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Tissue saturation & GF99

This page explains how the Bühlmann ZHL-16C model tracks dissolved inert gas in your body, what "M-value" means, and how to read the GF99 chart in the app.

The 16-compartment model

Bühlmann's model imagines your body as a set of hypothetical tissue compartments, each with a different speed of absorbing and releasing inert gas (nitrogen and helium):

  • Fast compartments (~2–10 minute half-times) — blood, brain, kidneys. They saturate quickly during the dive and release gas quickly on ascent.
  • Medium compartments (~20–60 min) — muscles, organs.
  • Slow compartments (~80–635 min) — fat, ligaments, joints, bone. They saturate slowly on a long dive but also off-gas slowly afterward.

ZHL-16C uses 16 compartments, each with separate nitrogen and helium half-times. At every moment, the model computes how much dissolved inert gas each compartment holds, based on the gas you're breathing and the depth pressure.

M-values

For each compartment at each depth, there's an M-value — the maximum tissue tension that compartment can hold without forming bubbles on direct ascent to the next stop. The M-value is depth-dependent (higher allowed at depth, lower near the surface).

Diving "right at the M-value" means using the entire available margin. Most divers want to stay well under it — hence gradient factors.

GF99 — the dashboard number

GF99 is the live percentage of M-value currently being used by the leading (most-saturated) compartment.

  • GF99 = 0 % — your tissues match ambient pressure; you're at equilibrium.
  • GF99 = negative — you're on-gassing; pressure is higher than tissue tension, so gas is flowing in.
  • GF99 = positive — you're off-gassing; tissue tension exceeds ambient pressure, so gas is flowing out.
  • GF99 > GF Hi — you've exceeded your chosen ceiling; mandatory stop.
  • GF99 = 100 % — raw Bühlmann limit; bubbling likely.

A useful target during deco stops is GF99 ≈ GF Hi / 2 (e.g. 42 % when running GF Hi = 85). That's roughly the "half-way decompressing" point — efficient but not panicky.

The GF99 chart

In the Deco plan card, below the depth profile, the GF99 chart shows tissue saturation across the dive:

  • Green areas — off-gassing (positive GF99); you're decompressing
  • Red areas — on-gassing (negative GF99); gas flowing into tissues
  • Solid green line — GF99 over time
  • Blue dashed line — target GF99 (= GF Hi / 2)
  • Red dashed line — GF Hi limit (the surface ceiling)

A well-shaped ascent has GF99 rising into the green zone shortly after leaving the bottom, hovering near the target during stops, and approaching GF Hi just before surface.

Deepest ceiling

The deepest ceiling value at the top of the Deco plan card is the deepest depth your tissue compartment ceilings allow. It's the depth where GF Lo is reached on the leading compartment — the first decompression stop in raw model terms. Stop intervals (e.g. 3 m) snap it to a valid stop depth.

Practical consequences

  • Faster ascent rates raise GF99 sharply during the ascent itself. AeroPlus Deco models ascent at 10 m/min by default (configurable in Settings → Ascent speed) and 3 m/min between deco stops (Settings → Deco ascent speed).
  • Slower ascent keeps the GF99 curve smoother but adds runtime.
  • Skipping a stop spikes GF99 above GF Hi — this is what dive computers warn about as "ceiling violation."

Why the model can be wrong

Bühlmann is dissolved-gas only — it assumes inert gas stays dissolved unless it bubbles outright. Reality includes silent micro-bubbles, venous gas emboli (VGE), and tissue-specific factors no model captures. The GF system is one way to add a margin; another is post-dive Doppler monitoring (which most divers never do). Always treat the planner's output as a plan, not a guarantee.