Dive computer

A dive computer, personal decompression computer or decompression meter is a device used by an underwater diver to measure the time and depth during a dive and use this data to calculate and display an ascent profile which according to the programmed decompression algorithm, will give a low risk of decompression sickness.
Most dive computers use real-time ambient pressure input to a decompression algorithm to indicate the remaining time to the no-stop limit, and after that has passed, the decompression required to surface with an acceptable risk of decompression sickness. Several algorithms have been used, and various personal conservatism factors may be available. Some dive computers allow for gas switching during the dive.
Audible alarms may be available to warn the diver when exceeding the no-stop limit, the maximum operating depth for the gas mixture, the recommended ascent rate or other limit beyond which risk increases significantly.
The display provides data to allow the diver to avoid decompression, to decompress relatively safely, and includes depth and duration of the dive. Several additional functions and displays may be available for interest and convenience, such as water temperature and compass direction, and it may be possible to download the data from the dives to a personal computer via cable or wireless connection. Data recorded by a dive computer may be of great value to the investigators in a diving accident, and may allow the cause of an accident to be discovered.
Dive computers may be wrist-mounted or fitted to a console with the submersible pressure gauge. A dive computer is perceived by recreational scuba divers and service providers to be one of the most important items of safety equipment. Use by surface-supplied divers is less widespread, as the diver's depth is monitored at the surface by pneumofathometer and decompression is controlled by the diving supervisor.
Purpose.
Dive computers address the same problem as decompression tables, but are able to perform a continuous calculation of the partial pressure of inert gases in the body based on the actual depth and time profile of the diver. As the dive computer automatically measures depth and time, it is able to warn of excessive ascent rates and missed decompression stops and the diver has less reason to carry a separate dive watch and depth gauge. Many dive computers also provide additional information to the diver including air and water temperature, data used to help prevent oxygen toxicity, a computer-readable dive log, and the pressure of the remaining breathing gas in the diving cylinder. This recorded information can be used for the diver's personal log of their activities or as important information in medical review or legal cases following diving accidents.
Because of the computer's ability to continually re-calculate based on changing data, the diver benefits by being able to remain underwater for longer periods of time at acceptable risk. For example, a recreational diver who plans to stay within "no-decompression" limits can in many cases simply ascend a few feet each minute, while continuing the dive, and still remain within reasonably safe limits, rather than adhering to a pre-planned bottom time and ascending directly. So-called multi-level dives can be planned with traditional dive tables, but the additional calculations become complex and the plan may be cumbersome to follow. Computers allow for a certain amount of spontaneity during the dive.
Dive computers are used to safely calculate decompression schedules in recreational, scientific, and military diving operations. There is no reason to assume that they cannot be valuable tools for commercial diving operations, especially on multi-level dives.
Operation.
Dive computers are 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, or the remaining pressure in the diving cylinder.
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. Based on these calculations, the computer estimates when a direct ascent 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.
Examples of decompression algorithms are the Bühlmann algorithms and their variants, the Thalmann VVAL18 Exponential/Linear model, the Varying Permeability Model, and the Reduced Gradient Bubble Model. The propitiatory names for the algorithms do not always clearly describe the actual decompression model. 
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. 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 so that the computer can measure the pressure profile that their body has undergone.
Many computers have some way for the user to adjust decompression conservatism. This may be by way of a "personal factor", which makes an undescribed change to the algorithm decided by the manufacturer, or the setting of gradient factors, a way of reducing the permitted supersaturation of tissue compertments by specific ratios, which is well defined in the literature, leaving the responsibility for making infotmed decisions on personal safety to the diver.
Algorithms.
The decompression algorithms used in dive computers vary between manufacturers and computer models. 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 may be marketed under a variety of brand names.
The algorithm used is intended to 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 and estimate remaining no-stop times at the current depth. An algorithm takes into account the magnitude of pressure reduction, repetitive exposures, rate of ascent, and time at altitude. Most algorithms are not able to directly 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 quantified, though some may attempt to compensate for temperature and workload by having sensors that monitor ambient temperature and cylinder pressure changes.
, the newest dive computers on the market use:
As of 2019: 
Display information.
Dive computers provide a variety of visual dive information to the diver.
Most dive computers display the following information during the dive on a LCD or OLED:
Many dive computers also display additional information:
Some computers are designed to display information from a diving cylinder pressure sensor, such as:
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.
Some information is only shown at the surface to avoid an information overload of the diver during the dive:
Audible information.
Many dive computers have warning buzzers that warn the diver of events such as:
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.
Temperature resolution for data records varies between 0.1°C to 1°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. 
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. 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.
Precautions.
The ease of use of dive computers exposes the diver to other dangers. Dive computers allow divers to perform complex dives with little planning. Divers may rely on the computer instead of dive planning and monitoring.
Many dive computers have menus, various selectable options and various display modes, which are controlled by a small number of buttons. Control of the computer display differs between manufacturers and in some cases between models by the same manufacturer. The diver may need information not displayed on the default screen during a dive, and the button sequence to access the information may not be immediately obvious. If the diver becomes familiar with the control of the computer on dives where the information is not critical before relying on it for more challenging dives there is less risk of confusion which may lead to an accident.
It is possible for a dive computer to malfunction during a dive. If the diver has been monitoring decompression status and is within the no-decompression limits, a computer failure can be safely managed by simply surfacing at the recommended ascent rate, and if possible, doing a short safety stop near the surface. If, however the computer could fail while the diver has a decompression obligation, or cannot make a direct ascent, some form of backup is prudent. The dive computer can be considered safety-critical equipment when there is a significant decompression obligation, as failure without some form of backup system can expose the diver to a risk of severe injury or death.
Some organisations such as the AAUS have recommended that a dive plan should be established before the dive and then followed throughout the dive unless the dive is aborted. This dive plan should be within the limits of the "decompression tables" to increase the margin of safety, and to provide a backup decompression schedule based on the dive tables in case the computer fails underwater. The disadvantage of this extremely conservative use of dive computers is that when used this way, the dive computer is merely used as a bottom timer, and the advantages of real time computation of decompression status are sacrificed.
The main problem in establishing decompression algorithms for both dive computers and production of decompression tables, is that the gas absorption and release under pressure in the human body is still not completely understood. Furthermore, the risk of decompression sickness also depends on the physiology, fitness, condition and health of the individual diver. The safety record of most dive computers indicates that when used according to the manufacturer's instructions, and within the recommended depth range, the risk of decompression sickness is low.
A diver wishing to further reduce the risk of decompression sickness can take additional precautionary measures such as one or more of: 
Many computers go into a "lockout" mode for 24 hours if the diver violates the computer's safety limits, to discourage continued diving after an unsafe dive. While in lockout mode, these computers will not function until the lockout period has ended. When this happens underwater it will leave the diver without any decompression information at the time when it is most needed. Other computers, for example Delta P's VR3, will continue to function, providing 'best guess' functionality while warning the diver that a stop has been missed, or stop ceiling violated. The Scubapro/Uwatec Galileo technical trimix computer will switch to gauge mode at 155 m after a warning, after which the diver will get no decompression information.
A single computer shared between divers cannot accurately record the dive profile of the second diver, and therefore the decompression status will be unreliable and probably inaccurate. In the event of computer malfunction during a dive, the buddy's computer record may be the best available estimate of decompression status, and has been used as a guide for decompression in emergencies. Further diving after an ascent in these conditions exposes the diver to an unknown risk. Some divers carry a backup computer to allow for this possibility. The backup computer will carry the full recent pressure exposure history, and continued diving after a malfunction of one computer will not affect risk. It is also possible to set the conservatism on the backup computer to allow for the fastest acceptable ascent in case of an emergency, with the primary computer set for the diver's preferred risk level. Under normal circumstances the primary computer will be used to control ascent rate.
Some dive computers are extremely sensitive to decompression stop depth violations. The HS Explorer is programmed to credit time spent even slightly (0.1 metre) above the stop depth at only 1/60 of the nominal rate. Others, such as the Shearwater Perdix, will fully credit any decompression done below the calculated decompression ceiling, which may be displayed as a user selectable option, and is always equal to or shallower than the indicated stop depth. The Ratio iX3M will provide a warning if the stop depth is violated by 0.1 m or more. In many cases the user manual does not provide information on how sensitive the algorithm is to precise depth, or what penalties may be incurred by minor discrepancies.
Special purpose dive computers.
Some dive computers are able to calculate decompression schedules for breathing gases other than air, such as nitrox, pure oxygen, trimix or heliox. The more basic nitrox dive computers only support one or two gas mixes for each dive. Others support many different mixes. When multiple gases are supported, there may be an option to set those which will be carried on the dive as active, which sets the computer to calculate the decompression schedule and time to surface based on the assumption that the active gases will be used when they are optimal for decompression. Calculation of tissue gas loads will generally follow the gas actually selected by the diver, unless there is multiple cylinder pressure monitoring to enable automatic gas selection by the computer.
Most dive computers calculate decompression for open circuit scuba where the proportions of the breathing gases are constant: these are "constant fraction" dive computers. Other dive computers are designed to model the gases in closed circuit scuba (rebreathers), which maintain constant partial pressures of gases by varying the proportions of gases in the mixture: these are "constant partial pressure" dive computers. These may be switched over to constant fraction mode if the diver bails out to open circuit.
There are also dive computers which monitor oxygen partial pressure in real time in combination with a user nominated diluent mixture to provide a constantly updated mix analysis which is then used in the decompression algorithm to provide decompression information.
Additional functionality.
Some dive computers provide additional functionality, generally a subset of those listed below:
History.
The Office of Naval Research funded a project with the Scripps Institute of Oceanography for the theoretical design of a prototype decompression analog computer. The Foxboro Decomputer, Mark I was manufactured by the Foxboro Company and evaluated by the US Navy Experimental Diving Unit in 1957. Confusion between the diffusivity coefficient and the then new concept of tissue half time resulted in a device that did not properly mirror decompression status. Had this error not occurred, the U.S. Navy Tables might never have been developed, and divers might have been using instrumentation to control their dives from 1957 on.
The first recreational mechanical analogue dive computer, the "decompression meter" was designed by the Italians De Sanctis & Alinari in 1959 and built by their company named SOS, which also made depth gauges. The decompression meter was distributed directly by SOS and also by scuba diving equipment firms such as Scubapro and Cressi. It was very simple in principle: a waterproof bladder filled with gas inside a big casing bled into a smaller chamber through a semi-porous ceramic cartridge (to simulate tissue in/out gassing). The chamber pressure was measured by a bourdon tube, calibrated to indicate decompression status. The device functioned so poorly that it was eventually nicknamed "bendomatic".
In 1965, Stubbs and Kidd applied their decompression model to a pneumatic analogue decompression computer, and in 1987 Brian Hills reported development of a pneumatic analogue decompression computer modelling the thermodynamic decompression model. It modelled phase equilibration instead of the more commonly used limited supersaturation criteria and was intended as an instrument for on-site control of decompression of a diver based on real-time output from the device. Hills considered the model to be conservative.
Several analogue decompression meters were subsequently made, some with several bladders for illustrating the effect on various body tissues, but they were sidelined with the arrival on the scene of electronic computers.
In 1983, the Hans Hass-"DecoBrain", designed by "Divetronic AG" a Swiss start-up, became the first decompression diving computer, capable of displaying the information that today's diving computers do. The DecoBrain was based on A. Bühlmann's 16 compartment (ZHL-12) tissue model which Jürg Hermann, an electronic engineer, implemented in 1981 on one of Intel's first single-chip microcontrollers as part of his thesis at the Swiss Federal Institute of Technology.
The 1984 Orca EDGE was an early example of a dive computer. Designed by Craig Barshinger, Karl Huggins and Paul Heinmiller, the EDGE did not display a decompression plan, but instead showed the ceiling or the so-called "safe-ascent-depth". A drawback was that if the diver was faced by a ceiling, he did not know how long he would have to decompress. The EDGE's large, unique display, however, featuring 12 tissue bars permitted an experienced user to make a reasonable estimate of his or her decompression obligation.
In 1984 the US Navy diving computer (UDC) which was based on a 9 tissue model of Edward D. Thalmann of the Naval Experimental Diving Unit (NEDU), Panama City, who developed the US Navy tables. "Divetronic AG" completed the UDC development – as it had been started by the chief engineer Kirk Jennings of the Naval Ocean System Center, Hawaii, and Thalmann of the NEDU – by adapting the Deco Brain for US Navy warfare use and for their 9-tissue MK-15 mixgas model under an R&D contract of the US Navy.
Orca Industries continued to refine their technology with the release of the Skinny-dipper in 1987 to do calculations for repetitive diving. They later released the Delphi computer in 1989 that included calculations for diving at altitude as well as profile recording.
Even by the late 1980s, the advent of dive computers had not met with what might be considered widespread acceptance. Combined with the general mistrust, at the time, of taking a piece of electronics that your life might depend upon underwater, there were also objections expressed ranging from dive resorts felt that the increased bottom time would upset their boat and meal schedules, to that experienced divers felt that the increased bottom time would, regardless of the claims, result in many more cases of decompression sickness. Understanding the need for clear communication and debate, Michael Lang of the California State University at San Diego and Bill Hamilton of Hamilton Research Ltd. brought together, under the auspices of the American Academy of Underwater Sciences a diverse group that included most of the dive computer designers and manufacturers, some of the best known hyperbaric medicine theorists and practitioners, representatives from the recreational diving agencies, the cave diving community and the scientific diving community.
The basic issue was made clear by Andrew A. Pilmanis in his introductory remarks: "It is apparent that dive computers are here to stay, but are still in the early stages of development. From this perspective, this workshop can begin the process of establishing standard evaluation procedures for assuring safe and effective utilization of dive computers in scientific diving."
After meeting for two days the conferees were still in, "the early stages of development," and the "process of establishing standard evaluation procedures for assuring safe and effective utilization of dive computers in scientific diving," had not really begun. University of Rhode Island Diving Safety Officer Phillip Sharkey and ORCA EDGE's Director of Research and Development, prepared a 12-point proposal that they invited the Diving Safety Officers (DSO) in attendance to discuss at an evening closed meeting. Those attending included: Jim Stewart (Scripps Institution of Oceanography), Lee Somers (University of Michigan), Mark Flahan (San Diego State University), Woody Southerland (Duke University), John Heine (Moss Landing Marine Laboratories), Glen Egstrom (University of California, Los Angeles), John Duffy (California Department of Fish and Game), and James Corry (United States Secret Service). Over the course of several hours the suggestion prepared by Sharkey and Heinmiller was edited and turned into the following 13 recommendations:
As recorded in "Session 9: General discussion and concluding remarks:" 
Mike Lang next lead the group discussion to reach consensus on the guidelines for use of dive computers. These 13 points had been thoroughly discussed and compiled the night before, so that most of the additional comments were for clarification and precision. The following items are the guidelines for use of dive computers for the scientific diving community. It was again reinforced that almost all of these guidelines were also applicable to the diving community at large.
After the AAUS workshop most opposition to dive computers dissipated, numerous new models were introduced, the technology dramatically improved and use of dive computers soon became standard scuba diving equipment.
In 2001, the US Navy approved the use of Cochran NAVY decompression computer with the VVAL 18 Thalmann algorithm for Special Warfare operations.
In 2008, the Underwater Digital Interface (UDI) was released to the market. This dive computer, based on the RGBM model, includes a digital compass, an underwater communication system that enables divers to transmit preset text messages, and a distress signal with homing capabilities.
By 2010 the use of dive computers for decompression status tracking was virtually ubiquitous among recreational divers and widespread in scientific diving. 50 models by 14 manufacturers were available in the UK.
The variety and number of additional functions available has increased over the years.
Validation.
The risk of the decompression algorithms programmed into dive computers may be assessed in several ways, including tests on human subjects, monitored pilot programs, comparison to dive profiles with known decompression sickness risk, and comparison to risk models.
Performance of dive computers exposed to profiles with known human subject results..
Studies at the University of Southern California Catalina Hyperbaric Chamber ran dive computers against a group of dive profiles that have been tested with human subjects, or have a large number of operational dives on record.
The dive computers were immersed in water inside the chamber and the profiles were run. Remaining no-decompression times, or required total decompression times, were recorded from each computer 1 min prior to departure from each depth in the profile. The results for a 40 msw “low risk” multi-level no-decompression dive from the PADI/DSAT RDP test series provided a range of 26 min of no-decompression time remaining to 15 min of required decompression time for the computers tested. The computers which indicated required decompression may be regarded as conservative: following the decompression profile of a conservative algorithm or setting will expose the diver to a reduced risk of decompression, but the magnitude of the reduction is unknown. Conversely the more aggressive indications of the computers showing a considerable amount of remaining no-decompression time will expose the diver to a greater risk of unknown magnitude.
Comparative assessment and validation.
Evaluation of decompression algorithms could be done without the need for tests on human subjects by establishing a set of previously tested dive profiles with a known risk of decompression sickness. This could provide a rudimentary baseline for dive computer comparisons. As of 2012, the accuracy of temperature and depth measurements from computers may lack consistency between them making this type of research difficult.
Ergonomic considerations.
If the diver cannot effectively use the dive computer during a dive it is of no value except as a dive profile recorder.
To effectively use the device the ergonomic aspects of the display and control input system are important. Misunderstanding of the displayed data and inability to make necessary inputs can lead to life-threatening problems underwater. The operating manual is not available for reference during the dive, so either the diver must learn and practice the use of the specific unit before using it in complex situations, or the operation must be sufficiently intuitive that it can be worked out on the spot, by a diver who may be under stress at the time. Although several manufacturers claim that their units are simple and intuitive to operate, the number of functions, layout of the display, and sequence of button pressing is markedly different between different manufacturers, and even between different models by the same manufacturer. Experience using one model may be of little use preparing the diver to use a different model, and a significant relearning stage may be necessary. Both technical and ergonomic aspects of the dive computer are important for diver safety. Underwater legibility of the display may vary significantly with underwater conditions and the visual acuity of the individual diver. If labels identifying output data and menu choices are not legible at the time they are needed, they do not help.
Several criteria have been identified as important ergonomic considerations:
Operational considerations for use in commercial diving operations.
If the decompression algorithm used in a series of dive computers is considered to be acceptable
for commercial diving operations, with or without additional usage guidelines, then there are
operational issues that need to be considered:
Bottom timer.
A bottom timer is an electronic device that records the depth at specific time intervals during a dive, and displays current depth, maximum depth, elapsed time and may also display water temperature and average depth. It does not calculate decompression data at all, and is equivalent to gauge mode on many dive computers.
Manufacturers.
Other retailers sell computer clones made by Seiko (Apeks, Cressi, Dive Rite, ScubaPro, Tusa, Zeagle) or Pelagic Pressure Systems (Beuchat, Genesis, Seemann, Sherwood) or Benemec Oy (A.P.Valves).
Value.
Along with delayed surface marker buoys, dive computers stood out in a 2018 survey of European recreational divers and diving service providers as highly important safety equipment.