What a dive computer does and what it cannot do

A dive computer is a wrist or console instrument that measures depth and elapsed time during a dive. It uses a decompression model to estimate how much more time you can spend at the current depth before a decompression stop is required. It turns readings from a depth sensor and a clock into guidance a diver can read without a separate timer and depth gauge. For recreational divers it is the instrument most people rely on to manage the dive profile, but it is an aid, not a replacement for training.

Knowing what the computer does well makes it easier to see where it stops. Its depth and time readings are direct measurements, and it records the dive for later review. Its estimates of no-decompression time and required stops come from a model, so they depend on the algorithm, the settings and the way the dive was actually conducted.

What the computer records

Most dive computers show current depth, maximum depth, elapsed dive time, water temperature and an ascent-rate indicator. Many also display a no-decompression time, which is the time left at the current depth before a stop is needed. When the computer detects an ascent faster than its threshold, it alerts the diver, and a safety stop prompt often appears near the end of the dive. The safety stop is commonly taught as a few minutes at about 5 meters, or 15 feet, and many computers show a countdown for it. Follow the prompt, but keep the rest of the ascent slow and controlled, because the display cannot judge how your buoyancy and ascent are actually going.

After the dive, the logged data can be reviewed on the display or downloaded. A stored log shows how depth and time changed across the dive, which is useful for reflecting on the profile and planning the next one. The record does not show everything, however. It does not capture how you felt or whether your buddy was keeping pace, and it does not capture how much air you used unless the computer is air-integrated.

How decompression models differ

Manufacturers use different decompression algorithms, and the results can differ between brands even for the same dive. A computer from one brand may show more no-decompression time than another for an identical profile. Do not compare readings across computers as if they were the same measurement.

The estimates also depend on the settings you choose. Conservative modes, where the model offers them, widen the margins the computer uses. Gas mix settings must match the air or enriched air you actually breathe. The computer can only calculate from what it has been told, so a wrong setting produces a wrong result, even though the display looks precise.

What the computer cannot tell you

A dive computer cannot detect your health, fatigue, dehydration, cold or the effects of recent activity, all of which can change how your body handles a dive. It cannot tell you whether a current is building or whether a site suits your experience. It also cannot see your buddy, so it cannot confirm that a partner is all right. A buddy system and a shared, planned dive profile still matter, because the instrument only reports on the diver wearing it.

Staying within a computer's no-decompression limits does not guarantee that you will avoid decompression sickness. The limits come from mathematical models of inert gas in the body, and those models cannot account for every person's physiology on every day, so a dive that stays inside the display's limits can still cause symptoms. Joint pain, a skin rash or itching, unusual fatigue, numbness, tingling, dizziness, weakness or changes in vision or balance after a dive call for prompt medical attention. Do not wait to see whether the symptoms pass, and tell the responders that you have been diving.

A computer also knows only about the dives it recorded. If you switch computers between dives on the same day, the second instrument has no record of the residual nitrogen from the earlier dive, so its no-decompression estimates can be too optimistic. Keep using the same computer for a series of dives, and follow the manufacturer's guidance before changing to a different one.

Air supply is another limit. Many computers show tank pressure only when they are paired with a wireless transmitter in the tank valve, and without one they give no information about how much gas remains. Even with a transmitter, you still need to monitor your pressure yourself and plan your turn-around pressure. The computer is one source of information among several, and training teaches you how to use all of them together.

Checking the computer before each dive

Before each dive, check the battery level and confirm the date and time. Make sure the mode is set for the dive you plan, and that the gas mix setting matches the gas in your tank if you dive enriched air. If the computer has an altitude setting, confirm it is correct for the site, since diving at altitude requires changed calculations. Do not rely on a computer you have not learned to read, and do not share one between divers, because each diver's profile needs to be tracked on that diver's own instrument.

If the computer shows a lockout or a decompression obligation, do not dive until the display clears or the required wait has passed. Treat its warnings seriously and follow the surface interval it recommends. For questions about how the computer works, ask a certified instructor or the manufacturer. For medical questions, such as whether a condition affects diving, consult a physician who understands diving medicine.

A dive computer makes many decisions easier, but its readings only help when the diver understands the limits behind them. Learn the settings, check them before every dive, and keep training and buddy practice at the center of your planning.