A Decompression Stop is a period of time a diver must spend at a constant depth in shallow water at the end of a dive in order safely to eliminate inert gases from the diver's body to avoid decompression sickness.

The diver uses decompression tables or dive computers to determine the stop depth and duration for a particular dive profile and breathing gas.

A no stop dive is a dive that requires no decompression stops during the ascent.

There are maximum safe ascent rates even for "no stop" dives. Normally the ascent for shallowest 6 metres to the surface will be slowed to at least 1 minute. Typically with tables, the maximum ascent rate is 10 metres per minute when deeper than 6 metres. Some dive computers have variable ascent rates, depending on depth.

During the stop, the "microbubbles" present after every dive leave the diver's body safely through the lungs. If they are not given enough time to leave safely or too many bubbles are created than can be eliminated naturally, the bubbles grow in size and number causing the symptoms and injuries of decompression sickness.

When diving with nitrogen based breathing gases, decompression stops are typically carried out in the 3 metres / 10 feet to 20 metres / 65 feet depth range. With helium based breathing gases, the stop depth is typically in the 20 metres / 65 feet to 40 metres / 130 feet range.

A "Pyle stop" is an additional short, deep water stop, which is often used in deep diving. Typically, the Pyle stop is 2 minutes long and is carried out at the depth where the pressure change of that ascent halves. For example, on an ascent from a maximum depth of 60 metres (7 bar) to a decompression stop at 20 metres (3 bar), the Pyle stop would take place at the halfway pressure which is 5 bar ( 40 metres).

A diver missing a decompression stop risks developing decompression sickness. The longer the stops missed, the greater the risk. The usual causes for missing stops are: not having enough breathing gas to complete the stops or accidentally losing control of buoyancy. An aim of most basic diver training is to prevent these two faults. There are less predictable causes of missing decompression stops. Diving suit failure in cold water forces the diver to choose between hypothermia and decompression sickness. Diver injury or marine animal attack also limit the duration of stops the diver is willing to carry out.

There are several pieces of safety equipment used to help divers carry out decompression stops. A diving shot, a surface marker buoy or a decompression buoy can be used to mark the underwater positions and act as a buoyancy control aid in low visibility or currents. A decompression trapeze is useful for comfortably carrying out long stops.

The length of "surface interval" between dives is also very important for decompression. It typically takes from 16 to 24 hours for the body to return to its normal, atmospheric, levels of inert gas saturation after a dive. The surface interval can be thought of as the final decompression stop of a dive.

Only divers breathing gas at high pressure, such as when using SCUBA, may need to do decompression stops. A diver who breathes gas at atmospheric pressure, such as in free-diving, snorkeling or when using an atmospheric diving suit, does not need to do decompression stops. However, it is possible to get decompression sickness from repetitive free-diving at depth with short surface intervals.

References

• Decompression sickness from free diving (Fred Bove, MD, PhD)