Decompression stop
From Free net encyclopedia
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 absorbed inert gases from the diver's body to avoid decompression sickness.
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Doing a stop
Image:Decompression stop sample profile.PNG The diver uses decompression tables or dive computers to determine, for a particular dive profile and breathing gas, if decompression stops are required and if so, the stops' depths and durations. A no stop dive is a dive that requires no decompression stops during the ascent.
Shorter and shallower decompression dives may only require a short and shallow decompression stop, for example 5 minutes at 3 metres (10 ft). Longer and deeper dives often require a series of decompression stops; each stop being at least as long but shallower than the previous stop. As a precaution against any unnoticed dive computer malfunction, diver error or physiological predisposition to decompression sickness, many divers do an extra "safety stop" in addition to those required by their dive computer or tables. A safety stop is typically 1 to 5 minutes at 3 to 6 metres (10 to 20 ft). They are even done after no-stop dives.
In addition to stops, the diver must not exceed a safe ascent rate. Normally the time to ascend the shallowest 20 feet (6 metres) to the surface will take 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 maximum ascent rates, depending on depth.
Mechanism
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 more 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.
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.
Breathing gas type
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.
Deep stops
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).
Missed stops
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.
Equipment
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.de:Dekompression nl:Decompressie pl:Dekompresja (informatyka) ru:Декомпрессия (дайвинг)