Dive computers

In short

A dive computer continuously measures depth and time and, using a decompression model, calculates how long you can stay without mandatory stops, which stops to make and how fast to ascend. It replaces tables for most recreational divers, provided you know how to read it, set it up correctly and follow its indications. Each diver has their own.

Why it matters

A computer follows your actual profile, metre by metre, whereas a table works from the maximum depth and the total time. It often gives you more time without stops on a multilevel dive, but it only protects you if you understand its information and alarms. A computer that is wrongly set, lent or ignored can give a false sense of safety.

What a computer calculates

• Current and maximum depth, dive time.
• No-stop time: the time left at the current depth before stops become mandatory.
• Stops: depth and duration of mandatory stops, and often a recommended safety stop of a few minutes at around 3 to 5 m.
• Total ascent time when stops are needed.
• Ascent rate, with an alarm if it is too fast.
• After the dive: desaturation time, surface interval and no-fly time (recommended delay before flying).
• With Nitrox: PPO2, maximum operating depth (MOD) and oxygen exposure (CNS).

Reading the screen and understanding alarms

Before diving with a new computer, read its manual and find where each piece of information is displayed. The most common alarms:

• Ascent rate: slow down immediately. A slow ascent is the first way to prevent decompression sickness.
• Missed stop or ceiling violation: go back down to the stop depth shown and follow the computer's instructions; tell your team and the dive leader after the dive.
• Maximum depth: you have gone deeper than the set depth; ascend a few metres.
• PPO2 or MOD (Nitrox): ascend above the MOD at once. The usual PPO2 limit is 1.4 bar and 1.6 bar is an absolute maximum.

After a dive with a missed stop, many computers restrict or lock the next dive for several hours: respect this.

Settings

• Gas: air, or Nitrox with its oxygen percentage, analysed before every dive (see the Nitrox guide). A wrong setting distorts every calculation.
• Conservatism: most computers offer several levels, or gradient factors, to make the calculations more cautious. Choose a more conservative setting when you are tired or cold, after exertion or on repetitive dives.
• Salt or fresh water: the conversion between pressure and depth depends on it.
• Units: metres and bar, or feet and psi; check them once and for all.
• Altitude: some computers adapt automatically, others need a setting (see the guide on specialty dives).

Good practice

• One computer per diver. The calculations depend on your profile and your previous dives; your buddy's profile is never quite yours.
• Do not lend it between dives and do not switch computers during a day of diving: the new computer would not know the nitrogen already absorbed.
• Follow your own computer, not your neighbour's or another diver's table. Within a team, follow the most restrictive computer.
• Do not dive to the limit of the no-stop time: keep a margin.
• Plan for a failure: know the procedure agreed with your club (slow ascent, extended safety stop, use of a second instrument or a table) and do not dive again with a computer that has lost its data.
• Care: rinse it and keep an eye on the battery (see the equipment care guide).

The app's watch

The app records your dives with an Apple Watch. During a scuba dive, the watch shows depth, time, water temperature, no-stop time and stops according to the chosen model: MN90, the default, or Bühlmann ZHL-16C with gradient factors (30/70 by default) with Dive Plus. It also shows the ascent rate, a compass and, with Nitrox, FO2, MOD, PPO2 and CNS.

It raises full-screen alerts: fast ascent, decompression or safety stop not respected, leaving the stop zone, oxygen alert, profile outside the MN90 table and sensor depth limit. After the dive, widgets show residual nitrogen and the recommended delay before flying. The session then syncs with the iPhone and the website, where you can review the profile and the events.

The watch has its limits: its sensor has a maximum depth, the app's Bühlmann model handles neither altitude, helium nor gas switches, and these calculations do not replace your training, a dedicated dive computer or your dive leader's instructions.

Key takeaways

• One computer per diver, set to your gas, never lent between dives.
• Ascent rate alarm: slow down; missed stop: follow the computer's instructions.
• Choose a more conservative setting when cold, tired or after exertion.
• Keep a margin from the no-stop time.
• Respect the no-fly time shown after the dive.

Further reading

The decompression guide explains what the computer calculates and why the ascent rate matters; the Nitrox guide details PPO2 and MOD; the deep diving guide covers the margins to plan beyond 30 m.