Editing Dive computer (section)

== Function ==
[[File:TC-en.svg|upright=1.8|thumbnail|Schematic structure of a dive computer]]
Dive computers are [[Battery (electricity)|battery]]-powered computers within a watertight and pressure resistant case. These computers track the dive profile by measuring time and [[pressure]]. All dive computers measure the ambient pressure to model the concentration of gases in the tissues of the diver. More advanced dive computers provide additional measured data and user input into the calculations, for example, the water temperature, gas composition, altitude of the water surface,<ref name="Validation workshop" /> or the remaining pressure in the diving cylinder. Dive computers suitable for calculating decompression for rebreather diving need to measure the oxygen partial pressure in the breathing loop. A dive computer may be used as the control unit for an electronically controlled closed circuit rebreather, in which case it will calculate oxygen partial pressure in the loop using the output from more than one oxygen sensor.<ref name="Predator manual" />

The computer uses the pressure and time input in a decompression [[algorithm]] to estimate the partial pressure of inert gases that have been dissolved in the diver's tissues.<ref name=UHMS44 /> Based on these calculations, the computer estimates when an acceptably low risk direct ascent to the surface is no longer possible, and what decompression stops would be needed based on the profile of the dive up to that time and recent hyperbaric exposures which may have left residual dissolved gases in the diver.<ref name=UHMS44/>

Many dive computers are able to produce a low risk decompression schedule for dives that take place at altitude, which requires longer decompression than for the same profile at sea level, because the computers measure the [[atmospheric pressure]] before the dive and take this into account in the algorithm. Many dive computers continuously monitor the pressure as long as the battery has a charge, so when divers travel before or after diving and particularly when they fly, they should transport their dive computer with them in the same pressure regime (carry on baggage, not checked in and carried in the hold) so that the computer can measure the pressure profile that their body has undergone and take it into account in consequent dives.{{citation needed|date=March 2016}} Older computers that are powered down completely when switched off will not benefit by this process.

Many computers have some way for the user to adjust [[Conservatism (diving)|decompression conservatism]]. This may be by way of a ''personal factor'', which makes an undisclosed change to the algorithm arbitrarily decided by the manufacturer, or the setting of [[gradient factor]]s, a way of reducing the permitted supersaturation of tissue compartments by specific ratios, which is well defined in the literature, leaving the responsibility for making informed decisions on personal safety to the diver.<ref name="Perdix manual" /><ref name="iX3M" />

=== Algorithms ===
<!-- [[Image:MaresM1simul.jpg|thumb|Mares M1 diving computer, showing simulated data]] -->
{{see also|Decompression theory}} 
The decompression algorithms used in dive computers vary between manufacturers and computer models. Examples of [[decompression algorithms]] are the [[Bühlmann algorithm]]s and their variants, the [[Thalmann algorithm|Thalmann VVAL18 Exponential/Linear model]], the [[Varying Permeability Model]], and the [[Reduced Gradient Bubble Model]].<ref name="Azzopardi and Sayer 2010" /> The proprietary names for the algorithms do not always clearly describe the actual decompression model. The algorithm may be a variation of one of the standard algorithms, for example, several versions of the  [[Bühlmann decompression algorithm]] are in use. The algorithm used may be an important consideration in the choice of a dive computer. Dive computers using the same internal electronics and algorithms may be marketed under a variety of brand names.<ref name="Ozygit and Egi 2012" />

The algorithm used is intended to inform the diver of a decompression profile that will keep the risk of [[decompression sickness]] (DCS) to an acceptable level. Researchers use experimental diving programmes or data that has been recorded from previous dives to validate an algorithm. The dive computer measures depth and time, then uses the algorithm to determine decompression requirements or estimate remaining no-stop times at the current depth. An algorithm takes into account the magnitude of pressure reduction, breathing gas changes, repetitive exposures, rate of ascent, and time at altitude. Algorithms are not able to reliably account for age, previous injury, ambient temperature, body type, alcohol consumption, dehydration, and other factors such as [[patent foramen ovale]], because the effects of these factors have not been experimentally quantified, though some may attempt to compensate for these by factoring in user input, and for diver peripheral temperature and workload by having sensors that monitor ambient temperature and cylinder pressure changes as a proxy.<ref name="DipnDive 2021" /> Water temperature is known to be a poor proxy for body temperature, as it does not account for the effectiveness of the diving suit or heat generated by work or active heating systems.<ref name="Pollock 2015a" />

====Choice of algorithm====

There is no conclusive evidence that any currently used algorithm  is significantly better than the others, and by selective setting of the constants, most of them can be made to produce very similar ascent and decompression profiles for a given ingassing profile. When used in the recreational diving range of no-stop exposures on factory settings, it is likely that they are all acceptably safe, though some will clearly be more conservative than others. The question of whether the diver gains anything of value from the higher conservatism is an open question, when the rate of symptomatic decompression sicknesss is very low and the external risk factors are not yet amenable to computational analysis.<ref name="Fogarty 2025" /> Some manufacturers use their unverified and  undisclosed modified algorithms as selling points, usually without making any specific claims about their effectiveness, and the user is in no position to make an educated choice due to the vagueness of the claims.<ref name="DiveGearExpress 2021" /> Some others use algorithms well-defined in the literature, allowing the user who has sufficient understanding of the specific decompression model to make an informed decision. Some of these also allow user modification of settings which modify algorithm conservatism following well defined methods such as gradient factors, further facilitating educated choice.<!--for example Shearwater -->

=====Use of algorithms by manufacturer and model=====

{{as of|2009}}, the newest dive computers on the market used:
* [[Liquivision]] X1: V-Planner Live: VPM-B [[Varying Permeability Model]] and GAP for X1: [[Bühlmann decompression algorithm|Bühlmann]] GF (Buhlman with Gradient Factors){{Citation needed|date=September 2012}}
* [[Mares (scuba gear company)|Mares]]: Mares-Wienke [[Reduced Gradient Bubble Model]]{{Citation needed|date=September 2012}}{{clarify|Is this one of the "folded RGBM" models like the Suunto-Wienke model?|date=June 2021}}
* [[Pelagic Pressure Systems]]: modified [[John Scott Haldane|Haldanean]]/[[Diving Science and Technology|DSAT]] Database or [[Bühlmann decompression algorithm|Bühlmann]] ZH-L16C(called Z+){{Citation needed|date=September 2012}}
* [[Seiko]]: [[Bühlmann decompression algorithm|Bühlmann]] ZH-L12 as modified by Randy Bohrer.<ref name="DH33" />
* [[Suunto]]: Suunto-Wienke [[Reduced Gradient Bubble Model]]. The Suunto folded RGBM is not a true RGBM algorithm, which would be computationally intensive, but a Haldanean model with additional bubble limitation factors.<ref name="Azzopardi and Sayer 2010" />
* [[Uwatec]]: [[Bühlmann decompression algorithm|Bühlmann]] ZH-L8 /ADT (Adaptive), MB (Micro Bubble), PMG (Predictive Multigas), [[Bühlmann decompression algorithm|Bühlmann]] ZH-L16 DD (Trimix){{Citation needed|date=September 2012}}
* Heinrichs Weikamp OSTC and DR5: [[Bühlmann decompression algorithm|Bühlmann]] ZH-L16 and Bühlmann ZH-L16 plus Erik Baker's [[gradient factor]]s deep stop algorithm both for open circuit and fixed set point closed circuit rebreather.{{Citation needed|date=September 2012}}

{{as of|2012}}:

* Cochran EMC-20H: 20-tissue Haldanean model.<ref name="Validation workshop" />
* Cochran VVAL-18: nine-tissue Haldanean model with exponential ongasing and linear offgassing.<ref name="Validation workshop" />
* Delta P: 16-tissue Haldanean model with VGM (variable gradient model, i.e., the tolerated supersaturation levels change during the dive as a function of the profile, but no details are provided as to how this is done).<ref name="Validation workshop" />
* Mares: ten-tissue Haldanean model with RGBM;<ref name="Validation workshop" /> the RGBM part of the model adjusts gradient limits in multiple-dive scenarios through undisclosed "reduction factors".<ref name="RGBM with Basis and Comparison" />{{rp|16–20}}
* Suunto: nine-tissue Haldanean model with RGBM;<ref name="Validation workshop" /> the RGBM part of the model adjusts gradient limits in multiple-dive scenarios through undisclosed "reduction factors".<ref name="RGBM with Basis and Comparison" />{{rp|16–20}}
* Uwatec: ZH-L8 ADT (Adaptive), MB (Micro Bubble), PMG (Predictive Multigas), ZH-L16 DD (Trimix).

{{as of|2019}}:

* Aqualung: Pelagic Z+ – a proprietary algorithm based on Bühlmann ZH-L16C algorithm.<ref name="DipnDive" />
* Cressi: Haldane and Wienke RGBM algorithm.<ref name="DipnDive" />{{clarify|Is this another pseudo RGBM modification?|date=June 2021}}
* Garmin: Bühlmann ZH-L16C algorithm.<ref name="DipnDive" />
* Oceanic: Dual Algorithm: Pelagic Z+ (ZH-L16C) and Pelagic DSAT.<ref name="DipnDive" />
* ScubaPro: ZH-L8 ADT (Adaptive), MB (Micro Bubble), PMG (Predictive Multigas), ZH-L16 DD (Trimix).
* Shearwater: Bühlmann ZH-L16C with user selectable gradient factors or optional VPM-B and VPM-B/GFS.<ref name="DipnDive" /><ref name="Perdix manual" />

{{as of|2021}}:

* Aqualung: Pelagic Z+ – a proprietary algorithm developed by Dr. John E. Lewis, based on Bühlmann ZH-L16C algorithm. Conservatism may be adjusted by altitude setting, deep stops, and safety stops.<ref name="DipnDive 2021" /> 
* Atomic: "Recreational RGBM" based on the Wienke model, using user input of age, selected risk level, and exertion level to adjust conservatism.<ref name="DipnDive 2021" /> 
* Cressi: RGBM. User settings for conservatism and optional deep and safety stops.<ref name="DipnDive 2021" /> 
* Garmin: Bühlmann ZH-L16C, with a choice of three preset conservatism settings or customisable gradient factors, and customisable safety stops.<ref name="DipnDive 2021" />
* Mares: RGBM or Bühlmann ZH-L16C GF (Gradient Factor) depending on model. Preset and customisable conservatism settings.<ref name="DipnDive 2021" />
* Oceanic: User option of dual algorithms: Pelagic Z+ (ZH-L16C) and Pelagic DSAT.<ref name="DipnDive 2021" />
* Oceans: Bühlmann ZH-L16C GF (Gradient Factor). Preset conservatism settings. 
* Ratio: Bühlmann ZH-L16B and VPM-B, user settable Gradient Factors (GFL/GFH) for Bühlmann and user settable Bubble Radius for VPM.
* ScubaPro: ZH-L16 ADT MB PMG. Predictive multi-gas modified algorithm, with various conservatism options with user inputs of experience level, age and physical condition, which are assumed to have some influence on gas elimination rate. Input from breathing rate, skin temperature and heart rate monitor is also available and can be used by the algorithm to estimate a workload condition, which is used to modify the algorithm.<ref name="DipnDive 2021" />
* Shearwater: Bühlmann ZH-L16C with optional VPM-B, VPM-B/GFS and DCIEM. The standard package is Bühlmann with user selectable gradient factors, and the option to enable VPM software which may be used in open-circuit tech and rebreather modes, or enable DCIEM which may be used in air and single-gas nitrox modes. VPM-B/GFS is a combination of the two models which applies the ceiling from the more conservative model for each stop.<ref name="DipnDive 2021" /><ref name="Scubadoc" /> The current decompression ceiling may be displayed as an option and the algorithm will calculate decompression at any depth below the ceiling. The GFS option is a hybrid that automatically chooses the decompression ceiling from the more conservative of the VPM-B profile and a Bühlmann ZH-L16C profile. For the Bühlmann profile a single gradient factor is used, adjustable over a range of 70% (most conservative) to 99% (least conservative), the default is 90%.  The DCIEM model differs from ZH-L16C and VPM which are parallel models and assume that all compartments are exposed to ambient partial pressures and no gas interchange occurs between compartments. A serial model assumes that the diffusion takes place through a series of compartments, and only one is exposed to the ambient partial pressures.<ref name="DiveGearExpress 2021" />
* Suunto: RGBM based algorithm with conservatism settings, known to be a comparatively conservative algorithm. There are various versions used in different models. The technical computers use an algorithm that claims flexibility through the use of continuous decompression, which means the current ceiling is displayed instead of a stop depth.
** RGBM
** Technical RGBM
** Fused RGBM: for deep diving, switches between "RGBM" and "Technical RGBM" for open circuit and rebreather dives to a maximum of 150&nbsp;m<ref name="DipnDive 2021" />
** Fused RGBM 2<ref>{{Cite web |url=https://www.suunto.com/Support/Suunto-rgbm-dive-algorithms/ |title=Suunto RGBM Dive Algorithms |access-date=2021-09-14 |archive-date=2021-09-14 |archive-url=https://web.archive.org/web/20210914034535/https://www.suunto.com/Support/Suunto-rgbm-dive-algorithms/ |url-status=live }}</ref>
** Bühlmann 16 GF (Gradient Factor) based on ZH-L16C<ref>{{Cite web |url=https://www.suunto.com/Support/Product-support/suunto_eon_steel_black/suunto_eon_steel_black/features/decompression-algorithms/ |title=Suunto EON Steel Black User Guide 2.5: Decompression algorithms |access-date=2021-09-18 |archive-date=2021-09-18 |archive-url=https://web.archive.org/web/20210918210657/https://www.suunto.com/Support/Product-support/suunto_eon_steel_black/suunto_eon_steel_black/features/decompression-algorithms/ |url-status=live }}</ref>

{{as of|2023}}:

:[[Shearwater Research]] has supplied dive computers to the US Navy with an exponential/linear algorithm based on the Thalmann algorithm since [[Cochran Undersea Technology]] closed down after the death of the owner. This algorithm is not as of 2024 available to the general public on Shearwater computers, although the algorithm is freely available and known to be lower risk than the Buhlmann algorithm for mixed gas and constant set-point CCR diving at deeper depths, which is the primary market for Shearwater products.<ref name="Doolette 2023" /><ref name="Blömeke 2024" />

=== 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 (Δ+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 – 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

=== Audible information ===
Many dive computers have warning buzzers that warn the diver of events such as:
* Excessive ascent rates.
* Missed decompression stops.
* Maximum operation depth exceeded.
* [[Oxygen toxicity]] limits exceeded.
* Decompression ceiling violation, or stop depth violation
Some buzzers can be turned off to avoid the noise.

=== Data sampling, storage and upload ===
Data sampling rates generally range from once per second to once per 30 seconds, though there have been cases where a sampling rate as low as once in 180 seconds has been used. This rate may be user selectable. Depth resolution of the display generally ranges between 1m and 0.1m. The recording format for depth over the sampling interval could be maximum depth, depth at the sampling time, or the average depth over the interval. For a small interval these will not make a significant difference to the calculated decompression status of the diver, and are the values at the point where the computer is carried by the diver, which is usually a wrist or suspended on a console, and may vary in depth differently to the depth of the demand valve, which determines breathing gas pressure, which is the relevant pressure for decompression computation.<ref name="Azzopardi and Sayer 2010" />

Temperature resolution for data records varies between 0.1&nbsp;°C to 1&nbsp;°C. Accuracy is generally not specified, and there is often a lag of minutes as the sensor temperature changes to follow the water temperature. Temperature is measured at the pressure sensor, and is needed primarily to provide correct pressure data, so it is not a high priority for decompression monitoring to give the precise ambient temperature in real time.<ref name="Azzopardi and Sayer 2010" />

Data storage is limited by internal memory, and the amount of data generated depends on the sampling rate. Capacity may be specified in hours of run time, number of dives recorded, or both. Values of up to 100 hours were available by 2010.<ref name="Azzopardi and Sayer 2010" /> This may be influenced by sampling rate selected by the diver.

By 2010, most dive computers had the ability to upload the data to a PC or smartphone, by cable, infrared or [[Bluetooth]] wireless connection.<ref name="Azzopardi and Sayer 2010" /><ref name="Perdix manual" />