M-Values, Half-Times, and Tissue Compartments

How Your Body Manages Nitrogen During a Dive

When you dive, nitrogen dissolves into different parts of your body at different speeds. Dive computers use tissue compartments to model this. Fast tissues (like blood) absorb gas quickly. Slow tissues (like fat) take longer. Each tissue has a safe limit — the M-value. Exceeding this limit during ascent increases the risk of decompression sickness. Dive computers track all 16 compartments continuously to keep you within safe limits.

Introduction: Your Body Isn't One Thing

At depth, your body doesn't absorb nitrogen uniformly. Different tissues absorb and release nitrogen at varying rates. To model this, scientists introduced tissue compartments — a concept first formalised by J.S. Haldane in 1908.

How the Science Developed

The decompression algorithms inside modern dive computers are the product of more than a century of refinement.

Tissue Compartments: A Model, Not a Map

Tissue compartments are mathematical models — they represent rate constants for gas exchange, not anatomical structures. A single compartment does not correspond to a specific organ. What it represents is a group of tissues with similar perfusion rates and similar gas exchange behaviour.

Most dive computers use 16 compartments with different half-times — the time it takes each tissue to absorb or release half of its gas load.

Half-Times: How Fast Tissues Load and Unload Gas

A half-time is how long it takes a tissue compartment to reach 50% of the difference between its current gas loading and the new equilibrium pressure. For example, a 10-minute half-time means 50% saturation after 10 min, 75% after 20 min, and 87.5% after 30 min.

After 6 half-times, a compartment is more than 98% equilibrated — effectively fully saturated or fully cleared. This is the "6 half-time rule."

The ZHL-16C Compartments

The Bühlmann ZHL-16C algorithm uses 16 nitrogen compartments spanning a wide range of half-times:

Helium half-times are approximately 1/2.65 of the corresponding nitrogen value — helium diffuses faster and both loads and unloads more rapidly.

M-Values: Your Tissue's Maximum Safe Pressure

Each compartment has an upper gas pressure limit — called an M-value. Robert Workman introduced M-values in 1965 as compartment-specific, depth-dependent limits on tolerated supersaturation.

The M-value formula (Bühlmann):

M = M₀ + ΔM × d

M₀ is the maximum tolerable inert gas tension at the surface (1 bar). ΔM is the rate at which the M-value increases with depth (the depth gradient coefficient). d is depth in bar (ambient pressure).

M-values increase with depth — at greater ambient pressure, tissues can tolerate more absolute supersaturation before bubble formation becomes likely. Fast compartments tolerate higher supersaturation ratios than slow ones. Workman's data for the six US Navy compartments showed M-values of approximately 104, 88, 72, 58, 52, and 51 fsw respectively — decreasing allowable supersaturation with increasing half-time.

If a tissue exceeds its M-value, gas may come out of solution as bubbles. Your computer constantly monitors tissue pressures and their M-values. If any compartment approaches its limit, the computer may extend your no-deco time, trigger a required decompression stop, or delay surface clearance or flying fitness.

Gradient Factors: Adjusting the M-Value Working Limit

Raw ZHL-16C M-values represent the theoretical maximum before DCS risk becomes likely. In practice, technical divers apply gradient factors (GF) — user-settable percentages that scale down the effective working M-value.

GF 85/85 means the computer will only allow tissue loading up to 85% of the raw M-value. GF 50/85 sets a conservative first stop (50% of M-value for the GF-Lo) and a slightly relaxed surface limit (85% of M-value for GF-Hi).

Gradient factors allow the diver to dial in the balance between efficiency and conservatism, without changing the underlying ZHL-16C algorithm.

How Your Dive Computer Thinks

Your dive computer runs an algorithm — typically the Bühlmann ZH-L16C — that updates every 2 to 4 seconds across all 16 compartments. It monitors depth and time, simulates nitrogen loading in each tissue compartment, checks compartment pressures against M-values (or GF-adjusted working limits), and determines your safe ascent ceiling — the shallowest depth at which the most-loaded compartment remains below its limit.

The controlling tissue (or leading compartment) is the one with the highest ratio of inert gas tension to its M-value at any given moment. It defines the decompression ceiling and the next possible ascent step.

Real-World Example

Dive: 30m for 18 minutes. Fast tissues are near saturation. Medium tissues are midway saturated. Slow tissues are still absorbing gas.

As you ascend: fast tissues dump gas quickly — risk of bubbling if the ascent is too fast. Slow tissues continue absorbing gas during the ascent — important for planning repetitive dives.

Ascent rate and safety stops are essential for managing these differences. More information in the NOAA Diving Manual.

How Do Bubble Models Compare?

ZHL-16C tracks dissolved gas only. It infers bubble risk through M-values but does not model bubble nuclei or growth physics.

RGBM (Bruce Wienke, 1995) and VPM (David Yount, 1986) explicitly model gas nuclei populations and bubble growth dynamics. Despite these theoretical differences, outcome data from a LANL comparison across 11,738 identical profiles showed ZHL-16C at 0.0135 DCS events per dive vs RGBM at 0.0175 DCS events per dive. The difference is not statistically significant. Both algorithms produce acceptable outcomes in practice.

Why This Matters for All Divers

Even recreational no-stop dives load nitrogen into different tissue compartments. Understanding tissue behaviour helps you respect no-deco limits and understand why they shrink as you stay deeper, make better ascent choices — the final 6 m are the highest-risk phase — and plan safe repetitive dives, where slow compartments carry residual loading from previous dives.

Dive computers assume ideal conditions — hydration, normal temperature, moderate exertion. Real-world factors (dehydration, cold, strenuous activity, PFO) all increase DCS risk beyond what the algorithm can see. A working knowledge of the model goes a long way toward using the computer intelligently rather than just following its numbers.

Key Findings

• Fast compartments tolerate higher supersaturation ratios than slow compartments (Workman 1965; Bühlmann 1992)
• M-values increase linearly with depth — greater absolute supersaturation is tolerable at depth, but the ratio of tissue tension to ambient pressure decreases (Workman 1965)
• The controlling tissue defines the decompression ceiling at every moment (Angelini et al. 2022)
• Half-times are governed mainly by blood flow rates, with diffusion contributing in poorly-perfused regions such as bone or spinal cord (Wienke 1994)
• After 6 half-times, a compartment is >98% equilibrated — effectively fully saturated or fully cleared
• ZHL-16C and RGBM produce equivalent DCS outcomes in controlled database comparison (LANL, 11,738 profiles)

References

Haldane JS, Boycott AE, Damant GCC (1908) — The prevention of compressed air illness — Journal of Hygiene

Workman RD (1965) — Calculation of decompression schedules for nitrogen-oxygen and helium-oxygen dives — US Navy Experimental Diving Unit Report NEDU-RR-11-65

Bühlmann AA (1983) — Decompression — Decompression Sickness — Springer-Verlag

Bühlmann AA (1992) — Tauchmedizin — Springer-Verlag

Wienke BR (1994) — Basic Diving Physics and Application — Best Publishing

Angelini SA, Tonetto L, Lang MA (2022) — Ceiling-controlled versus staged decompression: comparison between decompression duration and tissue tensions — Diving and Hyperbaric Medicine 52(1):7–15

Doolette DJ, Mitchell SJ (2013) — Recreational technical diving part 2: decompression from deep dives — Diving and Hyperbaric Medicine 43(2):96–104

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