Dive profile
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," 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 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, to calculate a decompression schedule for the dive, 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 sometimes be displayed directly on the dive computer or more often downloaded to a personal computer, tablet, or smartphone and displayed in graphic form as a dive profile.
Concept
The profile of a dive is the variation of depth, measured as ambient pressure, over time during that dive. The actual location of the diver at any time is generally not considered, as the dive profile is a tool for dive planning and decompression status calculation. Other data may be added to the depth graph, such as partial pressures of the breathing gas constituents, calculated estimates of accumulated gas concentrations in the theoretical tissue, gas consumption rates and cumulative gas consumption. These additional values are available when the dive computer uses them to estimate decompression status, to provide the diver with a recommend decompression schedule for the exposure of the actual dive.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 profile 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.Recorded actual dive profile
A dive computer will usually record the actual dive profile in real time, in a format that can be downloaded and viewed later. Depth values are recorded at intervals which are usually 30 seconds or less, along with other measured data at that time.If the pressure is measured manually by pneumofathometer by the diving supervisor it will also be recorded manually when it changes. The detailed profile does not generally need to be logged as the maximum depth is used with tables unless a multilevel dive profile is required, in which case the time will be logged for each change of depth, and for the start and end of each decompression stop.
Square profile
Or constant depth profile. The diver descends directly to maximum depth, spends most of the dive at maximum depth and then ascends directly at a safe rate, with any required decompression. 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. The term has also been used more loosely, for example DAN's definition of more than 40% of total dive time in the bottom zone which is within 15% of maximum depth.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 and be working while at the bottom, which is common practice for professional divers.
Multi-level diving
Multi-level diving, in the broader sense, is diving where the activity other than descent, direct ascent, and decompression, takes place in more than one depth range, where a depth range can be arbitrarily defined for convenience, and usually follows the depth graduations of the decompression tables in use. Most recreational diving is multi-level by this description. In the narrower sense, it implies that decompression is calculated based on the time spent in each of more than one depth range. Decompression calculations using dive tables for multi-level dives were moderately common practice for advanced recreational diving before dive computers were widely used.Where the dive site and underwater topography permit, divers often prefer to descend initially to maximum depth and slowly ascend throughout the dive. A slow ascent, and therefore slow pressure reduction, is a good decompression practice. Multi-level decompression calculation takes this into account and does not burden the diver with decompression obligation for all the time not spent at maximum depth, so the decompression schedule will be less conservative than for a square profile for the same maximum depth. Stepped multi-level decompression calculation uses local maximum depth for each sector of the dive, which is more conservative than real time calculations following instantaneous depth profile, but less conservative than for square profiles.
A practice developed of calculating decompression during the dive, using tables printed on a plastic card, to remain within no-decompression limits for multi-level dives. Although this procedure had very little controlled experimental verification, it did appear to be reasonably safe in the field. This may be attributable to the relative conservatism of the tables used.
Dive computers, unlike decompression tables, measure actual depth and time at short intervals during the dive and calculate the exact gas loading and decompression indicated by the decompression model, so their decompression calculations are inherently multi-level at a fine resolution.
Repet-Up profile
A commercial diving term for a multi-level dive in which each recorded change of depth is to a shallower depth range. This profile type is used to maximise dive time while limiting decompression time when using decompression tables, but could also use decompression software. At each change of depth range limit the nominal residual inert gas loading is recalculated for the dive to that point by the supervisor, and a new effective dive time established based on the most recent depth limit. The procedure has been shown to be acceptably safe, and is economically advantageous.Hang-off profile
A hang-off is a procedure used in commercial bounce diving to reduce unnecessary inert gas accumulation during idle periods when the diver is waiting for surface support activity to be completed before the diver's underwater work can continue. During a hang-off the diver ascends to a shallower depth, usually, at or below the first decompression stop depth, where ingassing is effectively stopped, and decompression obligation is put on hold, then descends back to the working depth to continue with the job. By its nature, this profile does not apply to recreational diving, but could be used in any surface oriented professional diving application.Repetitive diving
At the surface the remaining excess of absorbed inert gases from the dive is 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.
Reverse profile
Reverse profiles occur when a repeat dive is deeper than the earlier dive. 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. Many recreational diver training agencies discouraged or even prohibited reverse profiles for reasons that were not clearly expressed, until the American Academy of Underwater Sciences workshop on reverse dive profiles concluded there was no evidence to support prohibiting reverse dive profiles.Findings:Michael A. Lang and Charles E. Lehner, Co-Chairs of the Reverse Dive Profiles Workshop, October 29-30.
- Historically neither the U.S. Navy nor the commercial sector have prohibited reverse dive profiles.
- Reverse dive profiles are being performed in recreational, scientific, commercial, and military diving.
- The prohibition of reverse dive profiles by recreational training organizations cannot be traced to any definite diving experience that indicates an increased risk of DCS.
- No convincing evidence was presented that reverse dive profiles within the no-decompression limits lead to a measurable increase in the risk of DCS.