What to Do If Your Dive Computer Fails During a Dive

Your dive computer died underwater: what to do now

Stop the dive and go up with your buddy. Suunto tells divers whose computer malfunctions during a dive to follow their training agency's emergency procedures to ascend immediately and safely, and Garmin says to end the dive and return safely to the surface when a battery warning appears.

The problem is missing information. DAN's Alert Diver points out that without independent depth and pressure gauges you have no choice but to end the dive, because you cannot finish it safely without knowing your depth, time and gas. The steps below cover the ascent. Our dive computer and decompression guides explain the background.

Treat a partial failure the same way. Suunto says to abort the dive and return safely to the surface if you believe any computer function has a problem, and Mares tells Genius owners not to dive if the display looks unusual or unclear. A frozen depth reading, a flickering screen or an unexpected restart is reason enough to end the dive.

• Signal your buddy that the dive is over and point at the dead computer.
• Take depth and time from a backup: a depth gauge and timer, or your buddy's computer. DAN calls following a buddy's computer for ascent rate acceptable and safe, but not for your decompression status.
• Ascend slowly. DAN translates 9 m (30 ft) per minute as roughly 30 cm (1 ft) every two seconds; an anchor line or reference line makes it easier to hold.
• Make a safety stop if it is safe. Garmin's manual calls for 3 minutes at 3-5 m (10-16 ft); BSAC suggests at least one minute around 6 m, then at least one more minute to reach the surface from 6 m.
• If you think you may have passed the no-stop limit, treat the dive as an overshoot. Our no-decompression limits guide covers what the tables require.

What makers and agencies say about the ascent

They agree on the direction and differ on detail. Makers write for their own warning screens, so the wording ties to a specific alarm. The common thread is to end the dive, ascend under control and use a backup instrument for whatever the computer no longer shows.

How long should you stay out of the water after a computer failure?

At least 12 hours after a single no-stop dive and 18 hours after several dives, if you plan to start again with a different computer. In DAN's Alert Diver article, DAN's director of education says he would follow the flying-after-diving guidelines before treating himself as clear to start over. The reason is in DAN's guidance on computer use: you cannot move in and out of relying on a computer's decompression calculations unless it recorded all of your exposures. A spare or rented unit starts with empty tissues. Our flying after diving guide explains where the 12 and 18 hour figures come from.

Some makers set their own period. Mares says not to dive for 24 hours after a dive during or after which the Genius battery drained completely, because it loses the nitrogen loading in its tissues. Shearwater warns that tissue loading is lost if the battery dies between repetitive dives and tells divers to plan repetitive diving accordingly.

If you missed required stops or feel unwell after the dive, treat it as possible decompression sickness. DAN's advice is to call local emergency services first, then the DAN emergency line on +1-919-684-9111, staffed 24 hours a day.

Why do dive computers lock you out after a violation?

Because after a missed stop or a very fast ascent the model's numbers can no longer be trusted. Suunto calls its lock a safety feature showing that the algorithm information is no longer valid. Aqua Lung's i330R enters violation mode when a situation exceeds its capacity to predict an ascent procedure. Mares locks the Genius after an uncontrolled ascent because of the potential for harmful bubble formation.

A lockout is not a dead computer. Most still work as a depth gauge and timer, and they come back after the period in the table. Diving again during the lock restarts the clock on several models: the Suunto Zoop Novo resets to 48 hours and the i330R needs a full 24-hour surface interval again. Shearwater takes the opposite approach. Its computers have no lockout for violating decompression stops, and the manual says the policy is to warn clearly and leave decisions to your training.

Backup option 1: a second dive computer

A second computer is the only backup that keeps your full decompression history, provided you wear it on every dive. DAN's Alert Diver recommends two computers with compatible or identical algorithms, worn on every dive of a series. If one fails, you follow the other for the rest of the dive and the rest of the trip.

Makers say the same in their warnings. Scubapro tells Luna 2.0 AI owners to use a redundant dive computer and carry backup tables and depth and time instruments, and Shearwater's advice is never to risk your life on one source of information and to use a second computer or tables. A spare left in the bag does not count, because it has not recorded your dives. Garmin also warns against sharing one computer between divers, since profiles are user specific.

Backup option 2: timer, depth gauge and tables

This is the backup most manuals list. Mares tells Genius owners to use a depth gauge, an SPG, a timer or watch and dive tables alongside the computer, Garmin names a depth gauge, SPG and timer or watch, and DAN's article adds submersible tables. With a maximum-depth reading and your elapsed time, a table gives you a conservative ascent plan on the spot.

Worked example with the DECO 2000 table, the table used in CMAS training. Your computer dies 28 minutes into a dive. Your backup gauge shows a maximum depth of 22 m. The table rounds up to the next row, so you read 24 m and 31 minutes. The no-stop limit at 24 m is 23 minutes, so this row requires a 4-minute stop at 3 m and ends in group F. Ascend at the table's 10 m per minute and make that stop. Had the gauge shown 17 m, the same 28 minutes would read as 18 m and 35 minutes: no stop, group E. The maximum depth reading decides the answer, which is why a backup gauge needs a max-depth memory or needle.

The table assumes you spent all 28 minutes at 22 m. Your dead computer may have shown no stop at all on a multi-level profile, but without it you cannot know, so the table's answer is the one to follow. Our DECO 2000 planner does the same lookup, and the full DECO 2000 table is worth printing on a slate.

Battery and pre-dive checks

Check the battery before every dive, not only the first of the day. Suunto's pre-dive check is that the computer works, the display works, the battery level is fine, tank pressure is correct and the settings are right. Garmin's list is almost the same and adds a bubble check. Shearwater recommends switching the computer on by hand before each dive rather than relying on auto-on, so you can confirm it works and see the battery status.

Know what your battery warning means, because makers draw the line in different places. The Luna 2.0 AI will not start a dive at its critical level, and the Mares Genius tells you not to dive below 20%.