120 Rule

[mikemill]

Just so the layman can put this in context, "minimum deco" is what GUE calls their version of the RDP. It's just a table like any other. It also predicates that divers use EAN32 instead of air for their dives, which might explain the slightly longer NDLs.

Min deco is also used by UTD (of which I will now speak on how UTD does it) and:
1) The table itself is actually based on air and then adjusted for nitrox
2) It is more than a table, it is also an ascent strategy
3) The NDL limits vs the PADI NDL limits, for both air and 32%, are actually equally or more conservative.

 

[Diver0001]

Rob, without a "rule of thumb" like the "120 rule" what do you suggest is a method the "average diver" should use to double check their numbers whether derived from tables or some type of computer?

I train my students to both do their calculations independently from their buddy and then compare results. If the numbers are not the same then they have to re-do them.

During the OW class there good opportunities for that. During the check out days I ask the divers to each plan their dives and then I give them the opportunity to discuss them. I will reinforce that if mistakes *can* be made then they *will* be made and it's important that they are very careful to make sure that the planning is correct. This is a theme that I return to over and over again when it comes to dive planning.

Computers are a different beast because your mistakes, if you do make one, will become apparent during the dive. With tables, that's not the case.

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..

 

[Diver0001]

1) The table itself is actually based on air and then adjusted for nitrox

The table is a nitrox table. How it's derived doesn't seem overly relevant.

2) It is more than a table, it is also an ascent strategy

That's the case with all tables. Some ascent strategies are just more complicated than others.

3) The NDL limits vs the PADI NDL limits, for both air and 32%, are actually equally or more conservative.

Ok. I don't really care whose tables are more conservative than the other guy. Clearly at this point they're all good enough to be trusted. My point was about nomenclature. People are used to calling tables "tables" and I was just pointing out that calling it "minimum deco" doesn't make it something else.

R..

 

[AbyssalPlains]

I have never heard of this rule, but I feel very comfortable dismissing it outright and even suggesting it be dangerous. Here is why: One glance at this rule tells even a math dummy like myself that it is a very simple linear equation (120 minus depth = NDL). Knowing that physiological processes such as gas absorption in the body are almost never linear, but more complicated than that, makes me suspicious of using a linear rule to determine a non-linear relationship.

My hunch is further strengthened by looking at the table: For example, my NDL at 30 feet is 200 minutes. If I double the depth (to 60 feet), the NDL is only a quarter of the NDL at 30 feet, namely 50 minutes. If I double the depth again to 120 feet, the NDL is 10 minutes, even LESS than a quarter of the time at 60 feet. This tells me the relationship between depth and NDL is non-linear and that the "120 rule" HAS to be bogus. Not because I'm arrogant, but based on this very straightforward consideration.

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?

 

[mikemill]

The table is a nitrox table. How it's derived doesn't seem overly relevant.

Um, I'm looking right at my course materials from October and it is Air not Nitrox.

That's the case with all tables. Some ascent strategies are just more complicated than others.

Ok. I don't really care whose tables are more conservative than the other guy. Clearly at this point they're all good enough to be trusted. My point was about nomenclature. People are used to calling tables "tables" and I was just pointing out that calling it "minimum deco" doesn't make it something else.

Except that the PADI system basically lets you ignore the the ascent part and fall within the table. If you completely blow the min deco ascent you really can't just wait an hour and use the table again.

 

[Diver0001]

Um, I'm looking right at my course materials from October and it is Air not Nitrox.

Ok... the GUE one was 32%, the UTD one is air... Toe-may-toe Toe-mah-toe.

Except that the PADI system basically lets you ignore the the ascent part and fall within the table. If you completely blow the min deco ascent you really can't just wait an hour and use the table again.

As I said, ascent strategies differ... LOL

Incidentally nothing about the RDP "pohibits" students from learning or using proper ascent strategies. I'll be the first to admit that I think PADI is behind the curve on this, at least in terms of their materials.

However, the assumption that using the UTD table will automatically result in clean ascents every time is just about as absurd as saying that using the non-UTD table will automatically result in sloppy ascents.

So, I don't know why you're saying what you're saying, but YES there are differences in how students are taught to ascend from one agency to the other but NO you can't "ignore" the ascent with respect to *anyone's* tables. Not in the UTD system and not in non-UTD systems. What you do to recover from a sloppy ascent, however, may not be the same.

I know you want UTD to be the best because that's the team you're cheering for and I can understand that .... but frankly, it's about as rational as saying "my Ford is better than your Chevy".

R..

 

[Rainer]

The 120 rule was never meant to apply to all depth/time pairings (your 30' example is a perfect illustration). Over many smaller ranges, deco limits *are* linearly definable to depth/time pairings (e.g. 120 rule, ratio deco, etc).

I have never heard of this rule, but I feel very comfortable dismissing it outright and even suggesting it be dangerous. Here is why: One glance at this rule tells even a math dummy like myself that it is a very simple linear equation (120 minus depth = NDL). Knowing that physiological processes such as gas absorption in the body are almost never linear, but more complicated than that, makes me suspicious of using a linear rule to determine a non-linear relationship.

My hunch is further strengthened by looking at the table: For example, my NDL at 30 feet is 200 minutes. If I double the depth (to 60 feet), the NDL is only a quarter of the NDL at 30 feet, namely 50 minutes. If I double the depth again to 120 feet, the NDL is 10 minutes, even LESS than a quarter of the time at 60 feet. This tells me the relationship between depth and NDL is non-linear and that the "120 rule" HAS to be bogus. Not because I'm arrogant, but based on this very straightforward consideration.

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?

 

[mikemill]

So, I don't know why you're saying what you're saying, but YES there are differences in how students are taught to ascend from one agency to the other but NO you can't "ignore" the ascent with respect to *anyone's* tables. Not in the UTD system and not in non-UTD systems. What you do to recover from a sloppy ascent, however, may not be the same.

What I'm saying is that you can't say that min deco is *just* the tables so you can compare it directly to the RDP. IMO that is false.

I know you want UTD to be the best because that's the team you're cheering for and I can understand that .... but frankly, it's about as rational as saying "my Ford is better than your Chevy".

I cheer for no team. And of course it wouldn't be rational, Chevy is much better than Ford but I drive a Dodge

 

[Diver0001]

What I'm saying is that you can't say that min deco is *just* the tables so you can compare it directly to the RDP. IMO that is false.

I hear you. The minimum deco table is not just the table but you have to make a particular type of ascent in combination with the table.

I agree.

The point I've been trying to make is that this is no different than any other table. All tables, including the PADI RDP were developed and tested (at least those that were actually tested) with a particular ascent strategy in mind.

So we agree that the UTD tables have a distinct ascent strategy in mind.

And I'm trying to get you to understand that all other tables are no different in this regard. The ascent strategy might be as simple as "ascend at a constant rate of 18m/min to 5 metres and make a 3 min safety stop before ascending to the surface" but it *is* (as inadequate as it sounds given our modern perspective) an "ascent strategy".

I cheer for no team. And of course it wouldn't be rational, Chevy is much better than Ford but I drive a Dodge

You're one up on me. I own a Fiat.

R..

 

[Bob DBF]

I have never heard of this rule, but I feel very comfortable dismissing it outright and even suggesting it be dangerous.

Personally I don't dismiss anything out of hand just because I've never heard of it. I've picked up a lot of information over the years but I haven't heard everything yet.

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.

History of diving is not taught in diving classes, a point which paladin954 may be making with his question. Read my post #50 which shows the old dive table and why the 120 rule was used. Since the tables have changed it doesn't work out so well

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?"

The law requires seat belts be in vehicles from the manufacturer and the law requires everyone to use them. This is not the same as asking if you use a trick to remember part of the old dive tables.

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

Because I carry my mind with me and tables get lost, and computers puke. Some information is better than no information. Besides, when you dive old-school, you get a depth Gauge, a watch, and a J valve. And don't forget the knife.

Bob
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I may be old, but I'm not dead yet.