{ FIELD NOTES }

Field Notes — dive theory, one idea a day

Short, first-principles explainers on decompression theory, gas planning and rebreather practice — fact-checked, sourced, and each backed by a free tool. A new note posts most days.

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  1. 15 Jul 2026 At altitude, the surface itself is shallower pressure Start with the thing nobody says out loud: the danger in decompression isn't the depth you reach, it's the pressure drop you survive on the way out. Your…
  2. 12 Jul 2026 The surface interval is off-gassing time, not just lunch A surface interval is simply the time you spend out of the water between two dives — and it does real physiological work, even while you're eating a sandwich.…
  3. 11 Jul 2026 Your second dive starts with a head start — of nitrogen Every dive loads your tissues with inert gas, mostly nitrogen, and surfacing doesn't instantly clear it. Your faster tissues unload within an hour or so, but…
  4. 10 Jul 2026 Deco gas switch depths: EAN50 at ~21 m, O₂ at ~6 m A decompression gas is a separate cylinder of oxygen-rich mix you switch to during your ascent to off-gas faster. Each one can only be breathed down to a…
  5. 09 Jul 2026 Equivalent air depth, plainly Air is about 79% nitrogen. Nitrox (EAN) deliberately swaps some of that nitrogen for extra oxygen. Since it's the nitrogen that loads your tissues and drives…
  6. 08 Jul 2026 What TTS is actually telling you TTS stands for time to surface: your computer's running estimate of how long it would take to get you to the surface right now, doing everything correctly —…
  7. 07 Jul 2026 Why pure O₂ speeds up your deco Off-gassing is driven by a pressure difference. The inert gas dissolved in your tissues wants to move toward wherever its partial pressure is lowest — and the…
  8. 06 Jul 2026 M-values explained 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:…
  9. 05 Jul 2026 Your body isn't one sponge When you breathe gas under pressure, it dissolves into your tissues — but not evenly, and not all at the same speed. Blood-rich tissue like your brain loads…
  10. 04 Jul 2026 The deep-stops reckoning A decompression stop is a deliberate pause on the way up that gives your tissues time to release inert gas before the next drop in pressure. The long-running…
  11. 03 Jul 2026 Slow down 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…
  12. 29 Jun 2026 CCR bailout planning Bailout planning answers one blunt question, so let's define it plainly. On a rebreather, "bailout" is the open-circuit gas you carry to breathe your way to…
  13. 28 Jun 2026 The CCR setpoint To understand the setpoint, first understand what a closed-circuit rebreather (CCR) does. Instead of dumping every exhaled breath into the water like…
  14. 27 Jun 2026 Why log every dive (rec on-ramp) Most new divers think of a logbook as a scrapbook — a place to remember the turtle. It is far more useful than that. A dive log is a personal dataset, and…
  15. 25 Jun 2026 Fly after diving Here is the counterintuitive part first: you can finish a dive feeling perfectly fine and still get decompression sickness hours later, sitting in an airline…
  16. 24 Jun 2026 CNS% and OTU Oxygen is the one gas that helps you both stay down comfortably and decompress faster — richer oxygen mixes speed off-gassing because they leave less room for…
  17. 23 Jun 2026 Isobaric counterdiffusion First the name in plain words. Isobaric counterdiffusion (ICD) means two gases moving in opposite directions through your tissues at the same time, while the…
  18. 22 Jun 2026 Why we add helium Start with the problem, not the gas. As you go deeper, two things quietly get worse, and both come from breathing gas under pressure. The first is narcosis — a…
  19. 21 Jun 2026 What your computer means by NDL (rec on-ramp) Let's define the term before the acronym. Your no-decompression limit (NDL) is simply the longest you can stay at a given depth and still ascend slowly to the…
  20. 19 Jun 2026 Why 5 m for 3 minutes (rec on-ramp) First, the plain version. A safety stop is a short, deliberate pause near the end of a dive — most agencies teach roughly three minutes at about 5 m — before…
  21. 18 Jun 2026 Rock bottom gas Start with the plain idea. "Rock bottom" (also called minimum gas) is the amount of breathing gas you set aside so that, if everything goes wrong at the worst…
  22. 17 Jun 2026 SAC vs RMV Two divers describe their air consumption, one in bar per minute, one in litres per minute, and they're measuring genuinely different things. Confusing them…
  23. 16 Jun 2026 Why your fills come up short Ever topped a cylinder by the book and found the finished mix landed a point or two off? You probably didn't make a mistake. The simple formula made an…
  24. 15 Jun 2026 Partial-pressure blending basics Making nitrox or trimix isn't magic, it's stacking pressures. The key insight is that in a mixture of gases, each gas contributes its own share of the total…
  25. 14 Jun 2026 Best mix for the depth Most divers pick a gas out of habit or whatever the shop filled, then check whether it's safe. "Best mix" thinking flips that around: you start from the dive…
  26. 13 Jun 2026 The 5.2 g/L wall Here's a limit that kills divers who never knew it existed, and it has nothing to do with oxygen or nitrogen narcosis. It's about how heavy your breathing gas…
  27. 12 Jun 2026 Narcosis and END Go deep on air and your thinking gets fuzzy. That's nitrogen narcosis — a reversible impairment caused by inert gas dissolving into nerve tissue under…
  28. 11 Jun 2026 What nitrox really does (rec on-ramp) Nitrox, often labelled EAN for enriched air nitrox, is simply air with the recipe adjusted: more oxygen, less nitrogen. Ordinary air is about 21% oxygen and…
  29. 10 Jun 2026 Why 1.4 and 1.6 Oxygen keeps you alive, but at pressure it behaves like a drug, and the dose isn't the percentage in your tank, it's the partial pressure you're actually…
  30. 09 Jun 2026 Bubbles vs dissolved gas Every decompression algorithm is trying to answer the same question: how do I ascend without forming dangerous bubbles? They just start from different…
  31. 08 Jun 2026 What gradient factors actually do Think of decompression as climbing out of a pressurised world. While you're deep, your body quietly soaks up inert gas, mostly nitrogen, into its tissues.…

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