Decompression stop
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 20 feet (6 metres) to the surface will be slowed to at least 1 minute. Typically with tables, the maximum ascent rate is 30 feet (10 metres) per minute when deeper than 20 feet (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 10 to 65 feet (3 to 20 metres) depth range. With helium based breathing gases, the stop depth is typically in the 65 to 130 feet (20 to 40 metres) range.
A "Pyle stop" is an additional short, deep water stop, which is increasingly used in deep diving. Typically, a Pyle stop is 2 minutes long and is carried out at the depth where the pressure change halves on an ascent from the bottom to a shallow water decompression stop. For example, on an ascent from a maximum depth of 196 feet (60 metres) at 100 psi (7 bar) to a decompression stop at 65 feet (20 metres) at 40 psi (3 bar), the Pyle stop would take place at the halfway pressure which is 70 psi (5 bar) at 130 feet (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 Taravana from repetitive, deep free-diving with short surface intervals.