Editing Decompression sickness (section)

=== Underwater diving ===
{{See also|Decompression practice}}
[[File:Aladin-pdc.jpg|thumb|alt=Close-up of the LCD display of an Aladin Pro|The display of a basic personal dive computer shows depth, dive time, and decompression information.]]
[[File:Turning the bezel on a diving watch to mark a point in time.webm|thumb|Video: Setting the [[bezel (jewellery)|bezel]] of a diving watch to the start time of the dive at the beginning. Divers used this in conjunction with a depth gauge and a decompression table to calculate the remaining safe dive time during dives. Dive computers rendered this cumbersome procedure unnecessary.]]

To prevent the excess formation of bubbles that can lead to decompression sickness, divers limit their ascent rate—the recommended ascent rate used by popular decompression models is about {{convert|10|m|ft}} per minute—and follow a decompression schedule as necessary.{{sfn|Hamilton & Thalmann|p=471}} This schedule may require the diver to ascend to a particular depth, and remain at that depth until sufficient inert gas has been eliminated from the body to allow further ascent.{{sfn|Hamilton & Thalmann|p=455}} Each of these is termed a "[[decompression stop]]", and a schedule for a given bottom time and depth may contain one or more stops, or none at all. Dives that contain no decompression stops are called "no-stop dives", but divers usually schedule a short "[[safety stop]]" at {{convert|3 to 6|m|ft|0|abbr=on}},  depending on the training agency or dive computer.{{sfn|Hamilton & Thalmann|p=471}}{{efn|name=stops}}

The decompression schedule may be derived from [[decompression tables]], [[Dive tables#Alternatives|decompression software]], or from [[dive computers]], and these are generally based upon a mathematical model of the body's uptake and release of inert gas as pressure changes. These models, such as the Bühlmann decompression algorithm, are modified to fit empirical data and provide a decompression schedule for a given depth and dive duration using a specified breathing gas mixture.{{sfn|Hamilton & Thalmann|pp=456–457}}

Since divers on the surface after a dive may still have excess inert gas in their bodies, decompression from any subsequent dive before this excess is eliminated needs to modify the schedule to take account of the residual gas load from the previous dive. This will result in a shorter allowable time under water without obligatory decompression stops, or an increased decompression time during the subsequent dive. The total elimination of excess gas may take many hours, and tables will indicate the time at normal pressures that is required, which may be up to 18 hours.{{sfn|Hamilton & Thalmann|pp=471–473}}<!-- need to define [[bottom time]], [[decompression obligation]], [[surface interval]] by expanding [[Dive tables]] -->

Decompression time can be significantly shortened by breathing mixtures containing much less inert gas during the decompression phase of the dive (or pure [[oxygen]] at stops in {{convert|6|m|ft}} of water or less). The reason is that the inert gas outgases at a rate proportional to the difference between the [[partial pressure]] of inert gas in the diver's body and its partial pressure in the breathing gas; whereas the likelihood of bubble formation depends on the difference between the inert gas partial pressure in the diver's body and the ambient pressure. Reduction in decompression requirements can also be gained by breathing a [[nitrox]] mix during the dive, since less nitrogen will be taken into the body than during the same dive done on air.{{sfn|Hamilton & Thalmann|pp=474–475}}

Following a decompression schedule does not completely protect against DCS. The algorithms used are designed to reduce the probability of DCS to a very low level, but do not reduce it to zero.{{sfn|Hamilton & Thalmann|p=456}} The mathematical implications of all current decompression models are that provided that no tissue is ingassing, longer decompression stops will decrease decompression risk, or at worst not increase it. Efficient decompression requires the diver to ascend fast enough to establish as high a decompression gradient, in as many tissues, as safely possible, without provoking the development of symptomatic bubbles. This is facilitated by the highest acceptably safe oxygen partial pressure in the breathing gas, and avoiding gas changes that could cause counterdiffusion bubble formation or growth. The development of schedules that are both safe and efficient has been complicated by the large number of variables and uncertainties, including personal variation in response under varying environmental conditions and workload, attributed to variations of body type, fitness and other risk factors.