual nitrogen in slow compartments, so the WXYZ rule enforces extra waiting time to ensure those compartments genuinely clear before a repetitive dive.

Why does the 60-minute washout underestimate residual nitrogen in slow compartments?

For tissue compartments with half times longer than 60 minutes — such as 90-, 120- or 200-minute compartments — the 60-minute washout assumes they clear faster than they actually do. This means the RDP may calculate a lower residual nitrogen level in slow tissues after a surface interval than actually exists. The WXYZ rule exists specifically to compensate for this underestimation after long shallow dives, which load slow compartments the most.

Decompression Theory: Surface Interval Credit and the WXYZ Rule

Book your course now — Learn from Will Welbourn in person

IDC → Divemaster Course →

Will Welbourn explains surface interval credit and the WXYZ rule for the PADI IDC and Divemaster exams

Will Welbourn explains surface interval credit and the WXYZ rule — Part 4 of the GPC decompression theory series.

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 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.

Book your course now — Learn from Will Welbourn in person

IDC → Divemaster Course →