*If the inert gas pressure of a tissue is less than its maximum tolerated value for the water surface, it is possible to ascend directly to the surface. But how do we determine the maximum depth to which a diver may ascend when the no-decompression time has expired?*

This can be done by calculating the supersaturation tolerance using the formula

Ptol = Pamb * 1/b + a

As we have seen, the inert gas pressure (Ptol) that is tolerated by a tissue without bubble formation can be calculated for any depth or ambient pressure (Pamb). If the pressure at the water surface (1 bar) was utilized to calculate the no-decompression time for Pamb, the respective ambient pressure is used for the decompression stages.

This results in the following maximum tolerated inert gas pressures (Ptol) for the respective decompression stages for compartment 2:

9 m – deco stage: 3.92 bar (ambient pressure Pamb 1.9 bar)

6 m – deco stage: 3.46 bar (ambient pressure Pamb 1.6 bar)

3 m - deco stage: 3.00 bar (ambient pressure Pamb 1.3 bar)

surface: 2.54 bar (ambient pressure Pamb 1 bar)

Figure 1 shows this for compartment 2 for a dive with compressed air to 40 m for dive minute 20. At this time, the inert gas pressure in compartment 2 is 3.34 bar. As this value exceeds the maximum tolerated inert gas pressures for the water surface and the 3 m decompression stage, but remains below the corresponding values for the 6 m and 9 m decompression stages, it is permitted to ascend to 6 m, but no further.

We now assume that the ascent will take place at diving minute 20 and we observe 2 different compartments (comp. 2 and 6):

Already during the ascent, compartment 2 desaturates due to its very short half-life to such an extent that it may continue to ascend to the 3m deco level at minute 23 (Ptol < 3.0 bar). On the opposite, a further saturation takes place in compartment 6, reaching an inert gas pressure of 1.89 bar at dive minute 23. This exceeds the tolerated pressure for the water surface for this compartment (1.78 bar), but is less than that for the 3m decompression stage (2.13 bar). Decompression must therefore be carried out at 3 m. The inert gas pressure in compartment 2 continues to drop rapidly and falls below the tolerable inert gas pressure for the surface at minute 26 (2.54 bar). Meanwhile, the inert gas pressure in compartment 6 drops somewhat more slowly to 1.76 bar by minute 29, at which point it drops below the tolerated pressure for the water surface (1.78 bar). After a decompression time of 6 min. at 3 m, it is therefore safe to surface and the dive can be ended after 30 min.

For each deco stop, this makes it easy to determine when you are allowed to ascend to the next deco stage.

For NERDS: Calculating the deco time is somewhat more difficult when diving with mixed gases (trimix), as 2 different half-lives (for nitrogen and helium) have to be taken into account and therefore 2 logarithms have to be used for the calculation. Today, dive computers can do this easily by iterating over the saturation equation in small steps, taking into account the nitrogen and helium kinetics, and thus determining the deco time.

Iif you know how to calculate the supersaturation tolerance, determining the decompression stages is no rocket science.

But why are the decompression stages set at 3 m intervals? At the end of the 19th century, John Scott Haldane was the first to develop tables for safe decompression procedures. He calculated in feet with deco steps at 10, 20, 30 etc. feet of water. The corresponding decompression levels of 3 m, 6 m, 9 m etc. have survived until the present day.

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