Slow down near the surface

Posted 03 July 2026 · Field Note #21

Slow down — especially near the surface

Imagine a fizzy drink. Underwater, the inert gas in your body stays dissolved because the surrounding pressure holds it in solution, just like the cap holds gas in the bottle. Ascend, and you crack the cap — pressure falls and that gas wants to come out. Do it gently and the gas leaves quietly through your lungs. Do it fast and it forms bubbles in your tissues. That, in essence, is decompression sickness.

Ascent rate is simply how fast you let that pressure fall. Most agencies settled on roughly 9–10 m/min as a sane upper limit. Climb faster and your fast tissues — the ones that loaded up most during the dive — can't off-gas in time, so the dissolved gas turns to bubbles instead of leaving through your breath.

Here's the part many divers miss: pressure changes fastest, in proportion, near the surface. Going from 10 m to the surface halves your ambient pressure — 2 bar down to 1, a full 2:1 jolt. The same 10 m climbed deep, say 30 m to 20 m, only drops pressure from 4 bar to 3 — barely a quarter. So the last few metres are where a sloppy ascent does the most damage, even though it feels like the dive is already over.

The practical takeaway: treat the shallow zone with the most respect, not the least. Watch your computer's ascent-rate warning, keep an eye on a fixed reference like a line or the bottom, and deliberately decelerate in the final 10 metres. Your safety stop isn't a box-ticking ritual — it's a brake that lets your fast tissues settle before that final, steepest pressure drop.

Guidance suggests that if anything, you slow further as you get shallower. There's no prize for surfacing first.

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