A dive profile is a description of a diver's pressure exposure. It may be as simple as just a depth and time pair, as in: "sixty for twenty," (a stay of 20 minutes at a depth of 60 feet) or as complex as a second by second graphical representation of depth and time.
It is useful as an indication of the risks of decompression sickness and oxygen toxicity and also the volume of open-circuit breathing gas needed for a planned dive as these depend in part upon the depth and duration of the dive. A dive profile diagram is often drawn with time running from left to right and depth increasing down the page.
Planning and monitoring decompression
For planning and monitoring of nitrogen absorption, the data usually consists of the maximum depth reached during the dive and the length of time underwater. For repetitive dives it also includes the "surface interval", or the time spent above the water between the previous dive and the start of the current dive. This information is used to estimate the levels of residual nitrogen build-up in the diver's tissues during and after completing a dive or series of dives.
Types of dive profile
Some types of dive profile have been named.
The diver descends directly to maximum depth, spends most of the dive at maximum depth and then ascends directly at a safe rate. The sides of the "square" are not truly vertical due to the need for a slow descent to avoid barotrauma and a slow ascent rate to avoid decompression sickness.
This type of profile is common for dives at sites where there is a flat sea-bed or where the diver remains at the same place throughout the dive to work. It is the most demanding profile for decompression for a given maximum depth and time because inert gas absorption continues at maximum rate for most of the dive. Decompression tables are drawn up based on the assumption that the diver will follow a square profile.
Where the dive site and underwater topography permit, divers often prefer to do a more triangular than square dive profile; they descend to maximum depth and slowly ascend throughout the dive.
A slow ascent, and therefore slow pressure reduction, is a good decompression practice. When using a dive computer this type of profile often results in no need for deliberate decompression stops. Dive computers, unlike decompression tables, measure depth and time at short intervals during the dive and calculate the exact decompression required, which for a multilevel profile will be less than for the square profile with the same maximum depth and duration.
Repetitive diving occurs when two dives are separated by a short surface interval, during which the diver has not completely desaturated from the first dive. The gas loading from the first dive must then be taken into account when determining no stop times and decompression requirements for the second dive.
At the surface the absorbed inert gases from the dive are eliminated as time passes. When completely "desaturated" the levels of those gases in the diver's body have returned to those normal at atmospheric pressure. The interval to complete desaturation varies depending upon factors such as the depth and duration of the dive, the altitude of the dive, the gas mixtures breathed on the dive, and the decompression strategy used. The maximum interval until desaturation depends on the decompression algorithm in use. On the BSAC 88 dive table is 16 hours. The US Navy tables revision 5 considered the diver unsaturated in 12 hours for normal exposure, and the Buhlmann tables allow 24 hours for the slowest tissues to fully desaturate after a long dive.
Multiple decompressions per day over multiple days can increase the risk of decompression sickness because of the buildup of asymptomatic bubbles, which reduce the rate of off-gassing and are not accounted for in most decompression algorithms.
When no stop depth or time limits are exceeded the diver must do decompression stops to reduce the risk of decompression sickness. Stops when breathing gases containing nitrogen tend to be in shallow water, usually in 3-metre (10 ft) steps. Stops when breathing helium mixes tend to be in deeper water. The duration of the shallower stops is more than the duration of deeper stops. Stops tend to make a dive profile triangular.
Reverse profiles occur when a repeat dive is deeper than the earlier dive. Many diver training agencies discourage reverse profiles because they are not the best way to plan for safe decompression; it is better to do the deeper dive first when the body's tissues hold less absorbed nitrogen. The American Academy of Underwater Sciences workshop concluded there was no reason for the diving communities to prohibit reverse dive profiles for no-decompression dives less than 40 metres (130 ft) deep and depth differentials less than 12 metres (40 ft).
Saw tooth profile
In recreational diving terminology, in a bounce dive the diver descends directly to the maximum depth, spends very little time at maximum depth and ascends directly at a safe rate to the surface. In commercial diving a bounce dive is a surface oriented dive, where the diver is decompressed to surface pressure directly after the dive and does not transfer to a hyperbaric habitat where the diver lives at pressure between dives and only decompresses at the end of a tour of duty.
Ambient pressure on the surface
Sometimes changes in ambient pressure, while diver is on the surface, are drawn on a dive profile. The changes may be caused by flying, land travel involving changes in altitude, or atmospheric pressure changes due to weather. Ambient pressure changes before and after diving can be significant in decompression planning. Starting dives at altitude also has an impact on dive and decompression planning.
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