Dive Watch Computer

A dive computer or decompression meter is a device used by a scuba diver to measure the time and depth of a dive so that a safe ascent rate can be calculated and displayed so that the diver can avoid decompression sickness.

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, for example, the water temperature, data used to help prevent oxygen toxicity or the pressure of the remaining breathing gas in the diving cylinder.

Because of the computer's ability to continually re-calculate based on changing data, the diver benefits by being able to safely remain underwater for longer periods of time. 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 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.

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. It is interesting to consider that 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.

In 1965, Stubbs and Kidd applied their decompression model to a pneumatic analogue decompression computer.

The first recreational mechanical analogue dive computer, the "decompression meter" was marketed by Scubapro in 1972. 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), whose pressure was measured by a bourdon tube whose graduation indicated decompression status. The device functioned so poorly that it was eventually nicknamed "bend-O-meter".

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.

The Hans Hass DecoBrain and Orca Edge are early examples—from 1982 onwards—of digital electronic dive computers that became available for recreational divers.

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 an underwater communication system that enables divers to transmit text messages, also featuring SOS and homing capabilities, and digital 3D compass.

Display information

Dive computers provide a variety of visual dive information to the diver.

Modern dive computers display the following information on a LCD:

• Current depth.
• Maximum depth reached on this dive.
• No stop time, the time remaining at the current depth without the need for decompression stops on ascent.
• Dive time, often measured from the end of the last longer surface interval.

Many dive computers also display additional information:

• Required decompression stop depth and time.
• Water temperature.
• Ascent rate.
• Dive profile (often not displayed during the dive, but transmitted to a personal computer).

Some computers are designed to display information from a diving cylinder pressure sensor, such as:

• Gas pressure.
• Estimated remaining time based on available gas and rate of gas consumption.

Some information is only shown at the surface to avoid an information overload of the diver during the dive:

• "Time to Fly" display showing when the diver can safely board an airplane.
• A log of key information about previous dives.
• Maximum non-decompression bottom times for subsequent dives based on the partial pressure of the gases in the tissue.

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.

Operation

Modern dive computers are battery-powered computers within a watertight case. These computers track the dive profile by measuring time and pressure. All dive computers measure the surrounding pressure to estimate the partial pressure of gases in the human tissue. More advanced dive computers also include additional information into the calculations, for example, the water temperature or the diving cylinder pressure.

The computer then uses the profile and a decompression algorithm to estimate the partial pressure of inert gases that has 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.

Examples of decompression algorithms are Bühlmann tables, the Multi-Tissue Model, the Varying Permeability Model, and the Reduced Gradient Bubble Model.

Many dive computers are able to produce a safe decompression schedule for dives that take place at altitude, which requires a slower ascent than at sea level, because the computers measure the atmospheric pressure before the dive. 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.

Algorithms

Algorithms vary by model from each manufacturer but only a few algorithms have been developed, for example the Bühlmann decompression algorithm. The algorithm used may also be a modification of an existing algorithm and may be an important consideration in the choice of a dive computer. Dive computers using the same internal electronics are often marketed under an array of brand names.

The algorithm used helps to minimise a diver's risk of decompression sickness (DCS). Diving scientists use known data that is recorded and averaged from previous divers to create an algorithm. The computer measures depth and time, then uses the algorithm to determine decompression requirements and no-stop times. An algorithm tries to account for magnitude of pressure reduction, repetitive exposures, rate of ascent, and time at altitude. Some algorithms may not be able to account for age, previous injury, ambient temperature, body type, alcohol consumption, dehydration, and other factors such as patent foramen ovale, when determining the decompression requirements, while others may attempt to compensate for temperature and workload by having sensors that monitor ambient temperature and cylinder pressure.

As of 2009, the newest dive computers on the market use:

• Liquivision X1: V-Planner Live: VPM-B Varying Permeability Model and GAP for X1: Bühlmann GF (Buhlman with Gradient Factors)
• Mares: Mares-Wienke Reduced Gradient Bubble Model
• Pelagic: modified Haldanean / Diving Science and Technology Database
• Seiko: Bühlmann ZHL-12 + Randy Bohrer
• Suunto: Suunto-Wienke Reduced Gradient Bubble Model
• Uwatec: Bühlmann ZH-L8 /ADT (Adaptive), MB (Micro Bubble), PMG (Predictive Multigas)

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.

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 some '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.

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