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07 / 07 — Life-support stack

Dive Computer

A math cop on your wrist, tracking invisible nitrogen.

Every second, your computer measures depth and time, feeds them to a decompression algorithm, and tells you the one number that matters: how long you can stay before you owe the water a debt.

Maximum ascent rate — slower is always fine

9m/min

Sampling
Every 1–5 s
Tissues tracked
Up to 16
Depth rating
100–260 m
Algorithm
Bühlmann ZHL-16C

A pressure sensor with opinions

Drag or use the arrow keys to orbit. Press plus or minus to zoom, Home to reset. Use the parts manifest to move between callouts.

Dive Computer. This 3D plate needs WebGL; the parts list below carries the same content.

Drag sideways to orbit

FIG 7.1 — Wrist, Assembled

Scale: the bar is 10 centimetres on the model.

01Pressure sensor

A tiny piezoresistive transducer converts water pressure to depth with ~10 cm resolution. Depth over time is the raw feed for every decompression calculation the computer makes.

Parts list4 parts

  1. Pressure sensor

    A tiny piezoresistive transducer converts water pressure to depth with ~10 cm resolution. Depth over time is the raw feed for every decompression calculation the computer makes.

  2. The display

    Depth, dive time, and the number divers live by — NDL, the no-decompression limit: how many minutes you can stay before you would owe mandatory stops. Around it, an ascent-rate warning (9 m/min) and the 3-minute safety-stop countdown at 5 m.

  3. Air transmitter

    An optional pod screwed into a high-pressure port that beams tank pressure to the computer wirelessly. That unlocks the magic number: gas time remaining — minutes of gas at your current breathing rate and depth.

  4. Interface

    One to four buttons, operable with numb hands and 5 mm gloves. Good dive UI is a genuine engineering discipline — everything critical must be glanceable, and alarms must be unmissable.

Modelling nitrogen you cannot feel

  1. Nitrogen is the problem

    Under pressure, the gas you breathe dissolves extra nitrogen into your blood and tissues — Henry’s law, the same physics as CO2 in a soda bottle. Ascend too fast and it comes out of solution as bubbles: decompression sickness, “the bends”.

    Henry's law

    The amount of gas that dissolves in a liquid is proportional to the partial pressure of that gas above it. At 30 m you breathe nitrogen at 3.16 bar — four times the surface — and your tissues load towards that, each at its own speed. That is the whole reason a dive has a time limit. Old tables assumed the entire dive happened at maximum depth; a computer integrates the real profile, crediting every minute spent shallow — often doubling usable bottom time on a typical reef dive.

  2. Virtual tissues

    The computer models your body as up to 16 theoretical tissue compartments, each absorbing and releasing nitrogen at its own speed — half-times from 4 minutes to over 10 hours. It tracks all of them, continuously.

    Bühlmann and the M-value

    Each compartment has a maximum tolerated nitrogen pressure — its M-value — that grows with ambient pressure: M = P_amb ÷ b + a. Ascend until a tissue exceeds it and you have a ceiling you must not cross. Gradient factors dial the model back: at GF 85 you use only 85% of the gap between ambient and the M-value. This site’s simulator runs four compartments (5, 8, 12.5 and 38.3 min) with the real ZHL-16C coefficients at GF 85 — and it cannot represent repetitive diving, because it starts every dive with clean tissues.

  3. NDL: the countdown

    The no-decompression limit is the time until your most-loaded tissue could no longer ascend straight to the surface. Go deeper and it drains fast — 18 m gives you about 56 minutes, 30 m about 20, 40 m about 9. Ten metres can more than halve your no-stop time. Multi-level diving re-credits it as you ascend.

    Algorithms disagree

    Bühlmann, RGBM, VPM — different mathematical philosophies about how bubbles form. Conservative settings pad the margins (less bottom time); liberal ones trust the model more. Same diver, same dive, different NDLs.

  4. The ascent supervisor

    On the way up it enforces a maximum ascent rate — 9 m/min on most computers; slower is always fine — counts down your 3-minute safety stop at 5 m, and afterwards tracks your surface interval and no-fly time, typically 12–18 hours.

Watch the no-stop time drain

Dive profile — one timeline, seven instruments

T+12 minAL80 · SAC 18 L/min · 5 mm suit

Cylinder

136bar

Wing

1.6L

Suit

2.5mm

Mask air

113mL

NDL left

10min

Ceiling

0m

Ascent

0.0m/min

Depth 30.0 m · ambient 4.00 bar. Drag the needle, or use the arrow keys.

Drag or use the arrow keys to orbit. Press plus or minus to zoom and Home to reset the view.

A wrist dive computer showing depth, no-stop time and gas. This view needs WebGL.

Live dive · 1 s = 30 s of dive time

Bühlmann ZH-L16C · GF 85

Dive it. Don’t bend it.

Start the dive and fly the depth slider. Four tissues load nitrogen at their own half-times; the no-stop time is how long until the fastest of them could no longer ascend straight to the surface. Go too deep for too long and it hits zero — then you owe the water a ceiling.

Challenge

Take it to 30 m, come back before the no-stop time reaches zero, and log 3 minutes at 5 m without ever owing a ceiling.

0m

No-stop time

99min

Ceiling

0.0m

Runtime

0min

Ascent rate

0m/min

Cylinder

207bar

Gas time at depth

97min

Stop 5 m / 3 min

0:00

Ascent limit

9m/min

Tissue N₂ loading — % of the surfacing M-value

  • Fast · 5 min25%
  • Mid · 8 min29%
  • Slow · 12.5 min33%
  • Slowest · 38.3 min44%

Takeaway — locked

Finish the challenge to unlock what this proves.

Four compartments (5 · 8 · 12.5 · 38.3 min) against the sixteen a real computer tracks, and every dive starts with clean tissues — this cannot model a repetitive dive. Gas: AL80, 18 L/min surface rate.

Wrist, console or workstation?

Any of the three runs the same physics; you are choosing a screen size, a wrist or a hose, and how far the menus go. A first computer should be one you will read every minute and can operate in gloves — the algorithm inside is table stakes.

Wristwatch style

What it is
A full dive computer shrunk to daily-wear size — depth, NDL, compass, sometimes air integration.
For
  • Always with you, impossible to forget
  • Doubles as a smartwatch
  • Big bright OLED on premium models
Against
  • Small screen, shallow information hierarchy
  • Premium price
  • Battery anxiety (rechargeables)
Best for
Frequent divers and travellers who want one device for everything.

Console mount

What it is
Hose-mounted with your pressure gauge — big readouts on one console at the end of the HP hose.
For
  • Large, legible display
  • No wireless pairing to fail
  • Cheaper
Against
  • Dangles and drags if not clipped off
  • Easy to leave behind on the boat
  • Harder to glance at mid-kick
Best for
New divers, rental gear, and anyone who prefers physical hose routing.

Technical computer

What it is
Multi-gas, trimix, rebreather support, user-tunable gradient factors — a decompression workstation.
For
  • Plans and executes staged deco
  • Switch gases mid-dive
  • Bulletproof build, 260 m ratings
Against
  • Four-figure prices
  • Menu complexity demands training
  • Overkill for recreation
Best for
Technical, cave and rebreather divers — and rec divers growing into it.

What fails, and what you do

What the number on your wrist is not

NDL is not a target — diving your computer to zero every dive is how you accumulate subclinical bubbling. End dives with margin.

Two computers on one dive can disagree; you dive the more conservative one. Never average them.

Air integration changes behaviour: watching “gas time remaining” instead of bar makes gas planning intuitive and catches high-consumption spikes early.

Your computer does not know you: dehydration, fatigue, cold and age all raise decompression-sickness risk, and no algorithm models them fully.

No-fly time is real: cabin altitude after a day of diving is the last pressure drop your tissues did not plan for. Wait the 12–18 hours.

Where this chapter connects