Decompression equipment


There are several categories of decompression equipment used to help divers decompress, which is the process required to allow ambient pressure divers to return to the surface safely after spending time underwater at higher ambient pressures.
Decompression obligation for a given dive profile must be calculated and monitored to ensure that the risk of decompression sickness is controlled. Some equipment is specifically for these functions, both during planning before the dive and during the dive. Other equipment is used to mark the underwater position of the diver, as a position reference in low visibility or currents, or to assist the diver's ascent and control the depth.
Decompression may be shortened by breathing an oxygen-rich "decompression gas" such as a nitrox blend or pure oxygen. The high partial pressure of oxygen in such decompression mixes produces the effect known as the oxygen window. This decompression gas is often carried by scuba divers in side-slung cylinders. Cave divers who can only return by a single route, can leave decompression gas cylinders attached to the guideline at the points where they will be used. Surface-supplied divers will have the composition of the breathing gas controlled at the gas panel.
Divers with long decompression obligations may be decompressed inside gas filled hyperbaric chambers in the water or at the surface, and in the extreme case, saturation divers are only decompressed at the end of a project, contract, or tour of duty that may be several weeks long.

Planning and monitoring decompression

Equipment for planning and monitoring decompression includes decompression tables, depth gauges, timers, surface computer software, and personal decompression computers. There is a wide range of choice.

Decompression algorithms

A decompression algorithm is used to calculate the decompression stops needed for a particular dive profile to reduce the risk of decompression sickness occurring after surfacing at the end of a dive. The algorithm can be used to generate decompression schedules for a particular dive profile, decompression tables for more general use, or be implemented in dive computer software.

Choice of tables or algorithms

During the 1980s the US recreational diving community tended to move away from the US Navy tables to a range of tables published by other organisations, including several of the diver certification agencies.
Depending on the table or computer chosen the range of no-decompression limits at a given depth on air can vary considerably, for example for 100 fsw the no stop limit varies from 25 to 8 minutes. It is not possible to discriminate between "right" and "wrong" options, but it is considered correct to say that the risk of developing DCS is greater for the longer exposures and less for the shorter exposures.
The choice of tables for professional diving use is generally made by the organisation employing the divers. For recreational training it is usually prescribed by the certifying agency, but for recreational purposes the diver is generally free to make use of any of the published tables, and for that matter, to modify them to suit himself or herself.

Decompression tables

Dive tables or decompression tables are tabulated data, often in the form of printed cards or booklets, that allow people to determine a decompression schedule for a given dive or other hyperbaric exposure profile and breathing gas.
Decompression tables represent procedures recommended for decompression from hyperbaric exposures to 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 animal and human subjects, or combinations thereof, and may be empirically adjusted to reduce risk or improve 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. Some dive tables also assume physical condition or acceptance of a specific level of risk from the diver. Some recreational 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.

Commonly used decompression tables

The Recreational Dive Planner is a set of devices marketed by PADI with which no-stop time underwater can be calculated. The RDP was developed by DSAT and was the first dive table developed exclusively for recreational, no stop diving. There are four types of RDPs: the original table version first introduced in 1988, The Wheel version, the original electronic version or eRDP introduced in 2005 and the latest electronic multi-level version or eRDPML introduced in 2008.
The low price and convenience of many modern dive computers mean that many recreational divers only use tables such as the RDP for a short time during training before moving on to use a diving computer.

Decompression software

Decompression software such as Departure, DecoPlanner, Ultimate Planner, Z-Planner, V-Planner and GAP are available, which simulate the decompression requirements of different dive profiles with different gas mixtures using decompression algorithms.
Decompression software can be used to generate tables or schedules matched to a diver's planned dive profile and breathing gas mixtures. The usual procedure is to generate schedules for the intended profile and for the most likely contingency profiles, such as slightly greater depth, delayed ascent and early ascent. Sometimes an emergency minimum decompression schedule and a more conservative schedule will be generated to allow the diver further options.
Decompression software is available based on:
and variations of these.
V-Planner runs the variable permeability model, developed by D.E. Yount and others in 2000, and allows the choice of VPM-B and VPM-B/E, with six conservatism levels. GAP allows the user to choose between a multitude of Bühlmann-based algorithms and the full reduced gradient bubble model, developed by Bruce Wienke in 2001, in its five conservatism levels.

Personal decompression computers

The personal decompression computer, or dive computer, is a small computer designed to be worn by a diver during a dive, with a pressure sensor and an electronic timer mounted in a waterproof and pressure resistant housing and which has been programmed to model the inert gas loading of the diver's tissues in real time during a dive. Most are wrist mounted, but a few are mounted in a console with the submersible pressure gauge and possibly other instruments. A display allows the diver to see critical data during the dive, including the maximum and current depth, duration of the dive, and decompression data including the remaining no decompression limit calculated in real time for the diver throughout the dive. Other data such as water temperature and cylinder pressure are also sometimes displayed. The dive computer has the advantages of monitoring the actual dive, as opposed to the planned dive, and does not assume on a "square profile" – it dynamically calculates the real profile of pressure exposure in real time, and keeps track of residual gas loading for each tissue used in the algorithm.
Dive computers also provide a measure of safety for divers who accidentally dive a different profile to that originally planned. If the diver exceeds a no-decompression limit, decompression additional to the ascent rate will be necessary. Most dive computers will provide the necessary decompression information for acceptably safe ascent in the event that the no-decompression limits are exceeded.
The use of computers to manage recreational dive decompression is becoming the standard and their use is also common in occupational scientific diving. Their value in surface supplied commercial diving is more restricted, but they can usefully serve as a dive profile recorder.

Decompression using a personal decompression computer

The personal decompression computer provides a real time modelling of the inert gas load on the diver according to the decompression algorithm programmed into the computer by the manufacturer, with possible personal adjustments for conservatism and altitude set by the user. In all cases the computer monitors the depth and elapsed time of the dive, and many allow user input specifying the gas mixture.
Most computers require the diver to specify the mixture before the dive, but some allow the choice of mixture to be changed during the dive, which allows for the use of gas switching for accelerated decompression. A third category, mostly used by closed circuit rebreather divers, monitors the partial pressure of oxygen in the breathing mix using a remote oxygen sensor, but requires diver intervention to specify the inert gas constituents and ratio of the mix in use.
The computer retains the diver's pressure exposure history, and continuously updates the calculated tissue loads on the surface, so the current tissue loading should always be correct according to the algorithm, though it is possible to provide the computer with misleading input conditions, which can nullify its reliability.
This ability to provide real-time tissue loading data allows the computer to indicate the diver's current decompression obligation, and to update it for any permissible profile change, so the diver with a decompression ceiling does not have to decompress at any specific depth provided the ceiling is not violated, though the decompression rate will be affected by the depth. As a result, the diver can make a slower ascent than would be called for by a decompression schedule computed by the identical algorithm, as may suit the circumstances, and will be credited for gas elimination during the slower ascent, and penalised if necessary for additional ingassing for those tissues affected. This provides the diver with an unprecedented flexibility of dive profile while remaining within the safety envelope of the algorithm in use.