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The no decompression limit (NDL) is the maximum allowable dive time that you can remain at a specific depth and ascend directly to the surface without requiring staged decompression stops on the way up. Remember the higher the partial pressure of nitrogen (ppN2), the shorter the dive time (NDL).

You can use air dive tables to determine your NDL, but the best way is to consult your PDC and plan the depth for your dive. It is important to know how to manually calculate NDL in case the PDC is not working or has a dead battery before the dive.

The table below shows the sliding open circuit partial pressure of both oxygen and nitrogen as applied to air.

Table 1
Depth Pressure Oxygen (%) Oxygen (pp) Nitrogen (%) Nitrogen (pp) Total Pressure
Sea Level 1 bar/ata 21% 0.21 79% 0.79 1 bar/ata
10 m/33 ft. 2 bar/ata 21% 0.42 79% 1.58 2 bar/ata
20 m/66 ft. 3 bar/ata 21% 0.63 79% 2.37 3 bar/ata
30 m/99 ft. 4 bar/ata 21% 0.84 79% 3.16 4 bar/ata
40 m/132 ft. 5 bar/ata 21% 1.05 79% 3.95 5 bar/ata

If you accidentally exceed your NDL, follow the recommended stops displayed by the PDC. If required decompression stops were omitted, exit the water. Explain your situation to the surface support (boat crew or your dive buddy, for example). Do not continue diving, breathe 100% oxygen, drink plenty of water, and monitor for signs and symptoms of DCS. Seek medical attention by contacting emergency services or the closest medical facility as soon as possible.

NEVER delay medical treatment.

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