Dive computer
A dive computer, personal decompression computer or decompression meter is a device used by an underwater diver to measure the elapsed 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. A secondary function is to record the dive profile, warn the diver when certain events occur, and provide useful information about the environment. Dive computers are a development from decompression tables, the diver's watch and depth gauge, with greater accuracy and the ability to monitor dive profile data in real time.
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 minimum 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, and some monitor the pressure remaining in the scuba cylinders. Audible alarms may be available to warn the diver when exceeding the no-stop limit, the maximum operating depth for the breathing gas mixture, the recommended ascent rate, decompression ceiling, or other limit beyond which risk increases significantly.
The display provides data to allow the diver to avoid obligatory decompression stops, or to decompress relatively safely, and includes depth and duration of the dive. This must be displayed clearly, legibly, and unambiguously at all light levels. 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. Profile information may also be useful to help decide a suitable treatment for an injured diver, so the computer should accompany the injured diver to the treatment facility.
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. It is one of the most expensive pieces of diving equipment owned by most divers. Use by professional scuba divers is also common, but 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. Some freedivers use another type of dive computer to record their dive profiles and give them useful information which can make their dives safer and more efficient, and some computers can provide both functions, but require the user to select which function is required.
Purpose
The primary purpose of a decompression computer is to facilitate safe decompression by an underwater diver breathing a suitable gas at ambient pressure, by providing information based on the recent pressure exposure history of the diver that allows an ascent with acceptably low risk of developing decompression sickness. Dive computers address the same problem as decompression tables, but are able to perform a continuous calculation of the theoretical partial pressure of inert gases in the body based on the actual depth and time profile of the diver and the decompression model used by the computer. 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 ambient temperature, partial pressure of oxygen in the breathing gas at ambient pressure, accumulated oxygen toxicity exposure data, 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. The exposure record may also help guide decisions on appropriate treatment for an injured diver, if it is made available to the relevant medical personnel.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 at acceptable risk. For example, a recreational diver who plans to stay within "no-decompression stop" 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 then ascending directly. Multi-level dives can be pre-planned with traditional dive tables or personal computer and smartphone apps, or on the fly using waterproof dive tables, but the additional calculations become complex, and the plan may be cumbersome to follow, and the risk of errors rises with profile complexity. Computers allow for a certain amount of spontaneity during the dive, and automatically take into account deviations from the dive plan.
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.
Components
Some components are common to all models of dive computer as they are essential to the basic function. Some have more than one commonly accepted name:Additional components may be necessary for additional or extended features and functionality.
Function
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 scuba 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.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 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.
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 computers have some way for the user to adjust decompression conservatism. This may be by way of a personal factor, which makes an undisclosed change to the algorithm decided by the manufacturer, or the setting of gradient factors, 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.
Algorithms
The decompression algorithms used in dive computers vary between manufacturers and computer models. 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 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.The algorithm used is intended to inform the diver of a decompression profile that will keep the risk of decompression sickness 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. 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.