Decompression Algorithms

Decompression Algorithms

Decompression in the context of diving derives from the reduction in ambient pressure experienced by the diver during the ascent at the end of a dive or hyperbaric exposure and refers to both the reduction in pressure and the process of allowing dissolved inert gases to be eliminated from the tissues during this reduction in pressure.

When a diver descends in the water column the ambient pressure rises. Breathing gas is supplied at the same pressure as the surrounding water, and some of this gas dissolves into the diver's blood and other fluids. Inert gas continues to be taken up until the gas dissolved in the diver is in a state of equilibrium with the breathing gas in the diver's lungs, (see: "Saturation diving"), or the diver moves up in the water column and reduces the ambient pressure of the breathing gas until the inert gases dissolved in the tissues are at a higher concentration than the equilibrium state, and start diffusing out again.

In a manner similar to the fizzing of a carbonated beverage when opened, dissolved inert gases such as nitrogen or helium can form bubbles in the blood and tissues of the diver if the partial pressure of the gas in the diver gets too high when compared to the ambient pressure. These bubbles (and perhaps blood clots caused by the bubbles) can cause damage to tissues known as decompression sickness or the bends. The immediate goal of planned decompression is to avoid development of symptoms of bubble formation in the tissues of the diver, and the long term goal is to also avoid complications due to sub-clinical decompression injury.

The symptoms of decompression sickness are known to be caused by damage resulting from the formation and growth of bubbles of inert gas within the tissues and by blockage of arterial blood supply to tissues by gas bubbles and other emboli consequential to bubble formation and tissue damage.

The precise mechanisms of bubble formation and the damage they cause has been the subject of medical research for a considerable time and several hypotheses have been advanced and tested. Tables and algorithms for predicting the outcome of decompression schedules for specified hyperbaric exposures have been proposed, tested, and used, and usually found to be of some use but not entirely reliable. Decompression remains a procedure with some risk, but this has been reduced and is generally considered to be acceptable for dives within the well tested range of commercial, military and recreational diving.

During effective decompression, the venous microbubbles present after most dives are eliminated from the diver's body through the lungs. If they are not given enough time, or more bubbles are created than can be eliminated safely, the bubbles grow in size and number causing the symptoms and injuries of decompression sickness.

Decompression may be continuous or staged, where the ascent is interrupted by stops at regular depth intervals, but the entire ascent is part of the decompression, and ascent rate can be critical to harmless elimination of inert gas. What is commonly known as no-decompression diving, or more accurately no-stop decompression, relies on limiting ascent rate for avoidance of excessive bubble formation.

When diving with nitrogen-based breathing gases, decompression stops are typically carried out in the 3 to 20 metres (10 to 70 ft) depth range. With helium-based breathing gases the stop depths may start deeper.

The period at surface pressure after dives is also an important part of decompression and can be thought of as the last decompression stop of a dive. It typically takes up to 24 hours for the body to return to its normal atmospheric levels of inert gas saturation after a dive. When time is spent on the surface between dives this is known as the "surface interval" and is considered when calculating decompression requirements of the subsequent dive.

Divers breathing gas at high pressure may need to do decompression stops, but a diver who only breathes gas at atmospheric pressure when free-diving, does not usually need to do decompression stops. However, it is possible to get taravana, thought to be a form of decompression sickness, from repetitive deep free-diving with short surface intervals.

Divers who use a snorkel to free-dive near the surface or use an atmospheric diving suit will not need to decompress.