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Dive computer
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=== Display information === [[File:TechDiving NOAA.jpg|thumb|upright=1.2|[[Technical diving|Technical diver]] wearing a dive computer on his left wrist during a decompression stop.]]<!-- Not a very useful image, replace if something better becomes available --> [[Image:Suunto D9 Dive Computer.jpg|thumb|A [[watch]] sized dive computer incorporating an electronic compass and the ability to display cylinder pressure when used with an optional transmitter ([[Suunto]] D9)]] [[File:IDive DAN dive computer dive log P3050610.JPG|thumb|Dive computer dive profile display]] [[File:High PO2 warning on Shearwater Perdix PA190199.jpg|thumb|High oxygen partial pressure warning on Shearwater Perdix dive computer]] [[File:Shearwater Perdix low battery warning P1080460.jpg|thumb|Shearwater Perdix dive computer low battery warning display]] Dive computers provide a variety of visual dive information to the diver, usually on a [[Liquid-crystal display|LCD]] or [[OLED]] display. More than one screen arrangement may be selectable during a dive, and the primary screen will display by default and contain the safety critical data. Secondary screens are usually selected by pressing one or two buttons one or more times, and may be transient or remain visible until another screen is selected. All safety critical information should be visible on any screen that will not automatically revert within a short period, as the diver may forget how to get back to it and this may put them as significant risk. Some computers use a scroll through system which tends to require more button pushes, but is easier to remember, as eventually the right screen will turn up, others may use a wider selection of buttons, which is quicker when the sequence is known, but easier to forget or become confused, and may demand more of the diver's attention, :<ref name="Perdix AI" /><ref name="iX3M" /> Most dive computers display the following basic dive profile and no-stop status information during the dive. This information includes safety critical information, and is usually displayed on the default underwater display, and some may be shown on all underwater displays:<ref name="AandA" /><ref name="Perdix manual" /> *Current depth (derived from ambient pressure). *Maximum depth reached on the current dive. *No-stop time, the time remaining at the current depth without the need for [[decompression stop]]s on ascent. *Elapsed dive time of the current dive. Many dive computers also display additional information. Some of this is safety-critical for decompression, and would usually be displayed on all screens available underwater, or have a timed default return to the primary screen: Most of the non-critical information is likely to be useful on at least some dives, and may be displayed on a secondary screen layout which can be selected during the dive.<ref name="Predator manual" /> *Total ascent time, or time to surface (TTS) assuming immediate ascent at recommended rate, and decompression stops as indicated. When multiple gases are enabled in the computer, the time to surface may be predicted based on the optimum gas being selected, during ascent, but the actual time to surface will depend on the actual gas selected, and may be longer than the displayed value. This does not invalidate the decompression calculation, which accounts for the actual exposure and gas selected.<ref name="iX3M" /><ref name="Perdix AI" /> *Required decompression stop depth and time, also assuming immediate ascent at recommended rate. The depth and duration of the first stop are usually displayed prominently.<ref name="iX3M" /><ref name="Perdix manual" /> *Ambient temperature, (actually temperature of the pressure transducer). This may be a default display or a user selected setting, and may not be on the primary display, as it is not safety-critical information.<ref name="Wright et al 2016" /> *Current ascent rate. This may be displayed as an actual speed of ascent, or a relative rate compared to the recommended rate.<ref name="Perdix AI" /> *Dive profile (often not displayed during the dive, but transmitted to a personal computer). Not a safety-critical information, so usually on a temporary secondary display if available<ref name="iX3M" /> *Gas mixture in use, as selected by the user.<ref name="iX3M" /><ref name="Perdix AI" /> *Oxygen partial pressure at current depth, based on selected gas mixture.<ref name="iX3M" /><ref name="Perdix AI" /> *Cumulative oxygen toxicity exposure (CNS), computed from measured pressure and time and selected gas mixture.<ref name="iX3M" /><ref name="Perdix AI" /> *Battery charge status or low battery warning.<ref name="iX3M" /><ref name="Perdix AI" /> *Time of day, often with a 12hour or 24 hour format option.<ref name="Perdix manual" /> *Compass heading, using a flux gate sensor, with tilt corrections. When available this is usually combined with displays of all safety critical data, so that it does not have to automatically revert to the primary display layout.<ref name="Perdix AI" /> A few computers will display additional information on decompression status after the no-stop limit has been exceeded. These data may be selected as optional display settings by the diver, and may require a more comprehensive understanding of decompression theory and modelling than provided by recreational diver training. They are intended as information that may help a technical diver make a more informed decision while dealing with a contingency that affects decompression risk. <ref name="Shearwater 2020" /> *At depth + 5 minutes, (@+5), shows the effect on time to surface of remaining at the current depth on the current breathing gas for five more minutes. The display will show the amended TTS.<ref name="Shearwater 2020" /> *Delta + 5 (Ξ+5) is the change in time to surface if remaining at the same depth on the same gas for 5 minutes longer. This value will be positive if ingassing, negative if outgassing, and 0 if the extra exposure has no net effect on computed decompression obligation. This is useful for multi-level dives, where it helps estimate whether there will still be enough breathing gas for the ascent.<ref name="Shearwater 2020" /> *Decompression ceiling, the depth at which calculated supersaturation of the controlling tissue is at the maximum permissible level according to the algorithm. This is the shallowest depth to which the diver can ascend with acceptable decompression risk according to the chosen constraints. This depth will be equal to or shallower than the current obligatory stop depth and deeper than the next obligatory stop. When decompression is completed, the ceiling will be zero.<ref name="Shearwater 2020" /> *Current gradient factor (GF99), an indication of the diver's current proximity to the baseline M-value of the algorithm in the limiting tissue. If it exceeds 100% then the diver is oversaturated according to the algorithm's least conservative setting. This value will slowly decrease at each decompression stop, and increase during the ascent to the next stop. This functionality may be useful in a contingency when the diver needs to exit the water as soon as possible but at a reasonable decompression risk. Responsible use of this feature requires a good understanding of the theory of decompression and how it is modeled by the computer.<ref name="Shearwater 2020" /> *Surfacing gradient factor, The calculated gradient factor for the controlling tissue if the diver were to surface directly from the current depth, without any stops. The figure shown is a percentage of the calculated M-value at that stage of the dive. If it exceeds 99%, the risk of DCS is higher than for the baseline M-value, and if lowe, then the risk is lower than for the baseline M-value, When indicated decompression clears, it will be at the GF-Hi value the diver selected, This is an optional way of monitoring decompression status which could be useful in an emergency.<ref name="Shearwater 2020" /> Some computers, known as air-integrated, or gas-integrated, are designed to display information from a [[diving cylinder]] pressure sensor, such as: *Gas pressure.<ref name="iX3M AI" /><ref name="Perdix AI" /> *Estimated remaining air time (RAT) based on available gas, rate of gas consumption and ascent time.<ref name="iX3M AI" /><ref name="Perdix AI" /> Some computers can provide a real time display of the oxygen partial pressure in the rebreather. This requires an input from an oxygen cell. These computers will also calculate cumulative oxygen toxicity exposure based on measured partial pressure.<ref name="Predator manual" /> Some computers can display a graph of the current tissue saturation for several tissue compartments, according to the algorithm in use.<ref name="iX3M AI" /><ref name="Perdix AI" /> Some information, which has no practical use during a dive, is only shown at the surface to avoid an information overload of the diver during the dive:<ref name="Predator manual" /> *"Time to Fly" display showing when the diver can safely board an airplane. *Desaturation time, the estimated time required to return all tissues to surface pressure dissolved gas equilibrium. *A log of key information about previous dives β date, start time, maximum depth, duration, and possibly others. *Maximum non-decompression bottom times for subsequent dives based on the estimated residual concentration of the inert gases in the tissues. *Dive planning functions (no decompression time based on current tissue loads and user-selected depth and breathing gas).<ref name="HSE Manual" /> Warnings and alarms may include:<ref name="iX3M" /><ref name="Perdix manual" /> *Maximum operating depth exceeded *No decompression limit approaching *No decompression limit exceeded *Excessive ascent rate *Decompression ceiling violation *Omitted decompression *Low cylinder pressure (where applicable) *Oxygen partial pressure high or low *Maximum depth violation
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