Decompression tables, also known to divers as dive tables, are tabulated data, often in the form of printed cards or booklets, that allow people to determine a decompression schedule that is acceptably safe for a given dive or other hyperbaric exposure profile and breathing gas.
Decompression tables represent procedures recommended for decompression from hyperbaric exposures in a continuum of possibilities by schedules tabulated for discrete cases of such exposures chosen for practicality and convenience of the user. They may represent data from theoretical decompression models or empirical data from series of tests on human subjects, or combinations thereof, and may be empirically adjusted to reduce risk or improve decompression efficiency. The availability of dive computers which can apply algorithms in real time to calculate the personal decompression status of a diver have to a large extent supplanted decompression tables for recreational and scientific divers, but tables remain a practical and convenient method for deciding decompression schedules for people exposed to similar but not necessarily identical pressure profiles, particularly when they are to be decompressed as a group, such as saturation divers and compressed air workers.
With decompression tables, it is generally assumed that the hyperbaric exposure, or dive profile, is a square profile, meaning that the diver descends to maximum depth immediately and stays at the same depth until resurfacing (approximating a rectangular outline when drawn in a coordinate system where one axis is depth and the other is duration). Some dive tables also assume the physical condition or acceptance of a specific level of risk from the diver. Some recreational dive tables only provide for no-stop dives at sea level sites, but the more complete tables can take into account staged decompression dives and dives performed at altitude.
Types of table Tables have been produced for several modes of decompression, including surface oriented in-water and surface decompression profiles for air and mixed gases, with and without oxygen accelerated decompression, for constant oxygen fraction diving and for constant oxygen partial pressure diving, for repetitive dives, and for saturation diving. No-stop tables are also available for repetitive dives. Tables for nitrox diving with a single mixture and for altitude diving may be published as separate tables or as equivalent depths to be used with standard sea level air tables.
Examples of decompression tables and procedures
1% Risk tables 1% Risk tables are tables which satisfy the condition of a high probability (c0.95) of a low risk (c0.01) of producing symptomatic decompression sickness if followed according to the instructions. The condition may be stipulated for the no-stop limit, or may include profiles requiring staged decompression. Similar tables for other risk conditions like 5% risk tables have also been produced. Statistical analysis of a large number of actual relevant exposures is necessary for reliable results, and these may only be available for a subset of the desired range.
Bassett tables The Bassett tables of 1985 reduced the no stop limits of the US Navy tables as recommended by Bruce Bassett and changed the rules for use. They also added decompression stops in case of accidentally exceeding the no-stop limits. The ascent rate was reduced to 10 msw (33 fsw) per minute, and a safety stop of 3 to 5 minutes at 3 to 5 msw (10 to 17 fsw) recommended for dives deeper than 9 msw (30 fsw), and Repetitive group was calculated using total dive time instead of bottom time.
Brazilian tables For air diving and bounce diving on mixed gases, the Brazilian regulations authorise the use of US Navy tables.
Brazilian saturation procedures The Brazilian saturation procedures are laid out in NORMAM-15/DPC Revision 2, of 2016. They specify continuous decompression without a rest stop from storage depth at a constant oxygen partial pressure of 500mbar until 20msw. The Brazilian procedures are strongly influenced by Comex experience and the North Sea standards.
Brazilian procedures for decompression from saturation recognise three depth ranges: Standard saturation refers to depths less or equal to 180 msw. Deep saturation refers to maximum depths between 180 and 300 msw. Exceptional saturation refers to maximum depth between 300 and 350 msw.
British Sub-Aqua Club tables
A version of the RNPL tables was used by the British Sub-Aqua Club (BS-AC) until the production of the BSAC'88 tables in 1988.
In 1968, the Royal Naval Physiological Laboratory (RNPL) at Alverstoke, England, produced a set of Air Diving Tables which were never put general use.These tables were developed by Hempleman and Hennessey and laid out similarly to the BS-AC '88 tables, but were considerably more conservative.
The early BS-AC tables only allowed two dives per day, in comparison with those based on Buhlmann and US Navy models. An interim third dive table was produced which limited depth to 9 msw, and the BS-AC'88 tables were developed as a longer term solution. The tables comprise four sets of seven tables, with each set intended for a different range of atmospheric pressures, more than 984 mb, 898 to 984 mb, 795 to 899 mb, and 701 to 795 mb, which are equivalent to ranges of altitudes.
Each table in a set is associated with a repetitive dive status. For the first dive, table "A" for the current ambient pressure is used. After a surface interval which determines the repetitive status, the table corresponding to that repetitive status is used.
The BS-AC decided to have a new set of tables developed to replace the RNPL/BS-AC Table, as the old table tended to be misunderstood and lacked suitable flexibility for recreational diving use. They wanted tables that would sufficiently versatile to function usefully in company with divers using personal dive computers.
The new table was designed by Tom Hennessy, who modified the RNPL model slightly to take into account multiple repetitive dives, up to four per day, and the RNPL model had not been tested for multiple repetitive dives, and was said to be marginal for long deep dives. The model also included the assumption of maximum depth for safe ascent with a saturated tissue of 7msw, that bubbles form after every decompression, and affect the subsequent gas intake on repetitive dives, in which tissues may saturate more rapidly.
The model assumes different rate of gas uptake during repetitive dives while the bubbles are redissolving, after which the uptake will be similar to on the initial dive of the series, so the model treats repetitive dives differently for predicting safe ascents. The tables were designed to get more conservative as the number of dives, depth and duration increase.
Unlike the US Navy model of the time, this model does not produce a residual nitrogen value based on a single tissue compartment. Seven tables are provided to account for different profiles. Table A assumes a fresh diver, clear of residual nitrogen. The diver surfaces with a tissue code based on the dive profile, which changes during the surface interval and indicates the table to be used for the next dive. Th dive times in the tables are from start of descent of start of decompression stop, including descent and initial ascent at the specified rates. There are no 3 m stops – the last stop is at 6 m, and the final ascent to surface is to take at least one minute.
The same procedure is used for further repetitive dives, and no calculations are required.
Comparison with other tables in use at the time (US Navy, Buhlmann, and DCIEM) show that the BS-AC '88 tables are more conservative than some and less than others, and this variation is in detail such as ascent rates and stop depths and times, as well as overall. There is also variation between repetitive dives planned on these tables. The BS-AC '88 tables were criticised as being less safe, untested, and based on an unpublished model.
Bühlmann decompression tables
The Bühlmann decompression model is a neo-Haldanian model which uses Haldane's or Schreiner's formula for inert gas uptake, a linear expression for tolerated inert gas pressure coupled with a simple parameterised expression for alveolar inert gas pressure and expressions for combining nitrogen and helium parameters to model the way inert gases enter and leave the human body as the ambient pressure and inspired gas changes. Different parameter sets are used to create decompression tables and in personal dive computers to compute no-decompression limits and decompression schedules for dives in real-time, allowing divers to plan the depth and duration for dives and the required decompression stops.
The model (Haldane, 1908) assumes perfusion limited gas exchange and multiple parallel tissue compartments and uses an exponential formula for in-gassing and out-gassing, both of which are assumed to occur in the dissolved phase. Bühlmann, however, assumes that safe dissolved inert gas levels are defined by a critical difference instead of a critical ratio.
Multiple sets of parameters were developed by Swiss physician Dr. Albert A. Bühlmann, who did research into decompression theory at the Laboratory of Hyperbaric Physiology at the University Hospital in Zürich, Switzerland. The results of Bühlmann's research that began in 1959 were published in a 1983 German book whose English translation was entitled Decompression-Decompression Sickness. The book was regarded as the most complete public reference on decompression calculations and was used soon after in dive computer algorithms.
In 1986 two sets of dive tables based on the Bühlmann model for recreational divers were published for altitude ranges from sea level to 700m above sea level, and for 701 m to 2500 m The repetitive group designators were based on the 80 minute tissue compartment.
DCIEM tables
In 1984 the DCIEM (Defence and Civil Institute of Environmental Medicine, Canada) released no-decompression and decompression tables based on Kidd/Stubbs serial four-compartment model. Each compartment had a half time of about 21 minutes, and allowable supersaturation ratios of 1.92 and 1.73 for the first two compartments. The last two compartments are not considered limiting during ascent. The tables were validated with extensive ultrasonic testing.
In 1990 the DCIEM sport diving tables were released. An unusual feature of these tables is the use of a repetitive multiplier, a factor read off a table for the surface interval, which is multiplied by the planned actual dive time of the repetitive dive to give the equivalent effective dive time.
French commercial diving tables
In 1973 the French civilian Tables du Ministère du Travail 1974 (MT74) were published. In 1992 the French civilian Tables du Ministère du Travail 1992 (MT92) were published, which include air tables and saturation procedures equivalent to those in the Comex 1986 diving manual. Two options were given for saturation decompression: For depths less than and equal to 155 msw, using 600 mbar oxygen partial pressure in the chamber, and for depths greater than 155 msw using 500 mbar chamber oxygen partial pressure. When the French diving regulations were revised in 2016, the saturation procedures remained unchanged.
French Navy MN90 tables
In 1965 the French Navy GERS (Groupe d'Etudes et Recherches Sous-marines) 1965 table was published. In 1990 the French Navy Marine Nationale 90 (MN90) decompression tables published.
The mathematical model used for the development of the MN 90 tables is Haldanian, and was also used for the GERS (Groupe d'Etudes et Recherches Sous-marines) 1965 table.
German tables The German tables, also known as the Bühlmann/Hahn tables.
Huggins tables The Huggins tables were a more conservative modification of the US Navy air table with tabular layout to make calculation of repetitive dives easier.
Jeppesen tables