Safety stops, ascent rates and no-decompression limits explained

It's the time you can spend at a depth and still ascend directly to the surface without required decompression stops. DAN explains that the original U.S. Navy limits came from experiments with square profiles, and that dive computers use mathematical models which are not based on controlled trials with DCS as the endpoint — so different computers give different limits on the same dive.

Not guaranteed. DAN calls DCS a probabilistic condition, in which injury risk increases with proximity to the limits — and exertion, cold, age and fitness also play a part. Stay well within your limits; no fraction of the displayed time guarantees freedom from DCS.

Slowly. DAN cites the U.S. Navy ascent rate of 30 feet (9 metres) per minute. Rapid ascents risk lung over-expansion injuries and arterial gas embolism, especially if you hold your breath. Watch your computer, vent your BCD gradually, and if you start rising too fast, spread your arms and legs to create drag.

A safety stop near the end of the dive adds time at shallow depth. DAN notes some computers extend the standard three minutes to five or more, especially when you're within about five minutes of the limit or have dived deeper than 30 metres. Cold water, hard work and deep dives are reasons to stay further from the limits and stop for longer.