No decompression limit (NDL) means that it is possible to ascend from depth directly to the surface without affecting the supersaturation tolerance in a tissue compartment and causing bubble formation or decompression sickness.

As we have seen in the previous articles, the supersaturation tolerance of a tissue in Bühlmann's (and Keller's) model can be calculated very easily using the inert gas pressure in the tissue and two tissue-specific coefficients (A.A. Bühlmann. Dekompression - Dekompressionskrankheit. Springer 1983).

The formula

provides the tolerated ambient pressure (Pamb.tol) for a specific inert gas pressure (p) in the tissue of a compartment. Because the inert gas pressure (p) varies during a dive as a result of saturation or desaturation, the tolerated ambient pressure is also permanently changing. As long as it is lower than the ambient pressure at the water surface, you can ascend directly without decompression stops. The dive is within the NDL. Graphically, this looks as outlined below:

We make a dive to 40 m with compressed air and look at 2 compartments. Compartment 2 saturates nitrogen faster than compartment 7. The ambient pressure at the water surface is 1 bar. The curve for the tolerated ambient pressure in compartment 2 intersects the 1 bar axis after 10 min. Because the tolerated ambient pressure then exceeds 1 bar, it is no longer permitted to ascend directly to the water surface.

The graph for the maximum tolerated ambient pressure in compartment 7 intersects the 1 bar axis later. The NDL for compartment 7 is only surpassed after 28 minutes. Compartment 2 is the leading compartment in this example and determines the NDL. It is therefore referred to as the leading compartment. A diving computer can easily calculate the NDL continously for all compartments of a model as well as indicate the remaining time of the NDL.

Of course, this also works for the continuation of the dive with a change in depth (Figure 2):

After 15 minutes, we ascend from 40 m to 30 m (4 bar ambient pressure) and remain there for 10 minutes (until minute 25 of the dive). We have been under decompression obligation since minute 10 of the dive, as the tolerated ambient pressure for compartment 2 (dashed blue line) has been above the surface pressure of 1 bar since then. The inert gas pressure in compartment 2 remains almost constant from minute 15, but compartment 7 continues to saturate. After 25 minutes of diving, we ascend to 20 m (3 bar). The inert gas pressure in compartment 2 drops again. As a result, the tolerated ambient pressure of compartment 2 falls below the surface ambient pressure (1 bar) again after 38 minutes of diving. As the tolerated ambient pressure of compartment 7 (orange dashed line) is still below 1 bar at this point, a direct ascent to the surface would be possible again. The NDL starts to run again from this point.

While compartment 2 is desaturated and therefore no longer relevant for decompression, the inert gas pressure in compartment 7 continues to rise. 8 minutes later, at dive minute 46, the tolerated ambient pressure of compartment 7 exceeds the ambient pressure at the surface (1 bar). This means that the NDL for compartment 7 has expired after 8 minutes and decompression is required again. Compartment 7 has become the leading tissue for decompression.

This example illustrates how the guidance for decompression is transferred from the "faster" tissue (compartment 2) to the "slower" tissue (compartment 7) when a shallower depth is reached. This has consequences for decompression tactics (e.g. deep stops). More on this in a future article. The shifts described here between a decompression obligation and the NDL can also be observed during real dives, where dive computers calculate with many more compartments.

That's all it takes to keep the NDL under control during a dive. There is no witchcraft to it.

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