120 Rule

[Blackwood]

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

I'm not sure you're giving him enough credit here.

It's obvious on the face that using the 120 rule at any depth for which they are learning to [-]plan dives[/-] determine max no-stop bottom time doesn't work. What do they do at 130 feet? Stay for negative 10 minutes and then be on their way?

Obviously not.

I have to assume that Peter covers it in a way that makes sense within PADI's paradigm of dive education.

 

[diver 85]

don't forget 90-30......That one & the 60/60 were the 2 I used to go by......

EDIT: that was the old Navy tables, when they(PADI etc) 'upgraded' the tables in ???what the late 80's, they made our old numbers 'ineffective'.....The only table(s) I have are the old 120 ones, never got one of the never ones, hope my computer kept up better than me.......

 

[Crush]

While I don't plan to implement the 120 rule, I am very happy to have learned about it. Thanks.

 

[Mr Carcharodon]

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

R..

Well I can and do use it for repetitive dives as do many other people. That does mean extending min deco stops for short surface intervals. It is a lot simpler than the convoluted mess that dive tables are.

 

[Mike Boswell]

I think at some point in this otherwise very interesting discussion, someone should point out that there are more than a few ex scuba divers alive today who have suffered permanent neurological damage or paralysis because they didn't follow proper dive planning or decompression procedures.

While the "Rule of 120" may (sort of accidentally) produce useable NDL numbers within a narrow range of depths for a non-repetitive dive, it is subject to misunderstanding, misuse and subsequent error, with disastrous consequences. Producing a "no-brainer" rule that requires "brainer" exceptions and interpretations is bad instructional policy.

We have always to keep in mind that a lot of scuba divers aren't gifted mathematicians... or even gifted.

 

[Mr Carcharodon]

Can you give an example where "disastrous consequences" resulted? Saying that someone someplace got badly hurt because they followed no plan at all is hardly relavant to the present discussion.

To be fair some of us are thinking of the 120 rule in the context of ratio deco which puts addtional constraints on the ascent. No one is suggesting diving a rule that is not fully understood.

 

[Mike Boswell]

Can you give an example where "disastrous consequences" resulted? Saying that someone someplace got badly hurt because they followed no plan at all is hardly relavant to the present discussion."

Well I can and do use it for repetitive dives as do many other people. That does mean extending min deco stops for short surface intervals. It is a lot simpler than the convoluted mess that dive tables are.

To be fair some of us are thinking of the 120 rule in the context of ratio deco which puts addtional constraints on the ascent. No one is suggesting diving a rule that is not fully understood.

Hi Carcharodon,

No, I can't give you a specific example where someone followed the "120 rule" and was injured because of it. I just think it is a bad "rule" which can easily be misunderstood.

Can you give me an example of how you plan multiple repetitive dives using the 120 rule, calculating surface intervals and "extending min deco stops" and "ratio deco"? I really don't see how this is possible.

 

[dumpsterDiver]

Can you give an example where "disastrous consequences" resulted? Saying that someone someplace got badly hurt because they followed no plan at all is hardly relavant to the present discussion.

To be fair some of us are thinking of the 120 rule in the context of ratio deco which puts addtional constraints on the ascent. No one is suggesting diving a rule that is not fully understood.

I've seen a person become permananetly paralyzed following an exposure which was considerably less than the 120 rule (Navy Tables) would allow and the guy did a slow conntrolled ascent and a safety stop and I was with him on the entire dive.

I hope you don't mean to imply that all the people who get crippled failed to follow a well thought out and statisfically "safe" profile.

 

[Mr Carcharodon]

I hope you don't mean to imply that all the people who get crippled failed to follow a well thought out and statisfically "safe" profile.

No I was asking if there were examples of the 120 rule not working when min deco asents are used. I doubt there are significant differences. No one will say any system is zero risk.

 

[gcbryan]

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..

Most of the people advocating the usefulness of rules of thumb do have Vplanner, computers and tables.

If you are on a dive and your computer fails I'm sure the common answer is the dive is over but what does it hurt for the diver to actually understand what the computer would be showing at the moment if it was working?

Additional knowledge is always good. Being able to have enough info to cross check is always good.

You might be a student of martial arts and perhaps you have a long list of tried and true techniques for when you are suddenly confronted with danger. I would guess that a good rules of thumb might be if you forget everything else...run!

What is the issue with rules of thumb?