Question Ratio : Where does the 30 msw / 20 min NDL come me from ?

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Hello,

Can you help me in finding the origin of the simplified rule that says that one can dive NDL (no-deco) for 20min @ 30msw on air / 45à50min @ 20msw ?

This seems to be a generally accepted rule of thumb in ratio calculations (Gue,UTD,ISE,...)
Except for the IANTD OW & NAUI tables, that come close to it, I cannot find the rationale behind it?

Thank you in advance

Tom
For many years the accepted NDL times in the imperial system for recreational depths were Depth(ft)+NDL=120, thus the so-called "Rule of 120." It was a crude straight-line approximation (for a first dive of the day) to the old Navy tables, and where wrong (at the very shallow and the deeper depths) was conservative. I even have an old watchband somewhere with those numbers printed on it. An equivalent "rule" for the PADI RDP is 110....accurate at 70 and 80 feet and conservative deeper and shallower than that. For 32% Nitrox, on the PADI RDP, it would be a rule of 125, accurate at 80 and 90 feet and conservative deeper and shallower. Easiest possible first-dive planner!
 
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