120 Rule

[LeadTurn_SD]

Lastly, the fact that I have never even heard of this in any of my many, many diving courses and cert classes makes me think there must be a reason.

To me asking the question, "Does anyone still use the 120 rule" is perhaps a little bit like asking, "Is anyone still driving a car without seat belts?"

Sure, maybe, but why would anyone do that given that seat belts (read: dive tables and computers) are readily available?

The rule pre-dates your training. I am surprised it was never even mentioned in any of your training.... maybe ask for a refund? Just kidding!!!! But it was taught to us in Basic OW (PADI) in 1976....

As mentioned, the basis is a "happy coincidence" in the Navy dive tables (the older tables).

It is easy to remember: "60 feet for 60 minutes" has remained in my old brain since I learned the rule as a teenager in the 1970's. I don't use it for dive planning any longer, other than as a "reality check" for any other method I might employ. As already stated, it is a quick and dirty memory aid that DOES work for certain depths on the first dive of the day, square profile.

Best wishes.

 

[Mr Carcharodon]

In my case I don't have the luxury of "teaching" the 120 rule because our tables are metric, but even if I did, then I believe in teaching proper dive planning, which means making the square-wave diagrams if you're using tables and filling in the blanks. I don't believe in short cuts when it comes to dive planning. It's a recipe for screw ups.

R..

What makes 120 rule planning improper? There is a degree of arbitrariness in all the tables and computers which is why the results are all over the map. It is just strange to claim that one arbitrarily derived approach is more valid than others which have a long track record of working. You may consider using a square profile dive table a proper way plan multilvevel dives but in practice very few divers do.

 

[gcbryan]

I don't understand how there can be a problem with "rules of thumb" either. For one thing nothing is exact whatever you are using just given the nature of what we are talking about...decompression.

Planning using a rule of thumb is just as valid as using a table or a computer since there is nothing exact behind them either.

The fact of the matter is that the more one knows regarding decompression the more it's apparent that this is a grey area. The ability to think on your feet in a given situation in addition to your ability to read a dive table and glance at a computer has to be all for the good.

It really makes no difference how you reach your own conclusions as long as you reach one.

The rules of driving are such that you are to stop well before a stop sign. However, if stopping at that stop sign doesn't allow you to see around the tree on that corner then you have to actually stop well into the intersection. The ability to understand why the rule is there is more important than always following the rule.

It's the same with diving.

 

[Diver0001]

What makes 120 rule planning improper? .

As a rule of thumb for making a single dive to certain depth, the coincidence is that the 120 "rule" is just a way of remembering what the NDL's of the Navy tables are between 60ft and 100ft. For a single dive in tht depth range there's nothing wrong with using that fact.

But for any other kind of planning involving multiple dives, it's useless and that's the rub.

R..

 

[Paladin]

As a rule of thumb for making a single dive to certain depth, the coincidence is that the 120 "rule" is just a way of remembering what the NDL's of the Navy tables are between 60ft and 100ft. For a single dive in tht depth range there's nothing wrong with using that fact.

But for any other kind of planning involving multiple dives, it's useless and that's the rub.

R..

Actually, there is another "rule" that is used to calculated surface interval credit. Unfortunately, I don't remember how it goes. I've only used the 120 Rule a few times over the years for single, non-repetitive dives and have relied primarily on my Navy tables for planning the vast majority of my dives.

 

[gcbryan]

It's not useless. For those who rely on these things there are other rules (which I can't recall specifically). I don't usually rely on such things other than as rules of thumb but certainly taking a 2 hour surface interval and keeping your second dive shallower and shorter would do the trick.

The fact of the matter is that there are a lot of ways to skin the cat. Actually, just using Nitrox and a single tank goes a long way toward making it hard to even hit the NDL's in many cases (not that this is advice I personally take but it is true).

 

[Diver0001]

I don't see the value in using a bunch of "rules of thumb" to plan a dive as opposed to a table where the diver can use specific tested and verfied information instead.

It's certainly not like using a table is very complicated so I don't see what you *gain* by using a rule of thumb for planning instead. I do, however, see what you can *lose* by doing this, namely, a certain amount of specificity and the ability to accurately plan specific dives outside of the narrow bandwidth that works with 120.

I suppose you'll reply with how the 120 rule can be made to work outside of it's "coincidental" bandwidth but I would say again.... if we already have tables, then what do you *gain* by it..... it just comes across to me as forcing a square peg into a round hole when we have a perfectly good round peg already.

YYMV, I don't care if people want to keep using it, but I certainly don't see it as a strong basis for dive planning as compared to the alternatives, which *are*, in fact, a strong basis for dive planning. Give people a computer, software like Vplanner or a table. Teach students how to use those tools. I really don't see what we gain from a "rule of thumb". That's all I'm saying.

R..

 

[Blackwood]

Point being since he teaches a PADI class, teaching to exceed the RDP NDLs would be a standards violation.

You aren't allowed to discuss other stuff in PADI courses?

We learned about the 120 Rule (which is short for "rule of thumb", not to be confused with a regulation) in my PADI OW class. Did we use it to plan dives? No.

 

[Walter]

You aren't allowed to discuss other stuff in PADI courses?

I'm sure you are. I'm also sure you aren't allowed to teach your students to exceed the RDP NDLs.

 

[Blackwood]

Of course, but the term NDL (No Decompression Limit) refers to a point on the tables. A time longer than that at the NDL point requires staged decompression. A time at that point or shorter does not.

Really? Then why are there maximum ascent rates specified?

"Min Deco" and "Max Ascent Rate" are the same thing.