Dive Computers: How They Actually Work

Part 1 of 3 · Part 2: Living With Your Computer's Algorithm · Part 3: Reading Your Computer in Real Time

What it's doing every few seconds

Your dive computer is not a depth gauge with a timer bolted on. Every two to four seconds, start to finish, it runs a full decompression model. Usually that is Bühlmann ZH-L16C, spread across 16 imaginary tissue compartments that each soak up and release nitrogen at a different speed. It works out how much nitrogen is dissolved in each one right now, checks those against the most each can safely hold at your current depth, and updates your no-stop time or your ceiling on the spot.

That is why the number on your wrist is not a table value frozen at the start of the dive. It moves with every metre, every minute, and every change in how fast you come up. The computer is running a live model of the nitrogen in your body, or rather a careful mathematical stand-in for it. If you want the theory under that model, it is in how decompression models work.

From tables to computers

Before computers, divers ran printed tables: the US Navy tables, PADI's Recreational Dive Planner, and various national versions. For a given depth and time, the table told you whether you owed a stop and how long.

Tables worked, but they were blunt. They treated the whole dive as if you sat at maximum depth the entire time, so a multi-level dive wasted most of its credit. Repetitive planning meant juggling slates, and any deviation from the plan left you guessing.

The first commercial computer, the Orca Edge, arrived in 1983 running a modified Haldane model and showing a single number: no-stop time remaining. Early units were conservative and they worked. Lippmann's review of 44,277 dives on Dacor Microbrain computers turned up just one case of decompression sickness. The real turning point came in 1992, when Bühlmann published ZH-L16 with its full coefficient tables, giving manufacturers a documented, peer-vetted algorithm they could all implement the same way. In 2007 Shearwater's Predator went a step further and handed the conservatism dial to the diver, exposing gradient factors on top of ZHL-16C.

What the display is telling you

Every computer lays it out differently, but the core readings are the same, and they are all the same model wearing different clothes.

No-stop limit (NDL) is the time you have left at this depth before the computer starts demanding a stop. It shrinks as you go deeper or stay longer, and it already accounts for leftover nitrogen from earlier dives. Zero means you are at the edge: any more bottom time and you owe decompression.

Ceiling appears once you are in deco. It is the shallowest depth you are allowed to be, set by whichever tissue is currently closest to its limit. You cannot go above it until that tissue has offgassed enough, and it keeps moving as you hang there.

CNS% is your running tally of oxygen exposure. It climbs faster the higher your oxygen pressure, so the computer has to know your gas to get it right. The technical working limit is 80% in a single dive, not the table's 100%.

Tissue bars, where shown, picture the nitrogen load across the 16 compartments. The fast ones on the left fill and empty quickly; the slow ones on the right load gently and stay loaded long after you surface.

Ascent-rate indicator nags you when you come up too fast. Most alarm above 9 to 10 m/min, and the part that matters most is the last few metres, where each metre is a bigger slice of the pressure you are leaving behind. That zone is its own article: safe ascents.

An NDL dive vs a decompression dive

The biggest thing the screen tells you is which kind of dive you are on, and the two look quite different. Here is the same computer, a Shearwater Peregrine, on each:

On a no-stop dive, the headline number is your NDL: the minutes of bottom time left before you would owe a stop. It ticks down while you stay, and as long as it is above zero you can head up (at a sensible rate) whenever you like. Nothing is hanging over you.

On a decompression dive, that corner changes job entirely. Instead of an NDL it now shows a deco stop: the depth to hold at (here 6 m) and how long to stay there. You have lost the freedom to just leave. Go straight to the surface and you skip decompression you genuinely owe. The task switches from watching a clock run down to climbing a ladder of stops back up, which is exactly what safe ascents covers.

Set it to the gas you're breathing

The computer models your decompression from the gas you tell it you are on. On air it assumes 79% nitrogen. On EAN32 it should be working from 32% oxygen, and therefore a higher oxygen pressure at every depth.

The most common computer mistake in the incident records is running the air setting while actually breathing nitrox. The computer then undercounts your oxygen dose, because it is using air's oxygen pressure, not the real, higher one. On EAN32 at 30 m the true oxygen pressure is 0.32 × 4 = 1.28 bar; on air at 30 m it is only 0.84 bar, and those two sit in completely different exposure bands. So enter your gas before every dive, and if your computer handles gas switches, program your deco gases in so it knows when to change its sums. There is more on the gas itself in the benefits of nitrox.

Don't share a computer, don't swap mid-trip

Two rules get broken all the time.

Don't dive someone else's computer. It has been tracking your tissue loading across the whole series. Hand it to a buddy and the algorithm now models their dive on top of your residual nitrogen. They start the dive already in a deco state that isn't theirs, and the numbers no longer describe anyone real.

Don't switch computers partway through a multi-day series. A fresh computer starts from zero. It has no memory of yesterday's dives, so the nitrogen still sitting in your slow tissues is invisible to it, and your first dive on the new unit is under-protected.

Flying after diving

Current DAN guidance: wait at least 12 hours after a single no-stop dive, at least 18 hours after repetitive or decompression dives, and 24 to 48 hours after a deep multi-day trimix series.

A cabin is pressurised to roughly 1,500 to 2,400 m of altitude, not sea level. Get on a plane before your slow tissues have cleared and you create the same supersaturation that causes the bends on a too-fast ascent, just gentler and slower. It is real, if subtle, and it is behind cases of divers developing DCS in the air after feeling fine at the gate.

What this part doesn't cover

How the algorithms actually differ, and the theory underneath them, lives in the decompression series: how decompression models work for compartments and M-values, and bubble trouble for the bubble models. The practical side, conservatism settings, living with a bubble-model computer, and mixed-computer teams, is Part 2: Living With Your Computer's Algorithm.

References

• Bühlmann AA. Decompression: Decompression Sickness. Springer-Verlag, 1984 (revised 1992).
• Lippmann J, Bugg S. DAN Asia-Pacific dive safety data. 1992.
• Sayer MDJ, et al. A comparison of nitrox dive computer output in real dive conditions. SPUMS Journal. 2003;33(1):12–17.
• Wienke BR, O'Leary TR. Understanding Modern Dive Computers and Operation. Springer, 2018.

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