When making computer-assisted dives, your computer tracks depth and time in real time, calculating how much dissolved gas your body has absorbed and how long you can safely stay at depth. This replaces the old method of planning square profiles from printed tables, giving you a personalized decompression schedule that updates with every change in depth. But the accuracy of that schedule depends entirely on how you set up, follow, and respond to your computer before, during, and after the dive.
How Dive Computers Calculate Your Limits
Every dive computer runs a decompression algorithm, a mathematical model that estimates how nitrogen (or other inert gases) loads into and leaves your body’s tissues. These tissues absorb gas at different rates: some saturate quickly, others take hours. The computer tracks all of them simultaneously, using your real-time depth to update its predictions every few seconds.
The two main families of algorithms work differently. Dissolved-gas models, based on the work of Swiss physician Albert Bühlmann, track how much nitrogen dissolves into tissues and set limits on how much supersaturation each tissue can tolerate before bubbles are likely to form. The widely used ZHL-16C version divides the body into 16 theoretical tissue compartments, each with its own absorption rate. The other family, dual-phase or bubble models like VPM-B and Suunto’s RGBM, go a step further by also modeling the formation and growth of tiny gas bubbles that dissolved-gas models ignore. Suunto’s RGBM uses 9 compartments, while VPM-B uses 16.
These aren’t just cosmetic differences. The algorithms differ in how they model gas leaving your tissues during ascent, which directly affects stop depths and total decompression time. Dissolved-gas models generally assume gas leaves tissues in a simple mirror of how it entered. Some dual-phase models add delays to that off-gassing process, producing more conservative schedules. Neither type has been proven definitively safer than the other, but the practical result is that two computers on the same dive can display different no-decompression limits depending on which algorithm they run.
Setting Up Your Gas Mix Correctly
If you’re diving on regular air, your computer’s default settings handle the gas math automatically. But when diving nitrox (enriched air with more oxygen and less nitrogen), you need to manually input the oxygen percentage of your mix before every dive. This single setting changes two critical calculations.
First, it determines your maximum operating depth. Oxygen becomes toxic at high partial pressures, and the recreational diving community sets the limit at 1.4 atmospheres absolute for the working portion of a dive. On a 32% nitrox mix, that caps your depth at 112 feet. On 36% nitrox, it drops to 95 feet. Your computer will alarm if you exceed these depths, but only if you’ve entered the correct mix.
Second, the oxygen percentage tells your computer the nitrogen fraction, which is what actually drives your no-decompression limits. Less nitrogen means slower tissue loading, which means more bottom time at a given depth. If you forget to switch your computer from air to your actual nitrox mix, you’ll still get a safe nitrogen calculation (since the computer assumes more nitrogen than you’re actually breathing), but you’ll lose the extra bottom time you paid for. The reverse mistake, leaving a nitrox setting active when you’ve switched to air, is genuinely dangerous because the computer would underestimate your nitrogen exposure.
Understanding No-Decompression Limits
The number counting down on your computer’s display is your no-decompression limit (NDL): the minutes you have left at your current depth before a direct ascent to the surface is no longer considered safe. If you exceed it, you’ll owe mandatory decompression stops, turning a recreational dive into something more complex and risky.
Your NDL isn’t fixed. It recalculates constantly based on your actual depth profile, residual nitrogen from previous dives, and the conservatism settings you’ve chosen. A dive computer tracks your real path through the water, so a gradual descent to 80 feet gives you a different NDL than dropping straight to 80 feet and staying there.
Several personal factors can make your NDL more generous than your body can safely handle. Age, dehydration, fatigue, excess body fat, and physical exertion all increase susceptibility to decompression sickness without changing what the computer displays. The computer doesn’t know you slept poorly, skipped water, or kicked hard into a current. This is why most experienced divers build in a buffer rather than riding their NDL down to zero.
Adjusting Conservatism With Gradient Factors
Many computers based on the Bühlmann algorithm let you adjust conservatism through gradient factors (GF), expressed as two numbers like 85/85 or 40/85. These reduce the theoretical limits the algorithm uses, essentially telling the computer to pretend your tissues can tolerate less supersaturation than the math predicts.
A setting of 100/100 means you’re using the original algorithm values with no added safety margin. Dropping to 90/90 reduces those values by 10%, giving you shorter no-decompression limits and longer required stops. At 80/80, the reduction is 20%. The lower you set these numbers, the more conservative your dive profile becomes.
The two numbers serve different purposes. The first (GF Low) controls the depth of your first decompression stop on dives that require stops. The second (GF High) affects your total decompression time and, crucially, your no-decompression limits. Here’s what catches many recreational divers off guard: on no-stop dives, only GF High matters. Setting your computer to 45/95 gives you exactly the same no-stop time as 95/95, because GF Low only kicks in when you’ve already exceeded your NDL and owe stops. If you’re diving recreationally and want a wider safety margin, focus on lowering GF High.
Some computers simplify this with a single conservatism dial. Suunto’s RGBM, for instance, offers settings of 0 (default), +1, and +2, with higher numbers reducing your allowed bottom time. The principle is the same regardless of interface: you’re choosing to surface with less dissolved gas in your tissues than the baseline algorithm requires.
Ascent Rate and Safety Stops
Ascending too fast is one of the most common ways divers get into trouble, and your computer monitors this continuously. Recommendations vary: the U.S. Navy and NOAA set the limit at 30 feet per minute, while some recreational training agencies allow up to 60 feet per minute. Most modern dive computers default to somewhere in that range and will flash warnings or sound alarms if you exceed the programmed rate.
Check which ascent rate your computer uses. Many models let you adjust it. If you find the default faster than you’re comfortable with, slow it down. In practice, most divers ascending at 30 feet per minute describe it as feeling almost unnervingly slow, but slower ascents give dissolved gas more time to leave your tissues through normal breathing rather than forming bubbles.
Risky dive profiles also matter beyond raw ascent speed. Yo-yo profiles (repeatedly ascending and descending), sawtooth patterns, and rapid ascents from depth all increase decompression stress in ways that simple algorithms may not fully capture. Your computer calculates based on physics, not on the biological messiness of how your particular circulatory system handles gas under stress.
Why You Cannot Share a Computer
Each dive computer builds a continuous tissue-loading model based on every second of depth data it records. That model is specific to the person wearing it. If you hand your computer to another diver between dives, or if your buddy checks your screen to plan their dive, the numbers are meaningless for anyone but you. Your computer doesn’t know their residual nitrogen from their previous dives, their actual depth profile, or their surface interval.
This also means that if your computer dies mid-trip, you can’t simply borrow one. A replacement computer starts with a clean slate, assuming zero residual nitrogen, which could dramatically overestimate how much bottom time you safely have left. The standard practice is straightforward: if your computer fails during a multi-dive day, you’re done diving until it resets or you’ve waited long enough for your tissues to fully off-gas, typically 18 to 24 hours.
What to Do if Your Computer Fails Underwater
A blank screen or erratic display mid-dive means you’ve lost your decompression data. Signal your buddy immediately, because their computer becomes your only real-time reference. Stay at or above their depth, follow their ascent profile, and make a conservative safety stop. If you were already close to your NDL, extend your safety stop beyond the usual three minutes.
If you’re diving solo or your buddy’s computer has also failed, begin a slow, controlled ascent at the slowest rate you can manage. Without a depth gauge or timer, visual references and controlled breathing are your tools. The goal is to avoid a rapid ascent, which is the single most dangerous outcome of a computer failure.
Pre-Dive Checks That Matter
Before entering the water, verify a few things on your computer. Confirm the battery level is sufficient for the planned dive, including any repetitive dives. A dying battery mid-dive means a lost decompression profile. Check that the correct gas mix is programmed, especially if you’ve switched between air and nitrox or between different nitrox blends. Verify your conservatism settings haven’t been accidentally changed. And confirm the computer is in dive mode, not freedive or gauge mode, which don’t calculate decompression.
If your computer has an integrated air pressure sensor, confirm it reads a plausible tank pressure before you jump in. A reading of zero or a wildly fluctuating number suggests a loose transmitter or a dead sensor battery.
Flying After Computer-Assisted Dives
Your dive computer will display a countdown timer telling you when it considers flying safe, but these countdowns vary by manufacturer and algorithm. The conservative approach, recommended by the Divers Alert Network, is simpler: wait at least 12 hours after a single no-decompression dive, at least 18 hours after multiple days of repetitive diving, and substantially longer after any dive requiring decompression stops.
DAN’s general guidance for recreational divers is to wait a full 24 hours after any diving before boarding a flight. This is longer than some computers will suggest, but cabin pressure at cruising altitude is equivalent to roughly 6,000 to 8,000 feet of elevation, which can cause residual nitrogen to form bubbles that wouldn’t be a problem at sea level. Your computer’s “time to fly” countdown is a useful reference, but treating 24 hours as your minimum keeps things simple and adds a practical safety margin that costs you nothing but an extra day on the ground.