120 Rule
- Thread starter Paladin
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Kevrumbo
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It was never a rule, but was a way to help memorize the tables. The tables have changed. NDL times are shorter. I'm older. I dive various mixes now. I haven't used it in years.
Troutmaster
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Troutmaster
Contributor
There are always two celebrations a day. It works across multiple time zones. It is just another excuse for celebration.
I find it a little concerning that someone who represents an organization dedicated to teaching children to dive would post something that seems to endorse use of drugs as "celebration"
I have a slightly different perspective on this.
The 120 "rule" is actually a 120 "coincidence" and while it might have some application in a small number of dives the *need* to use the 120 "rule" assumes one of two preconditions:
1) that you have either not planned the dive ahead of time and just jumped in the water using the 120 rule the way people "ride" the computer. I don't think this practice would be recommended, even by people who say they use the 120 rule. I can see divers doing this for one dive, however. When I dove on tables (for about the first 600 dives I made) then I eventually had the base NDL's memorized for all depths, which I guess is comparable to using the 120 rule. I guess you could say if you just "know" the NDL then you don't have to consult the table. This only works for a small number of dives, however. Most divers these days are trained and accustomed to making multiple dives on a day.
or
2) you did, in fact, plan the dive, which would involve consulting the NDL's for the planned depth/time and maybe a couple of contingency items like the NDL at the hard-bottom if there is one or for the next couple of 10ft increments. These things would ordinarily be written down or memorized by divers using tables. I recommend to my OW students who might be using the tables to actually write them down on a wrist slate for the simple reason that memorizing the base NDL's or the 120 rule is vastly inadequate in terms of planning repetitive dives. The physical act of writing it down helps with memorization too and doing it like this (thinking about where the hard-bottom is, for example) is an essential part of responsible dive planning.
In my case I didn't use the 120 "rule". My solution was to memorize the table.
R..
The 120 "rule" is actually a 120 "coincidence" and while it might have some application in a small number of dives the *need* to use the 120 "rule" assumes one of two preconditions:
1) that you have either not planned the dive ahead of time and just jumped in the water using the 120 rule the way people "ride" the computer. I don't think this practice would be recommended, even by people who say they use the 120 rule. I can see divers doing this for one dive, however. When I dove on tables (for about the first 600 dives I made) then I eventually had the base NDL's memorized for all depths, which I guess is comparable to using the 120 rule. I guess you could say if you just "know" the NDL then you don't have to consult the table. This only works for a small number of dives, however. Most divers these days are trained and accustomed to making multiple dives on a day.
or
2) you did, in fact, plan the dive, which would involve consulting the NDL's for the planned depth/time and maybe a couple of contingency items like the NDL at the hard-bottom if there is one or for the next couple of 10ft increments. These things would ordinarily be written down or memorized by divers using tables. I recommend to my OW students who might be using the tables to actually write them down on a wrist slate for the simple reason that memorizing the base NDL's or the 120 rule is vastly inadequate in terms of planning repetitive dives. The physical act of writing it down helps with memorization too and doing it like this (thinking about where the hard-bottom is, for example) is an essential part of responsible dive planning.
In my case I didn't use the 120 "rule". My solution was to memorize the table.
R..
dumpsterDiver
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I remember bringing it up in a lecture within my NAUI scuba instructor class and they indicated they really didn't want that taught to students, in 1978.
On the other hand, it is an excellent rule to memorize, as a very simple guide to help judge if your computer is going wacko or something.
On the other hand, it is an excellent rule to memorize, as a very simple guide to help judge if your computer is going wacko or something.
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Yeah ... way better to shelter kids from the realities of the world so they can be more easily taken advantage of by priests and pedophiles ...
Can we get back to the topic now?
... Bob (Grateful Diver)
While I may be way off here, I like the 120 rule, and present it to students, as a quick and dirty way of doing some dive planning. By using it, they get a basic idea of the NDL for their planned depth -- and then (I hope) they'll do a quick air supply calculation.
Hmm -- 60 feet for 60 minutes -- OK, how much gas do I need -- 3 x .7 x 60 ~ 120 ft3 -- oops, I'm diving an HP 100 -- OK -- NDL not a limiting factor -- plan the dive for 40 minutes -- 3 x .7 x 40 ~ 80 ft3.
It is also handy, as someone mentioned, as a quick check to make sure you either read the table correctly or that your computer is working correctly.
Hmm -- 60 feet for 60 minutes -- OK, how much gas do I need -- 3 x .7 x 60 ~ 120 ft3 -- oops, I'm diving an HP 100 -- OK -- NDL not a limiting factor -- plan the dive for 40 minutes -- 3 x .7 x 40 ~ 80 ft3.
It is also handy, as someone mentioned, as a quick check to make sure you either read the table correctly or that your computer is working correctly.
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