Decompression stop
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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 to safely eliminate absorbed inert gases from the diver's body to avoid decompression sickness. The practice of making decompression stops is called staged decompression, as opposed to continuous decompression.
[edit] Doing a stop
The diver uses decompression tables or dive computers to find, for his planned dive profile and breathing gas, if decompression stops are needed, and if so, the depths and durations of the stops. A "no stop" dive is a dive that needs no decompression stops during the ascent.
Shorter and shallower decompression dives may only need one single short shallow decompression stop, for example 5 minutes at 3 metres (10 ft). Longer and deeper dives often need a series of decompression stops, each stop being longer 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 ordered 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 during no-stop dives.
In addition to stops, the diver must not exceed a safe ascent rate during the whole of the ascent from depth. 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.
[edit] 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 between 65 and 130 feet (20 and 40 metres). High partial pressures of oxygen are commonly used in decompression gases to use the effect of the oxygen window.
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 last decompression stop of a dive.
[edit] 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.
[edit] 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 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).
[edit] 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.
Technical diving education organizations define special procedures to be done if decompression stops are missed. These procedures may need repeating one or several stops.
[edit] 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.
Decompression may be shortened (or accelerated) by breathing an oxygen-rich "deco gas" such as a nitrox with 50% or more oxygen. This decompression gas is sometimes carried in small diving cylinders called "pony cylinders" or in larger side-slung cylinders.
