A dive profile is a description of a diver's pressure exposure over time. 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 recorded by a personal dive computer. Several common types of dive profile are specifically named, and these may be characteristic of the purpose of the dive. For example, a working dive at a limited location will often follow a constant depth (square) profile, and a recreational dive is likely to follow a multilevel profile, as the divers start deep and work their way up a reef to get the most out of the available breathing gas. The names are usually descriptive of the graphic appearance.
The intended dive profile is useful as a planning tool as an indication of the risks of decompression sickness and oxygen toxicity for the exposure, and also for estimating 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 conventionally drawn with elapsed time running from left to right and depth increasing down the page.
Many personal dive computers record the instantaneous depth at small time increments during the dive. This data can often be downloaded to a personal computer or tablet and displayed in graphic form as a dive profile.
Types of dive profile
Some types of dive profile have been named. An analysis of dive profiles logged by dive computers by the Divers Alert Network used categorization rules which were based on the fraction of the dive time spent in four depth zones: descent, bottom, multilevel, and decompression. The descent zone was defined as the part of the dive between the surface and first reaching 85% of the maximum depth. The bottom zone is the part of the dive deeper than 85% of maximum depth. The multilevel zone is ascent from 85% to 25% of maximum depth, and the decompression zone is less than 25% of maximum depth. A square dive was defined as having more than 40% of the total dive time in the bottom zone and not more than 30% in the multilevel and decompression zones. A multilevel was defined as having at least 40% of the total dive time in the multilevel zone. All other dives are considered to be intermediate.
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 may follow a square profile.
Where the dive site and underwater topography permit, divers often prefer to 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 may not require additional decompression stops. Dive computers, unlike decompression tables, measure depth and time at short intervals during the dive and calculate the exact decompression indicated by the decompression model, which for a multilevel profile will be less than for the square profile with the same maximum depth and duration.
At the surface the remaining excess of 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 is considered to have occurred depends on the decompression algorithm in use. On the BSAC 88 dive table it is deemed to take 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.
Repetitive diving occurs when two dives are separated by a short surface interval, during which the diver has not completely outgassed 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. 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 decompress more extensively than allowed for in the recommended maximum ascent rate to reduce the risk of decompression sickness. This is conventionally done as decompression stops, which are pauses in ascent at specified depths for specified times derived from the decompression algorithm and based on the dive profile history and breathing gas composition. Depth and duration of obligatory decompression stops are specified by the decompression model used. Stops are usually specified in 3-metre (10 ft) steps. The depth of the deepest (first) stop for the same profile history will depend on the algorithm, as some decompression models start decompression at lower supersaturation (lower M-values) than others. The duration of the shallower stops is generally more than the duration of deeper stops on a specific dive. Stops extend the dive profile graph along the time axis.
Reverse profiles occur when a repeat dive is deeper than the earlier dive. Many diver training agencies discourage reverse profiles because they use a decompression model which is not efficient for safe decompression of a reverse profile. 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). The term is also sometimes used to refer to a single dive profile where the depth generally increases during the bottom phase of the dive until the start of the ascent.
In a saw tooth profile the diver ascends and descends a number of times during the dive. Each ascent and descent increases the risk of decompression sickness if there are any bubbles already in the diver's tissues. The increase in risk depends on the ascent rate, magnitude and duration of the upwards excursion, the saturation levels of the tissues, and to some extent the time spent after returning to depth. Accurate assessment of the increase of risk is not currently (2016) possible, but some dive computers make an adjustment to the decompression requirement based on violations of recommended maximum ascent rate as an attempt to compensate.
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 to the surface, preferably at an ascent rate recommended by the decompression model used, and making any necessary decompression stops. In commercial diving a bounce dive is any surface oriented dive, in which the diver is decompressed to surface pressure at the end of 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. The duration of bottom time is not relevant in this usage.
A saturation profile is one which all the tissues considered by the decompression model have become saturated with inert gas from the breathing mixture. Most decompression models will take this to be at six tissue half-times for the slowest tissue considered. Further bottom time at the same depth will not affect the inert gas loading of any tissue and will not affect the decompression required.
Applications of a dive profile
A simple record of depth and time for a dive is useful as a legal record of a diving operation, where this is required, and in the case of an accident during the dive, an accurately recorded dive profile can provide useful diagnostic information for treatment of the injured diver and for analysis of the circumstances leading to the accident and the action taken during and after the incident. A proposed dive profile is necessary for effective dive planning, both for estimating the required breathing gas composition and quantities, for planning decompression, and for choosing suitable diving equipment and other logistical aspects.
Calculation of gas requirements
The breathing gas mixtures appropriate to a dive depend to a large extent on the maximum depth and the decompression obligations incurred by the planned duration of the dive and the time spent at each depth. The quantity of gases required for scuba will depend on the time spent at each depth, the breathing rate of the diver, the type of breathing apparatus to be used, and reasonable allowances for contingencies.
Planning and monitoring decompression
For planning and monitoring of inert gas absorption using decompression tables, the data usually consists of the maximum depth reached during the dive, the bottom time as defined by the dive table in use and the length of time underwater (total dive time). For repetitive dives it also includes the "surface interval" – the time spent above the water between the previous dive and the start of the next dive. This information is used to estimate the levels of residual inert gas build-up in the diver's tissues during and after completing a dive or series of dives. Residual gas is often expressed as "repetitive group", which is an important input value for planning the decompression for the next dive when using tables. A more detailed and extensive set of residual gas data is stored in the memory of a dive computer, and automatically applied as initial conditions to subsequent dives.
When decompression planning software is used to produce a schedule for a planned dive, the necessary input includes a definition of the dive profile. This may be in as much detail as the user is prepared to provide and the program is capable of using, but will always specify at least maximum depth and bottom time, and may go into detail regarding recent dive history, multiple levels, gas switches, altitude and personal conservatism factors. Many dive computers have a dive planning function for which the diver selects a maximum depth and the computer displays the maximum bottom time for which no decompression stops are required.
Ambient pressure at the surface
Atmospheric pressure changes due to change of altitude before or after diving can have a significant influence on decompression risk. Diving at high altitude requires special consideration in decompression planning. Such variations in ambient pressure caused by flying or surface travel involving changes in altitude will affect decompression and should be considered during dive planning and therefore may influence a planned dive profile.
The dive profile is often recorded in some way as part of a permanent record of the dive. Maximum depth, bottom time and any decompression required are routinely logged by most professional divers, for whom it may be a legal requirement, and by many recreational divers, for whom it is usually a recommendation of the training agencies.
Recreational diving paper logbooks frequently provide a simple graphic representation of a dive profile for recording the details of a dive which are necessary for planning a repetitive dive using a specified set of dive tables. Digital diving logs such as the freeware Subsurface and various proprietary packages from diving computer manufacturers may display a graphic representation of the dive profile downloaded from the dive computer.
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