You will sometime hear divers (especially older ones) refer to the Rule of 120. this was an approximation to working out No-Decompression-Limits based on the old U.S. Navy tables. The "Rule of 120" was based on the sum of depth (in feet) and NDL (in minutes), which turned out to be 120 for the old Navy tables, exact between 60 and 90 ft. For shallower depths, or deeper depths, the 120 rule was quite conservative...for example it would suggest 10 minutes at 110 ft and zero minutes at 120 ft, whereas the actual table gave 20 and 15 minutes, respectively, at 110 and 120 ft. So, the rule of 120 kept you "safe." It was for the

__first dive__, only. It was even printed on the watchbands of the day. You who are mathematically inclined will recognize this as a tangent-linear approximation to the deco curve.

This same "Rule of XXX" could be applied to pretty much any dive table or dive computer,

__for the first dive.__Find that depth at which the sum of depth and time is the minimum, and that is your new rule. The PADI RDP gives 110, valid for 70 feet; at any other depth the Rule of 110 is conservative.

__For 32% Nitrox__, the PADI RDP rule would be 125, "exact" for depths of 80 and 90 ft, conservative for any other depth.

__For air__, here are some other "Rules;" remember, your depth must be in feet for these to be valid:

The newest Navy table would have a Rule of 118, "exact" at 70 ft, conservative at any other depth.

DCIEM would have a Rule of 105.

My old DiveRite DUO would have a Rule of 104.

My old DiveRite PLUS would have a Rule of 106.

My old Zeagle N2Ition 3 would have a Rule of 105.

My Oceanic OC1 (DSAT) has a Rule of 110, just like the PADI RDP. Surprise!

My Oceanic OC1 (PZ+) has a Rule of 105.

My Shearwater in Rec Mode with Low Conservatism (GF 45/95),Rule of 105.

My Shearwater in Rec Mode with Med Conservatism (40/85),Rule of 98..

My Shearwater in Rec Mode with High Conservatism (35/75),Rule of 92.

My Shearwater in Tec Mode with GF 30/60, Rule of 81.

My Shearwater in Tec Mode with GF 30/70, Rule of 89.

My Shearwater in Tec Mode with GF 20/80, Rule of 96.

My Shearwater in Tec Mode with GF 49/99, Rule of 109.

On a typical resort/liveaboard trip, where Nitrox is made with a membrane so is usually between 31 and 33%, I just leave my computer set on 31% to track the nitrogen, and stay above 100 ft (30m) to satisfy the MOD. This way, I don't have to keep changing my computer for each dive.

For 31% Nitrox, my Shearwater set at GF 70/85 (my usual default) would give a Rule of 115, exact at 80 ft, and conservative shallower and deeper.

How is this useful?

It gives a quick estimate of your NDL without looking at a table or plan mode on your computer.

I just remember 115 for 31%. NDL for 80 ft is thus 35 mins, with more conservatism deeper and shallower.