Decompression theory explains why divers must ascend slowly and make stops after breathing compressed gas at depth. This page covers the operational aspects: what causes decompression sickness, how dive tables encode safe ascent profiles, and how to apply the theory in practice.

Why This Exists

Every diver working with compressed gas at depth is subject to decompression obligations. Understanding the theory behind those obligations allows supervisors and divers to plan dives safely, recognize when tables apply, and respond appropriately when exposures exceed planned limits.

Who This Is For

  • Diving supervisors planning and approving dive plans
  • Commercial divers understanding their decompression obligations
  • Safety officers reviewing dive plans and incident reports
  • Medical staff treating potential decompression sickness

Inert Gas Uptake and Elimination

How Gas Enters Tissues

When breathing compressed gas at depth, inert gas (nitrogen or helium) dissolves into tissues under pressure. The amount dissolved depends on:

  • Partial pressure of the gas — Higher pressure = more gas dissolved
  • Time at depth — Tissues absorb gas at different rates (compartments)
  • Tissue type — Different tissues (blood, fat, muscle) absorb and release gas at different rates

Tissue Compartments

Decompression models use multiple theoretical tissue compartments with different half-times:

  • Fast compartments (e.g., 5-minute half-time) — Absorb and release gas quickly; limiting at shorter exposures
  • Slow compartments (e.g., 120-minute half-time) — Absorb gas slowly but retain it for longer; limiting for repetitive or long dives

Operational implication: The slowest compartment that has absorbed significant gas determines the required decompression.

Supersaturation and Bubble Formation

During ascent, pressure decreases and inert gas comes out of solution. If ascent is too fast, gas forms bubbles in tissues and blood — the cause of decompression sickness (DCS).

Tolerated supersaturation: Tissues can tolerate some level of supersaturation without forming symptomatic bubbles. Decompression tables are designed to keep supersaturation within tolerable limits.

USN Decompression Model

The U.S. Navy Diving Manual uses the Workman M-value model:

  • M-values define the maximum tolerable inert gas pressure in each tissue compartment at each ambient pressure
  • Decompression stops are required whenever inert gas pressure in any compartment approaches its M-value during ascent
  • Tables encode pre-calculated ascent profiles that keep all compartments within safe limits

Table Structure

USN dive tables are organized by:

  • Bottom depth — Maximum depth reached during the dive
  • Bottom time — Time from leaving the surface until beginning ascent
  • Repetitive group — Letter designation indicating residual nitrogen carried into the next dive

No-Decompression Limits (NDL)

The NDL is the maximum time a diver can remain at a given depth and ascend directly to the surface without required decompression stops. Exceeding the NDL creates an obligatory decompression stop requirement.

See No-Decompression Limits (Tables 9-7 & 2A-1) for specific USN NDL values.

Decompression Stop Procedures

When a dive exceeds the NDL:

  1. Begin ascent at the prescribed rate (typically 9 m/min or 30 ft/min)
  2. Stop at the first required decompression stop depth
  3. Wait the prescribed time at each stop
  4. Ascend to the next stop (or surface) after completing each stop time
  5. Do not skip or shorten stops

Stop breathing gas: For air dives, stops are performed breathing air. Oxygen-enriched gas at stops accelerates inert gas elimination.

Repetitive Diving

Residual nitrogen from a previous dive adds to inert gas loading on subsequent dives. USN tables account for this through:

  • Repetitive group letters — Track residual nitrogen after a dive
  • Surface interval credit — Nitrogen eliminated during surface intervals is credited
  • Residual nitrogen time — Added to actual bottom time for table entry on the second dive

Operational rule: Always calculate cumulative nitrogen loading for multiple-dive days.

Decompression Sickness Recognition

Type I (Musculoskeletal/Skin)

  • Joint pain (“the bends”)
  • Skin mottling (cutis marmorata)
  • Lymphatic swelling

Type II (Neurological/Cardiopulmonary)

  • Weakness, paralysis, numbness
  • Visual disturbances
  • Respiratory distress (“chokes”)
  • Loss of consciousness

Any DCS symptoms require immediate recompression treatment. Do not wait to see if symptoms resolve.