Info The Rule of 120

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tursiops

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(This is an updated version of a post made 4 1/2 years ago....)

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.
 

boulderjohn

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Tursiops emphasized by underlining that this was for first dives. When I see people cite the rule of 120 (US Navy tables), they seem to be using it for all dives. If so, then it can be a significant misuse of the tables.

The US Navy tables yielding the rule of 120 used the 120 minute compartment to govern surface intervals. That leads to seriously long surface intervals. About 40 years ago the PADI tables were released, and these took several steps to decrease surface intervals and make the modern 2-tank dive schedule possible. One of those steps was decreasing first dive NDLs. For example, the US Navy 100 foot NDL was 25 minutes, and PADI made it 20. The other steps were using the 60 minute compartment for surface intervals and increasing the number of pressure groups to decrease rounding errors.

When modern diver's go on 2-tank dives, their surface intervals are consistent with PADI tables, and today's computer algorithms are also consistent with the PADI tables. Therefore, if someone following that schedule is not using a computer and claims to be using the rule of 120 for both dives, they are making the following errors.
  1. They are using a Navy first dive plan for a second dive, meaning they are not taking into account residual nitrogen from the first dive, which should reduce bottom time for the second dive.
  2. They are using a US Navy table first dive depth and combining it with a PADI surface interval, which will give them a very significant overestimation of bottom times according to the Navy tables. If they were truly using the Navy tables, their second dive would be significantly shorter than everyone else's.
Consequently, if people say they are using the US Navy tables because they use the rule of 120 for all their dives, they are not using the US Navy tables at all. They are really just winging it and hoping for the best. Fortunately, the US Navy tables' use of the 120 minute compartment is almost certainly far too conservative, so by winging it they are actually approximating other algorithms more closely than the Navy tables.
 

Akimbo

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You will sometime hear divers (especially older ones) refer to the Rule of 120.

That brings back memories. The 120 rule and 60 for 60 is burned into my brain. My first wetsuit had a No-D table decal on the sleeve.
 

oya

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I (loosely... because in practice I use a dive computer because I live in the year 2022) use the rule of 130. Obviously the same general idea. Also: for the second dive, after a 90 minute surface interval you get 75% of the time.
 
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tursiops

tursiops

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I (loosely... because in practice I use a dive computer because I live in the year 2022) use the rule of 130. Obviously the same general idea. Also: for the second dive, after a 90 minute surface interval you get 75% of the time.
Do you actually mean 130? Where does that come from?
 

oya

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Do you actually mean 130? Where does that come from?
I ripped it off of GUE. I don't know (or really care that much) what GF they used to figure it. I think, maybe 30/80?
Using average depth as the depth vector it's agreed with my computers within a minute or two for hundreds of dives. I use a 45/85 on my wrist.
 

oya

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I stopped reading here. Average depth is not a valid construct.
Nitrogen uptake and off-gassing does not depend upon average depth.

Super weird that all my Shearwaters seem to think so. You should write the programmers and get them to write a patch.
 
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tursiops

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Super weird that all my Shearwaters seem to think so. You should write the programmers and get them to write a patch.
Are you misunderstanidng on purpose? Average depth can be easily calculated, but using is for nitrogen uptake/offgassing is fiction. As I said.
 

oya

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Are you misunderstanidng on purpose? Average depth can be easily calculated, but using is for nitrogen uptake/offgassing is fiction. As I said.

I use a form of ballpark dive planning that is taught by one of the most well-regarded agencies on the planet.
As a cave diver following mandatorily non-square profiles (gotta be at whatever depth the cave dictates) that are symetrical (the cave going out is the same as the cave going in), so whatever your average depth is RIGHT NOW is going to be your average depth when you get to the surface.
Which is why I keep average depth on my primary screen of one of my computers (as, apparently, an option that enough people are interested in that it's an option on the computer). So I don't need to calculate it. I can just look down.
I have personally watched this play out as accurate to within a minute or two of what my dive computer, one of the most popular on the market, has calculated for 16 individual tissue compartments using math I couldn't possibly even describe.
I have seen this happen on hundreds if not thousands of dives.
I'm inclined to think that it works just fine.
 

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