Surface Intervals (SI) and Scuba Diving

A female diver relaxes on a tropical beach during a surface interval.
Surface intervals are a great time to relax and plan your next dive. ©

What Is a Surface Interval?

A surface interval (SI) is the time that a diver remains out of the water between two dives. During this time, the nitrogen absorbed during the first dive continues to off-gas, or to be released from a diver's body. A diver has less nitrogen in his body at the end of a surface interval than at the beginning of it.

When Does a Surface Interval Begin?

A surface interval begins when the diver reaches the surface of the water and is no longer breathing underwater from his regulator. Even floating on the surface of the water immediately after a dive can be counted as part of the surface interval. In fact, most dive computers will begin timing the surface interval the moment a diver reaches the surface.

When Does a Surface Interval End?

A surface interval ends when a diver descends to begin his next dive. At this point, he still has some nitrogen left in his body from the previous dive. To know how much nitrogen he has in his body after a surface interval, a diver will need to calculate his pressure group and residual nitrogen time.

How Long After a Dive Should a Diver Keep Track of His Surface Interval?

A small amount of nitrogen remains in a diver's system for many hours after a dive. This is why in recreational diving (technical diving may require different rules) divers are advised not to fly after diving for at least 12 hours after a single dive and 18 hours after repetitive dives.

For the purposes of calculating nitrogen absorption for repetitive dives, a recreational diver can consider himself clean of nitrogen after six hours, even if he has dived aggressively. (according to the PADI dive tables). This can be seen clearly on the PADI dive tables in the area listing surface intervals. The maximum surface interval listed ends after six hours. For less aggressive diving, the maximum listed surface interval may be shorter.

Is a Surface Interval Required After Every Dive?

Technically, a surface interval is not required after all dives. If a diver has not reached his no-decompression limit during the dive, he can descend and continue to dive immediately. However, this may not always be the best idea. Surface intervals allow a diver's body to release nitrogen, give the diver time to rest and warm up, and allow a diver to rehydrate. For these reasons, a surface interval between dives is always a good idea! A rested, comfortable and alert diver will be safer than an exhausted, mentally tired and dehydrated diver.

What Should a Diver Do During a Surface Interval?

Surface intervals are for rest and recuperation. Whether he feels it or not, diving is stressful on a person's body. Absorbing and releasing nitrogen, adventuring into a foreign environment, dealing with dive equipment, getting cold underwater, and becoming dehydrated all affect a diver. Wise divers will treat a surface interval as an enjoyable rest between dives.

For this reason, strenuous exercise, consumption of alcohol, and other activities that may compromise a diver's physical or mental ability to dive safely are not recommended during a surface interval. Instead, divers should focus on allowing their bodies to release nitrogen by resting, rehydrate by drinking water or other hydrating fluids, and have a light snack if desired. Food that makes a diver "gassy" should be avoided, as this can lead to significant discomfort during a dive. A surface interval is also a great time to plan your next dive!

The Take-Home Message About Surface Intervals

A diver should keep track of the time he spends on the surface between two dives, beginning with the moment he surfaces and ending with the time he begins the descent for the next dive. This is important because it allows a diver to calculate how much nitrogen he has in his system when planning repetitive dives. During surface intervals, even the fittest of divers should focus on recuperation. Drinking hydrating fluids, resting, and enjoying light snacks to recover energy will help to ensure that your subsequent dives are safe and comfortable.