These are the variety of ways a diver may decompress, depending on the
dive profile and the
mode of diving used. Decompression is a part of every ambient pressure dive. Modes of decompression range from "
no-stop dives" where a limited and controlled
ascent rate is sufficient decompression, to decompression from saturation over several days. Decompression can be continuous, where no stops are required, and the rate of ascent is limited to provide sufficient time to offgas safely, or staged, where ascent is made up to and between stops a limited rate, but most of the offgassing occurs during periods of constant depth, (pressure) called decompression stops. Continuous decompression rates depend on the theoretical gas loading of the controlling tissue, and may be fixed or, more often, variable with depth. Decompression can also be done entirely in the water, partly in the water and partly in a surface decompression chamber or entirely in a decompression chamber. It can also be classified by the type of breathing gases used while decompressing, whether there are changes in gas composition during decompression, and whether the changes are stepwise or continuous, or a combination of both. Air diving traditionally uses air as breathing gas for the entire dive, including for in-water staged decompression. It is simple, low cost, requires little or no special equipment, but is inefficient and limited to tolerable in-water exposures.
No-decompression stop dives A "no-stop dive", also commonly but inaccurately referred to as a "no-decompression" dive is a dive that needs no decompression stops during the ascent according to the chosen algorithm or tables, and relies on a controlled ascent rate for the elimination of excess inert gases. In effect, the diver is doing continuous decompression during the ascent.
No-stop limit The "no-stop limit", or "no-decompression limit" (NDL), is the time interval that a
diver may theoretically spend at a given depth without having to perform any
decompression stops while surfacing. The NDL helps divers plan dives so that they can stay at a given depth for a limited time and then ascend without stopping while still avoiding an unacceptable risk of decompression sickness. A dive within the no-stop limit may be referred to as a . The NDL is a theoretical time obtained by calculating inert gas uptake and release in the body, using a
decompression model. Although the science of calculating these limits has been refined since Haldane's original model, there is still much that is unknown about how inert gases enter and leave the human body, and the NDL may vary between decompression models for identical initial conditions. In addition, every individual's body is unique and may absorb and release inert gases at different rates at different times. For this reason, dive tables typically have a degree of conservatism built into their recommendations. Divers can and do suffer
decompression sickness while remaining inside NDLs, though the incidence is very low. On
dive tables a set of NDLs for a range of depth intervals is printed in a grid that can be used to plan dives. There are many different tables available as well as software programs and calculators, which will calculate no decompression limits. Most personal decompression computers (dive computers) will indicate a remaining no decompression limit at the current depth during a dive. The displayed interval is continuously revised to take into account changes of depth and elapsed time, and where relevant, changes of breathing gas. Dive computers also usually have a planning function which will display the NDL for a chosen depth taking the diver's recent decompression history, as recorded by that computer, into account.
Safety 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" (precautionary decompression stop) in addition to those prescribed by their dive computer or tables. A safety stop is typically 1 to 5 minutes at . They are usually done during no-stop dives and may be added to the obligatory decompression on staged dives. Many dive computers indicate a recommended safety stop as standard procedure for dives beyond specific limits of depth and time. The
Goldman decompression model predicts a significant risk reduction following a safety stop on a low-risk dive A safety stop can significantly reduce decompression stress as indicated by venous gas emboli, but if remaining in the water to do a safety stop increases risk due to another hazard, such as running out of gas underwater or a significant medical emergency then the overall safety of the diver may be best served by omitting the safety stop. A similar balancing of hazard and risk also applies to surfacing with omitted decompression, or bringing an unresponsive, non-breathing, diver to the surface. If the risk appears greater for completing the decompression then further decompression should be omitted. A bend can usually be treated, whereas drowning, cardiac arrest, or bleeding out in the water is likely to be terminal. A further complication arises when the buddy must decide whether they will also truncate decompression and put themself at risk in the interests of helping the diver in difficulty. In these situations the actual risk is seldom known with any accuracy, making the decision more difficult for the divers in the water.
Continuous decompression Continuous decompression is decompression without stops. Instead of a fairly rapid ascent rate to the first stop, followed by a period at static depth during the stop, the ascent is slower, but without officially stopping. In theory this may be the optimum decompression profile. In practice it is very difficult to do manually, and it may be necessary to stop the ascent occasionally to get back on schedule, but these stops are not part of the schedule, they are corrections. For example,
USN treatment table 5, referring to treatment in a decompression chamber for type 1 decompression sickness, states "Descent rate - 20 ft/min. Ascent rate - Not to exceed 1 ft/min. Do not compensate for slower ascent rates. Compensate for faster rates by halting the ascent." To further complicate the practice, the ascent rate may vary with the depth, and is typically faster at greater depth and reduces as the depth gets shallower. In practice a continuous decompression profile may be approximated by ascent in steps as small as the chamber pressure gauge will resolve, and timed to follow the theoretical profile as closely as conveniently practicable. For example,
USN treatment table 7 (which may be used if decompression sickness has reoccurred during initial treatment in the compression chamber) states "Decompress with stops every 2 feet for times shown in profile below." The profile shows an ascent rate of 2 fsw (feet of sea water) every 40 min from 60 fsw to 40 fsw, followed by 2 ft every hour from 40 fsw to 20 fsw and 2 ft every two hours from 20 fsw to 4 fsw.
Staged decompression at a decompression stop. Decompression which follows the procedure of relatively fast ascent interrupted by periods at constant depth is known as staged decompression. The ascent rate and the depth and duration of the stops are integral parts of the decompression process. The advantage of staged decompression is that it is far easier to monitor and control than continuous decompression.
Decompression stops A decompression stop is the period a
diver must spend at a relatively shallow constant depth during ascent after a dive to safely eliminate absorbed
inert gases from the body tissues sufficiently to avoid
decompression sickness. The practice of making decompression stops is called
staged decompression, as opposed to
continuous decompression. The diver or diving supervisor identifies the requirement for decompression stops, and if they are needed, the depths and durations of the stops, by using
decompression tables, software planning tools or a
dive computer. The ascent is made at the recommended rate until the diver reaches the depth of the first stop. The diver then maintains the specified stop depth for the specified period, before ascending to the next stop depth at the recommended rate, and follows the same procedure again. This is repeated until all required decompression has been completed and the diver reaches the surface. The intermittent ascents before the first stop, between stops, and from the last stop to the surface are traditionally known as "
pulls". Once on the surface, the diver will continue to eliminate inert gas until the concentrations have returned to normal surface saturation, which can take several hours. Inert gas elimination is considered in some models to be effectively complete after 12 hours, while other models show it can take up to, or even more than 24 hours. The depth and duration of each stop is calculated to reduce the inert gas excess in the most critical tissues to a concentration which will allow further ascent without unacceptable risk. Consequently, if there is not much dissolved gas, the stops will be shorter and shallower than if there is a high concentration. The length of the stops is also strongly influenced by which tissue compartments are assessed as highly saturated. High concentrations in slow tissues will indicate longer stops than similar concentrations in fast tissues. Shorter and shallower decompression dives may only need one single short shallow decompression stop, for example, 5 minutes at . Longer and deeper dives often need a series of decompression stops, each stop being longer but shallower than the previous stop.
Deep stops A deep stop was originally an extra stop introduced by divers during ascent, at a greater depth than the deepest stop required by their computer algorithm or tables. This practice is based on empirical observations by technical divers such as
Richard Pyle, who found that they were less fatigued if they made some additional stops for short periods at depths considerably deeper than those calculated with the currently published decompression algorithms. More recently computer algorithms that are claimed to use deep stops have become available, but these algorithms and the practice of deep stops have not been adequately validated. Deep stops are likely to be made at depths where ingassing continues for some slow tissues, so the addition of deep stops of any kind can only be included in the dive profile when the decompression schedule has been computed to include them, so that such ingassing of slower tissues can be taken into account. Nevertheless, deep stops may be added on a dive that relies on a personal dive computer (PDC) with real-time computation, as the PDC will track the effect of the stop on its decompression schedule. Deep stops are otherwise similar to any other staged decompression, but are unlikely to use a dedicated decompression gas, as they are usually not more than two to three minutes long. A study by
Divers Alert Network in 2004 suggests that addition of a deep (c. 15 m) as well as a shallow (c. 6 m) safety stop to a theoretically no-stop ascent will significantly reduce decompression stress indicated by
precordial doppler detected bubble (PDDB) levels. The authors associate this with gas exchange in fast tissues such as the spinal cord and consider that an additional deep safety stop may reduce the risk of spinal cord decompression sickness in recreational diving. A follow-up study found that the optimum duration for the deep safety stop under the experimental conditions was 2.5 minutes, with a shallow safety stop of 3 to 5 minutes. Longer safety stops at either depth did not further reduce PDDB. In contrast, experimental work comparing the effect of deep stops observed a significant decrease in vascular bubbles following a deep stop after longer shallower dives, and an increase in bubble formation after the deep stop on shorter deeper dives, which is not predicted by the existing bubble model. A controlled comparative study by the Navy Experimental Diving Unit in the NEDU Ocean Simulation Facility wet-pot comparing the VVAL18 Thalmann Algorithm with a deep stop profile suggests that the deep stops schedule had a greater risk of DCS than the matched (same total stop time) conventional schedule. The proposed explanation was that slower gas washout or continued gas uptake offset benefits of reduced bubble growth at deep stops.
Profile determined intermediate stops Profile-dependent intermediate stops (PDIS)s are intermediate stops at a depth above the depth at which the leading compartment for the decompression calculation switches from on gassing to off gassing and below the depth of the first obligatory decompression stop, (or the surface, on a no-stop dive). The ambient pressure at that depth is low enough to ensure that the tissues are mostly off gassing inert gas, although under a very small pressure gradient. This combination is expected to inhibit bubble growth. The leading compartment is generally not the fastest compartment except in very short dives, for which this model does not require an intermediate stop. The 8 compartment Bühlmann - based UWATEC ZH-L8 ADT MB PMG decompression model in the Scubapro Galileo dive computer processes the dive profile and suggests an intermediate 2-minute stop that is a function of the tissue nitrogen loading at that time, taking into account the accumulated nitrogen from previous dives. Within the Haldanian logic of the model, at least three compartments are off gassing at the prescribed depth - leading compartment and the 5 and 10-minute half time compartments. A PDIS is not a mandatory stop, nor is it considered a substitute for the more important shallow safety stop on a no-stop dive. Switching breathing gas mix during the ascent will influence the depth of the stop. The PDIS concept was introduced by Sergio Angelini.
Decompression schedule A decompression schedule is a specified ascent rate and series of increasingly shallower decompression stops—usually for increasing amounts of time—that a diver performs to outgas inert gases from their body during ascent to the surface to reduce the risk of
decompression sickness. In a decompression dive, the decompression phase may make up a large part of the time spent underwater (in many cases it is longer than the actual time spent at depth). The depth and duration of each stop is dependent on many factors, primarily the profile of depth and time of the dive, but also the
breathing gas mix, the interval since the previous dive and the altitude of the dive site. The diver obtains the depth and duration of each stop from a
dive computer,
decompression tables or dive planning computer software. A technical scuba diver will typically prepare more than one decompression schedule to plan for contingencies such as going deeper than planned or spending longer at depth than planned. Recreational divers often rely on a personal dive computer to allow them to avoid obligatory decompression, while allowing considerable flexibility of dive profile. A surface supplied diver will normally have a diving supervisor at the control point who monitors the dive profile and can adjust the schedule to suit any contingencies as they occur.
Missed stops A diver missing a required decompression stop increases the risk of developing decompression sickness. The risk is related to the depth and duration of the missed stops. 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 also less predictable causes of missing decompression stops.
Diving suit failure in cold water may force the diver to choose between
hypothermia and
decompression sickness. Diver injury or marine animal attack may also limit the duration of stops the diver is willing to carry out.
Omitted decompression procedures A procedure for dealing with omitted decompression stops is described in the US Navy Diving Manual. In principle the procedure allows a diver who is not yet presenting symptoms of decompression sickness, to go back down and complete the omitted decompression, with some extra time added to deal with the bubbles which are assumed to have formed during the period where the decompression ceiling was violated. Divers who become symptomatic before they can be returned to depth are treated for decompression sickness, and do not attempt the omitted decompression procedure as the risk is considered unacceptable under normal operational circumstances. If a decompression chamber is available, omitted decompression may be managed by chamber recompression to an appropriate pressure, and decompression following either a surface decompression schedule or a treatment table. If the diver develops symptoms in the chamber, treatment can be started without further delay.
Delayed stops A delayed stop occurs when the ascent rate is slower than the nominal rate for a table. A computer will automatically allow for any theoretical ingassing of slow tissues and reduced rate of outgassing for fast tissues, but when following a table, the table will specify how the schedule should be adjusted to compensate for delays during the ascent. Typically a delay in reaching the first stop is added to bottom time, as ingassing of some tissues is assumed, and delays between scheduled stops are ignored, as it is assumed that no further ingassing has occurred. This may be considered a special case of a
multi-level dive.
Accelerated decompression Decompression can be accelerated by the use of breathing gases during ascent with lowered inert gas fractions (as a result of increased oxygen fraction). This will result in a greater diffusion gradient for a given ambient pressure, and consequently accelerated decompression for a relatively low risk of bubble formation. Nitrox mixtures and oxygen are the most commonly used gases for this purpose, but oxygen rich trimix blends can also be used after a trimix dive, and oxygen rich heliox blends after a heliox dive, and these may reduce risk of
isobaric counterdiffusion complications. Doolette and
Mitchell showed that when a switch is made to a gas with a different proportion of inert gas components, it is possible for an inert component previously absent, or present as a lower fraction, to in-gas faster than the other inert components are eliminated (inert gas counterdiffusion), sometimes resulting in raising the total tissue tension of inert gases in a tissue to exceed the ambient pressure sufficiently to cause bubble formation, even if the ambient pressure has not been reduced at the time of the gas switch. They conclude that "breathing-gas switches should be scheduled deep or shallow to avoid the period of maximum supersaturation resulting from decompression". Raised partial pressures of oxygen in the saturation system atmosphere during decompression are standard practice, and minimum values are specified for the safe use of saturation decompression schedules. Since these schedules are typically several days long, the PO2 must be limited by pulmonary toxicity considerations, but in an emergency decompression a greater level of pulmonary function degradation may be accepted as less harmful than decompression sickness, as the pulmonary function usually recovers without treatment.
Oxygen decompression The use of pure oxygen for accelerated decompression is limited by
oxygen toxicity. In open circuit scuba the upper limit for oxygen partial pressure is generally accepted as 1.6 bar, equivalent to a depth of 6 msw (metres of sea water), but in-water and surface decompression at higher partial pressures is routinely used in surface supplied diving operation, both by the military and civilian contractors, as the consequences of CNS oxygen toxicity are considerably reduced when the diver has a secure breathing gas supply. US Navy tables (Revision 6) start in-water oxygen decompression at 30 fsw (9 msw), equivalent to a partial pressure of 1.9 bar, and chamber oxygen decompression at 50 fsw (15 msw), equivalent to 2.5 bar.
Repetitive dives Any dive which is started while the tissues retain residual inert gas in excess of the surface equilibrium condition is considered a repetitive dive. This means that the decompression required for the dive is influenced by the diver's decompression history. Allowance must be made for inert gas preloading of the tissues which will result in them containing more dissolved gas than would have been the case if the diver had fully equilibrated before the dive. The diver will need to decompress longer to eliminate this increased gas loading.
Repetitive group A repetitive group is a designation applied to a diver with residual gas loading after a dive, who intends to do a repetitive dive using decompression tables. As the gas loading reduces during the surface interval, the group designation changes according to a specified algorithm, generally laid out as a table for convenience, and considered part of the decompression tables. The repetitive group at the end of the surface interval is used to estimate residual tissue loading (residual nitrogen time) at the start of the following dive.
Surface interval The surface interval (SI) or surface interval time (SIT) is the time spent by a diver at surface pressure after a dive during which inert gas which was still present at the end of the dive is further eliminated from the tissues. This continues until the tissues are at equilibrium with the surface pressures. This may take several hours. In the case of the US Navy 1956 Air tables, it is considered complete after 12 hours. The US Navy 2008 Air tables specify up to 16 hours for normal exposure. However, other algorithms may require more than 24 hours to assume full equilibrium.
Residual nitrogen time For the planned depth of the repetitive dive, a bottom time can be calculated using the relevant algorithm which will provide an equivalent gas loading to the residual gas after the surface interval. This is called "residual nitrogen time" (RNT) when the gas is nitrogen. The RNT is added to the planned "actual bottom time" (ABT) to give an equivalent "total bottom time" (TBT), also called "total nitrogen time" (TNT), which is used to derive the appropriate decompression schedule for the planned dive. Equivalent residual times can be derived for other inert gases. These calculations are done automatically in personal diving computers, based on the diver's recent diving history, which is the reason why personal diving computers should not be shared by divers, and why a diver should not switch computers without a sufficient surface interval (more than 24 hours in most cases, up to 4 days, depending on the tissue model and recent diving history of the user). Residual inert gas can be computed for all modeled tissues, but repetitive group designations in decompression tables are generally based on only the one tissue, considered by the table designers to be the most limiting tissue for likely applications. In the case of the US Navy Air Tables (1956) this is the 120-minute tissue, while the Bühlmann tables use the 80-minute tissue.
Decompression at altitude The atmospheric pressure decreases with altitude, and this has an effect on the absolute pressure of the diving environment. The most important effect is that the diver must decompress to a lower surface pressure, and this requires longer decompression for the same dive profile. A second effect is that a diver ascending to altitude, will be decompressing en route, and will have residual nitrogen until all tissues have equilibrated to the local pressures. This means that the diver should consider any dive done before equilibration as a repetitive dive, even if it is the first dive in several days. The US Navy diving manual provides repetitive group designations for listed altitude changes. These will change over time with the surface interval according to the relevant table. Altitude corrections (Cross corrections) are described in the US Navy diving manual. This procedure is based on the assumption that the decompression model will produce equivalent predictions for the same pressure ratio. The "Sea Level Equivalent Depth" (SLED) for the planned dive depth, which is always deeper than the actual dive at altitude, is calculated in inverse proportion to the ratio of surface pressure at the dive site to sea level atmospheric pressure. :
Sea level equivalent depth = Actual depth at altitude × Pressure at sea level ÷ Pressure at altitude Decompression stop depths are also corrected, using the ratio of surface pressures, and will produce actual stop depths which are shallower than the sea level stop depths. :
Stop depth at altitude = Stop depth at sea level × Pressure at altitude ÷ Pressure at sea level These values can be used with standard open circuit decompression tables, but are not applicable with constant oxygen partial pressure as provided by closed circuit rebreathers. Tables are used with the sea level equivalent depth and stops are done at the altitude stop depth. The decompression algorithms can be adjusted to compensate for altitude. This was first done by Bühlmann for deriving altitude corrected tables, and is now common on diving computers, where an altitude setting can be selected by the user, or altitude may be measured by the computer if it is programmed to take surface atmospheric pressure into account. == Flying and ascent to altitude after diving ==