Currently the most popular type of dive computers are the ones worn on the wrist. There are also computers that attach to the regulator’s first stage high pressure hose / hoseless (also act as a pressure gauge) or to attach to the console along with the pressure gauge and possibly the compass, but nowadays this is the most common system used.
Like all computers, the underwater computer also has some parameters that the user must configure using the buttons on the housing, before diving, since it is usually not possible to change them during the dive:
The time and date, which will be associated with the dive parameters, will be used to complete the DiveLog (log).
Usable in salt or fresh water, to dive both in the ocean and in lakes.
Oxygen Percentage: for the use of Air (21%) or Nitrox (> 21%).
Altitude: for diving in mountain lakes.
As soon as you enter the water and begin the descent, the computer will be able to provide you with the data necessary for your dive:
Actual depth.
Dive time.
Maximum depth reached.
Water temperature.
No decompression time remaining.
All computers can also inform the diver of any abnormal or incorrect behavior with visual and audible alarms:
Excessive ascent speed, if you exceed the maximum recommended speed.
Exceed the maximum depth according to the partial pressure of oxygen allowed.
Skipped decompression stops
This last point deserves a more detailed analysis. The dive computer, in addition to providing the remaining no-decompression minutes to stay out of the non-stop curve, is capable of calculating the decompression profile if the limits are exceeded. At the end of this SNSI Advanced Open Water Diver course you will have the necessary training to be able to dive to a maximum depth of 39 meters / 130 feet within the no decompression limits. If these limits are inadvertently exceeded, the computer is able to provide you with all the useful information to enable you to perform the required decompression stops before you can return to the surface.
Once you have completed this course and have obtained the SNSI Recreational Nitrox Diver certification, SNSI recommends diving that involves decompression, you will be qualified to participate in the SNSI Recreational Deco Diver course.
Another feature that some wrist dive computers offer (often optional) is a transmitter to measure cylinder pressure. It is a device that you screw into a high pressure outlet of the first stage of the regulator that communicates wirelessly with the computer, it can provide you with the amount of residual mixture in the cylinder (like an analog manometer) and, based on your breathing rate, it offers an estimate of the gas remaining in the tank but in minutes.
Finally, it should be noted that some of the latest generation computers also include a digital underwater compass that, in addition to being used as a common analog compass, allows you to configure the “Way Points” (highlighted points) during the course of the dive in order to indicate the return route to the starting point.
The models
As we already mentioned, computers use a model (or decompression algorithm) to calculate a safe ascent: direct to the surface or, by making one or more decompression stops.
After the first model created by Haldane, in the early 20th century to the present, many scholars and scientists have developed and published their own decompression models. One of the most famous and longest-lived, still used today, is the Bühlmann model (whose development began in the late 1950s and was published in 1983) which, in addition to working with the Multi-tissue half-times (from fastest to slowest), also takes into account the “M value”, which is the maximum partial pressure of nitrogen that each theoretical compartment can hold without the formation of bubbles.
While the purpose of Haldane and the US Navy was to get the diver to the surface in the shortest amount of time possible (because these were created for military and professional diving, so each minute divers spend diving comes at a cost), the ultimate goal of Bühlmann’s research was diver safety. For this reason, your model is particularly conservative in terms of no-decompression and desaturation times.
All Haldane models consider that by respecting the decompression procedures there is no formation of bubbles and / or microbubbles at all and that therefore the absence of bubbles is synonymous with the absence of DCS. At the same time, since the 1960s, decompression models have been developed that consider the presence of bubbles as a more or less constant factor in each dive, and aim to contain the number and size of these bubbles. Indeed, as modern studies show, using cardiac Doppler ultrasound it has been observed that often, at the end of dives carried out according to standard procedures and within the safety curve, in the completely asymptomatic diver, bubbles are found in the right heart in proportions variables, the presence of bubbles in itself does not, therefore, imply the onset of decompression sickness.
VPM (Varying Permeability Model) y RGBM (Reducing Bubble Gradient Model) they are, to date, the most modern decompression models and aim to keep the gaseous nuclei below a certain critical radius and a certain critical number and were created to generate conservative decompression profiles also suitable for repetitive dives over several days.
How to Use the Dive Computer
The most important thing to remember when using a dive computer is that it cannot calculate the exact absorption and elimination of nitrogen in a person’s body. The dive computer is a tool that simulates nitrogen loading in theorized tissue compartments based on mathematical algorithms. It is not, as some believe, an “electronic brain” capable of understanding and processing unforeseen circumstances, or knowing how well a diver is hydrated or slept the night before.
Computers are unable to recognize if a diver is intoxicated, obese, taking medications or a smoker. It is the diver’s responsibility to account for these factors, and remember that unless he programmed these into the computer, it does not know about, nor account for them in its calculations. Also, it is important to follow the limitations that manufacturers have built into their dive computers. For example, the proper ascent rate is usually built into dive computers. Divers that choose not to follow the computer’s ascent guidelines, among others, may find themselves locked out of their computer for 24 hours. The lockout feature that is built into many dive computers and is usually triggered when a diver has exceeded its parameters.
Following are the main rules for the correct use of the dive computer:
Read the dive computer’s instruction manual. Every dive computer has its own set of features, some of them unique, and though, many dive computers have the same functions, each computer works differently. A good example is the rapid ascent alarm on a computer; it could be interpreted as an “okay to surface” alarm, when in reality it is telling the user to “slow down!” Read and understand your dive computer’s instructions before using it on a dive.
The fundamental purpose of a recreational scuba diving computer is to help the user avoid a decompression situation. Many dive computers are capable of calculating decompression dives; However, this type of dive is not included in the SNSI Advanced Open Water course, but rather in specific subsequent courses.
Remember, any time that you enter a decompression situation; you will not be able to surface immediately without significantly increasing the risk of DCS. Stay within your computer’s recommended no-decompression limits.
One diver, one computer. Every diver participating in a dive must have their own computer.
A dive computer should not be shared between divers, or dive buddies. Even if the dive team stays together for the entire dive, the computer only knows what its wearer has done before, during and after every dive. As such, the information the computer provides is valid for the wearer alone.
Follow the most conservative computer. As we have mentioned, each diver must have their own computer, even if diving is an activity that must always be done with a buddy. However, your buddy might have a different computer or have a slightly different profile than you (even by just one meter or staying just one more minute at a different depth than yours), so the no-decompression times may not match. In this case, both divers will follow the more conservative computer, ascending together, following its two prompts, even if the other computer is less conservative.
Respect the ascent speed. All computers have set a maximum ascent rate of approximately 9 meters / 30 feet per minute.
During the ascent, a diver is actually eliminating nitrogen from his tissues; if a diver ascends too fast, the nitrogen may form bubbles within the body. Research has shown that a slow ascent can help reduce, or eliminate micro-bubbles that sometimes form due to a sudden reduction of pressure during ascent.
Modern dive computers are programmed to provide data based on the diver following the predetermined ascent rate. An audible alarm will sound in conjunction with a visual alert if the computer senses that the diver is ascending faster than the recommended rate.
A safety stop should be a standard part of every dive. Some dive computers build the procedure into their programming, having you halt your ascent at a depth of 3 to 5 meters (between 10 and 15 feet) for 3 minutes at the conclusion of every dive. However, some computers do not have this provision. Even if yours does not require a safety stop as a standard procedure, make one anyway. It can do no harm, and may reduce your risk of decompression sickness.
Computer Failure. It happens very rarely, but the computer of one of the two dive buddies could shut down (for example due to a low battery that we forgot to replace or recharge) or suffers a breakdown during the dive. In this case, it will be mandatory to abandon the dive and return to the surface, following the indications of the computer that works, that of the other buddy and making a safety stop. If the rules of recreational diving have been followed, the computer crash problem can be solved easily and without consequences.
Follow the correct profiles. Plan and complete the deepest part of the dive first, beginning with a descent to the maximum depth and then continuing with a gradual ascent to shallower depths.
For repetitive dives within the same day, the deepest dive must be made first and each subsequent dive should be shallower than the previous one. Repetitive deep dives are not recommended.
Use Common Sense. We have already discussed that a computer is unable to factor in conditions it does not know about. Some of these factors increase a diver’s susceptibility to DCS even when diving within the recommendations of a dive computer. It is each diver’s responsibility to use common sense if these conditions exist.
Avoid pushing the limits of the computer; surface before the computer’s recommended time limit expires, ascend at the proper rate, avoid becoming fatigued and do not conduct decompression dives. Do not dive if you are physically sick, or if your mind is not as sharp as it should be.
Technology has revolutionized the way we dive. Dive computers allow us to experience the underwater world for longer periods of time, and help us manage our theoretical nitrogen tissue compartments.
However, as we have said many times, dive computers are not able to think autonomously. They can’t feel or understand specific physiological changes. They are simply very fast calculators based on mathematical models. It is up to the diver to use his dive computer responsibly, with good sense and conservatively.
Most importantly, every diver must understand that following a dive computer’s rules, recommendations and, or guidelines does not guarantee that he will not suffer from decompression sickness.
The best advise a diver should follow is to responsibly manage the risks through proper training and using common sense.