What decompression theory topics appear in the PADI IDC exam?

The PADI IDC decompression theory exam covers half times, tissue compartments, M-values, surface interval credit, the WXYZ rule, and the differences between the US Navy tables and the RDP.

How many tissue compartments does the RDP use?

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

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

The RDP uses a 60-minute gas washout tissue for surface interval credit calculations. After 6 half times (6 x 60 minutes = 6 hours) a tissue is considered fully cleared of nitrogen. This means a diver who has waited 6 hours is treated as a new diver with no residual pressure group — they fall below Pressure Group A.

Why are the US Navy tables and the RDP not interchangeable?

The Navy tables and the RDP were designed using different tissue compartment models, different M-values, and different safety criteria. Pressure groups from one table cannot be used with the other. Using Navy table pressure groups with the RDP surface interval table — or vice versa — produces incorrect residual nitrogen calculations and creates a real risk of decompression sickness.

In what order should I study PADI decompression theory?

Study the topics in sequence: Half Times first, then Tissue Compartments, then M-Values, then Surface Interval Credit, then Navy Tables vs the RDP. Each topic builds on the last. The Navy vs RDP comparison will not make sense unless you understand the preceding four topics first.

PADI Decompression Theory & RDP — Complete Study Guide

Understanding decompression theory is what separates a diver who uses the RDP from one who truly understands it. This series covers everything you need to answer decompression theory questions in the PADI Divemaster and IDC exams — including the one question almost everyone gets wrong: why Navy tables and the RDP are not interchangeable.

Watch in order Each video builds on the last. Half times → Compartments → M-values → Surface Interval Credit → Navy vs RDP. The final video will not make sense without the previous four.

Part 1

Half Times

How nitrogen dissolves into tissues, the teabag analogy, and why depth determines how much nitrogen loads — not how fast.

Part 2

Compartments

Why the body is modelled as 14 theoretical tissue compartments, and how fast and slow compartments behave differently on the same dive.

Part 3

M-Values

The maximum nitrogen a compartment can safely hold on surfacing, how M-values create no-decompression limits, and what the controlling compartment is.

Part 4

Surface Interval Credit

How the RDP calculates nitrogen washout, why the WXYZ rule exists, and why the surface interval table ends at exactly 6 hours.