Dive Computer

What a dive computer does

A dive computer measures depth with a pressure sensor and keeps time as you dive. From those two readings it works out how much nitrogen your body has probably absorbed, using a mathematical model of how gas enters and leaves different tissues. It then shows how long you can stay at your current depth before you would need decompression stops: your no-decompression limit.

Unlike printed dive tables, which assume you spend the whole dive at the deepest depth, a computer follows your real profile. A multilevel dive, deep at first and shallower later, earns credit for the time spent shallower. This usually gives longer dives than tables allow.

What it shows

• Current depth and maximum depth
• Dive time
• No-decompression time remaining, or decompression stops required
• Ascent rate, with an alarm if you rise too fast
• A safety stop countdown at the end of the dive
• Water temperature
• Surface interval and time to fly after the dive
• For nitrox, the gas mix, oxygen partial pressure and oxygen exposure

Air-integrated computers also show tank pressure, through a hose or a wireless transmitter on the regulator first stage, and can estimate how much gas time you have left.

Algorithms and conservatism

Every computer runs a decompression algorithm. Common ones include versions of the Bühlmann ZHL-16 model and various bubble models such as RGBM. Different models can give different limits for the same dive. Most computers also have conservatism settings that make the limits shorter. Many divers choose a more conservative setting when they are older, less fit, cold, tired or diving many days in a row.

A computer only models your body; it can't measure it. Following your computer doesn't guarantee you won't get decompression sickness, so stay well inside its limits rather than diving right up to them.

Using a computer well

1. Read the manual and learn the display before your first dive with it.
2. Set it to the gas you are breathing, such as air or nitrox.
3. Wear your own computer on every dive of a trip. Its calculations depend on your earlier dives.
4. Never share a computer with a buddy, even for one dive, because it can't tell two divers apart.
5. Don't follow your buddy's computer blindly; dive to the more conservative of the two.
6. Check the battery before each trip.
7. Know what to do if it fails: ascend slowly, make a safety stop and stay out of the water long enough for your body to clear, often 24 hours, unless you have a backup computer that has tracked the same dives.

Wrist, console and others

Most recreational computers are worn on the wrist, either as a watch-sized unit or a larger display. Some are built into a console with the pressure gauge. Technical divers often carry two computers for redundancy and use units that can switch between several gases.

Frequently asked questions