The purpose of using dive tables or a dive computer during a dive is to help us avoid Decompression Sickness (DCS). Dive tables and computers are based on mathematical models that calculate theoretical tissue nitrogen uptake in the various tissue compartments and its elimination.
Since its first appearance in the eighties, the dive computer has been progressively taken over the Dive Tables in recreational diving.
The amount of time a diver spends at a specific depth are the primary factors that determine nitrogen absorption, while the time spent at lower pressures and on the surface determine nitrogen elimination
Mathematical models that simulate the absorption and elimination of nitrogen must account for the factors that determine the velocity of saturation and desaturation of the various tissues.
The resulting algorithm must correlate the effects of pressure and time of exposure at the determinate pressure with the velocity and quantity of nitrogen absorption and elimination as it relates to each type of tissue. The algorithm must also be based on a number of different tissue half times. Of course, the aforementioned tissues are theoretical, and simply provide a simulation of what is actually happening in the human body.
The first researcher to develop a working model was John Scott Haldane at the beginning of the 20th century. Haldane’s model later represented the theoretical basis of the United States Navy dive tables.
Since then, improvements in technology and a better understanding of decompression theory have led us to create more realistic mathematical models. Using hyperbaric chambers and Doppler ultrasound technology, researches found that Haldane’s original model allowed for the development of micro-bubbles in the body.
Many different algorithms have been studied and tested in an effort to create new, more realistic and accurate decompression models. Many models have been developed, each with subtle differences that allow dive computers to function in varied ways.
Each model, while slightly different, has its own advantages and applications.
Multi-level diving
Before discussing the different decompression models and the theories on which they are based, it is important to understand what multi-level dives are, and how they affect the application of a particular model’s algorithm.
It should be noted that the dives made using the tables should be considered “square” because the diver must calculate the dive with a direct descent to the maximum depth, staying at this depth throughout the duration of the dive and the ascent at a speed of 9 meters / 30 feet per minute, plus the 3 minute safety stop at 5 meters / 15 feet before surfacing. Furthermore, when planning the dive, if the values of the depth desired time were not present in the table, it would be necessary to round them off, to those with the next higher value.
This type of profile is very similar to that of a dive made on a wreck, which requires staying at the same depth throughout the dive.
It is different to dive on a sandy bottom, along a wall or in depths that have a wider vertical topography since they offer different marine life at different depths where it is worth stopping at multiple depths. These dives will have a “multilevel” profile: at the beginning we will descend to the maximum depth and, in anticipation of the expiration of the no-decompression time limit, we will ascend to depths that gradually decrease to spend a few minutes exploring, and continuing the dive at different levels, decreasing the depths, until reaching the safety stop.
For multilevel profiles, the use of tables is certainly an outdated and inappropriate system when performing, for example, a dive at 30 meters / 100 feet for 5 minutes with a stop at 12 meters / 40 feet for 15 minutes does not result in the same degree nitrogen absorption as a dive at 30 meters / 100 feet for 20 minutes, if we would calculate with the tables. This in fact indicates the no decompression limit of a normal dive, but does not allow us to know how those limits change when ascending to shallower depths. A computer, “adapts” to different depths by continually calculating the new no-decompression limit as you vary depths (in real time), allowing you to dive longer.
Dive planning with dive tables involves laborious mathematical calculations that are now performed automatically and in a few seconds using dive computers. If we add to this the ease of use, this explains its diffusion and popularity among divers of all levels.