M-values explained

Posted 06 July 2026 · Field Note #24

How much is too much?

Every decompression model needs a line in the sand — a point past which dissolved gas in a tissue is likely to start forming bubbles. That line is the M-value: the maximum inert-gas pressure (tension) a given tissue compartment can tolerate at a given ambient pressure before the risk of decompression sickness climbs steeply.

Think of it as a pressure ceiling for each compartment. Your tissues can be supersaturated — holding more dissolved gas than the surrounding pressure would normally allow — and get away with it, up to a point. The M-value marks where "up to a point" runs out.

The key idea: that ceiling isn't fixed. It rises with ambient pressure. Go deeper and the surrounding water squeezes your tissues, so each compartment can safely hold more dissolved gas before it complains. Ascend and the ceiling drops with you. Each of Bühlmann's compartments has its own M-value line, with its own slope — fast tissues tolerate bigger gradients, slow ones less.

Decompression, stripped to its essence, is the art of ascending without letting any compartment poke through its M-value. Come up too fast and your dissolved-gas pressure outruns the falling ceiling; a compartment crosses its line, and you're gambling with a bend.

So where do gradient factors fit? They're simply how much of the gap between ambient pressure and the M-value you choose to use. A raw M-value is the theoretical edge. GF-high of 85 means you'll only let a compartment reach 85% of the way from ambient pressure to its M-value as you surface — a deliberate buffer below the line. GF-low sets the same kind of buffer at your first (deepest) stop, and the planner interpolates between the two as you ascend.

The practical takeaway: you don't dive to the M-value, you dive to a self-chosen fraction of it. Tighten that fraction (lower GFs) on cold, hard, or tired days, and understand that "conservative" really just means "leaving more daylight between you and the ceiling."

All Field Notes · daily on @DecoLogFieldNotes

⚠ Educational only — not a substitute for training or a dive computer. Verify every number independently and dive within your certification.