Author: Laura Vernotico
There are two approaches to calculating decompression:
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the system compartmental (developed by Prof. Buhlman starting from the studies of Prof. Haldane, creator of the US Navy tables),
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the system to control the formation and development of bubbles (Varying Permeability Model – VPM, Reduced Bubble Gradient Model – RGBM, Tissue Bubble Dynamics Model – TBDM)
Let's apply this reasoning to a dive computer that uses a compartmental algorithm. Let's keep in mind the partial pressure formula for nitrogen gas:
PN2 = FN2 x P
Where:
PN2 = partial pressure of nitrogen
FN2 = fraction of nitrogen
P = absolute pressure
Each time the diver begins an ascent, the computer calculates how much nitrogen is in the compartment it's considering. If it's less than 1,6 bar, it gives the OK to ascend. Diving within the no-decompression limits means that for each depth, the maximum time spent at the bottom is such that upon reaching the surface, there is a maximum of 1,6 bar of nitrogen in our bodies. If the computer determines that by ascending directly to the surface, there are more than 1,6 bar of nitrogen in our bodies (and therefore bubbles form), it plans the stops. How does the computer make these calculations? To determine if it's possible to ascend to at least three meters, it must determine if there is less than 2,1 millibars of nitrogen (0,8 bar + 1,3 bar). If it's more, then you'll need to stop at 6 meters. If it's more than 2,4 bar (0,8 bar + 1,6 bar), then you'll need to make the stop at 9 meters. This tells us that the more fabrics a computer takes into consideration and the greater the number of samples it takes per minute, the more precise the analysis performed and therefore the reliability of the computer will be.
Dive computers that use a "bubble-based" algorithm predict that as soon as the diver begins to ascend (the external pressure on the body decreases and accumulated nitrogen escapes from the tissues into the blood), 90% of the nitrogen contained in the tissues diffuses into the blood to be eliminated through exhalation, while 10% enters the bubbles that are always present in the circulation. If the diver descends too quickly from the bottom, a pressure difference is created between the tissues and the external environment, which favors the entry of nitrogen into the bubble nuclei, which then expand.
If we want to translate these theoretical concepts into practical terms, we can say that the market offers different types of computers that support one decompression model rather than another. However, this is not the time to address the issue of choosing a dive computer; we'll leave that topic for another chapter in our journey into the world of diving.
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