120 Rule

[TSandM]

Walter, you and I both know that "deco" is not a bright line. There is a gradation of likelihood of DCS symptoms, as you increase the depth/time product for a given dive.

The system in which I am educated simply jettisons the concept of a "no-deco" dive altogether, and says that, since all dives involve absorption of nitrogen and require some time for offgassing, we will simply incorporate a decompression strategy into all dives. The time limits we memorize are derived from depth/time products that produce a "minimum deco" profile in standard decompression software. This consists of one minute stops from half maximum depth to the surface.

Limits which generate those times generally conform to the 120 rule for air, or the 130 rule for 32% (or 20% EAD advantage, depending on how you want to do it). Diving those limits WITH that decompression strategy works well. Diving those limits and using direct ascents, or 3 minute safety stops, may not work. The limits were not derived using that kind of ascent strategy.

Minimum deco is walking the fuzzy line between the "a direct ascent is always an option" approach of standard recreational diving, and the staged decompression acceptance of having lost that option.

 

[Walter]

"Deco" in some fashion occurs on every dive, to some degree.

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.

The 120 rule will get you into deco by most tables today.

True. I made that point in post 23.

Is the rule of 120 perfect under PADI's RDP/eRDPml? Of course not:

Except at 100 ft, it doesn't apply at all.

The system in which I am educated simply jettisons the concept of a "no-deco" dive altogether, and says that, since all dives involve absorption of nitrogen and require some time for offgassing, we will simply incorporate a decompression strategy into all dives.

Since the 120 "rule" is about no decompression (PfcAJ, that refers to staged decompression) diving, you don't use it. You may use a variation of it, but you don't use the concept that in 10 ft increments from 60 to 100 ft, 120 minus the depth dives you the NDL.

 

[Diver0001]

I just want to make the observation that NDLs and ascent strategies are inextricably intertwined. Those of us who are using the memorized 120 table (which is not precisely the rule of 120 at all depths) are ALSO using minimum deco

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.

But Lynne is right. Each table *assumes* a particular ascent strategy. the PADI RDP is based on an 18m / min (60ft/min) ascent, which determines to a certain extent the NDL's they prescribe. The GUE table may use the "120 rule" but uses a different ascent strategy so it fits.

I say 6 of one, 1/2 dozen of the other. Choose a table with it's associated ascent speed and you'll be ok. If it's GUE, PADI or any other combination of 3 or 4 letters of the alphabet is completely irrelevant.

R..

 

[Walter]

Choose a table with it's associated ascent speed and you'll be ok. If it's GUE, PADI or any other combination of 3 or 4 letters of the alphabet is completely irrelevant.

Decompression sickness is rare with any of them and differences are probably not significant, but there are factors that reduce the risk of DCS. Shorter NDLs, longer surface intervals and slower ascent rates are all such factors. Since I like to reduce my odds of getting bent (I don't care if the differences are significant), I avoid tables with longer NDLs and shorter surface intervals.

 

[Diver0001]

Don't you think this is good enough for checking to see if your basic numbers are right? I do.

I don't.

It's a workable "ride the computer" type kludge but has no application outside of certain depths and the assumption that

- the diver hasn't made a recent dive
- they didn't bother planning their dive
- they don't work in metric
- they don't have any better options.

Personally, I think if ^^^ that's the case that the diver would be better advised to delay the dive until they are in a position to plan it responsibly (refer to my earlier post for details). YYMV

As instructors, it's our job to teach students how to plan *all* of their dives, not a limited sub-set of their dives.

R..

 

[Diver0001]

Decompression sickness is rare with any of them and differences are probably not significant, but there are factors that reduce the risk of DCS. Shorter NDLs, longer surface intervals and slower ascent rates are all such factors. Since I like to reduce my odds of getting bent (I don't care if the differences are significant), I avoid tables with longer NDLs and shorter surface intervals.

I'll just underscore what Walter says here. Some ascent strategies make more sense than others. The 18m/min recommendation from PADI is out-dated. Most divers now apply 10m/min. 3m/min is generally thought to be the boundary between "ascending" and making a multi-level dive.

An interesting side-bar to this thread would be to discuss how divers can assess their ascent rates on the fly.

R..

 

[dumpsterDiver]

I don't.

It's a workable "ride the computer" type kludge but has no application outside of certain depths and the assumption that

- the diver hasn't made a recent dive
- they didn't bother planning their dive
- they don't work in metric
- they don't have any better options.

Personally, I think if ^^^ that's the case that the diver would be better advised to delay the dive until they are in a position to plan it responsibly (refer to my earlier post for details). YYMV

As instructors, it's our job to teach students how to plan *all* of their dives, not a limited sub-set of their dives.

R..

I think the 120 Rule is most valuable as a simple device to help new divers INSTANTLY gain a gut feeling for the upper limits of no-deco diving within a resonably popular range of diving depths.. for a single dive. I would never use the 120 rule to plan a dive, but I have it in the back of my head. To me the 120 Rule is simply a couple of pairs of NUMBERS that serve as a reality check:

60/60
70/50
80/40
90/30

someone might even extrapolate these numbers as follows and it probably would not get anyone hurt:

40 ft for 80 min
50 ft for 70 min

and

100 ft/ 20 min
110/ 10
120/ zero minutes


I think that the discussion of minimum deco and other strategies and application of nitorx and varaible ascent rates and deep stops and shallow stops and repet dives. and on and on. is extrapolating WAY past the basis of the "RULE". Of course, all ascent strategies (deco or otherwise) are going to incorporate some type of numerical guidlines that will be somewhat consistent with the 120 Rule, but they are not the "120 Rule".. at least to me anyway.

We have some organizations teaching the use of computers and not tables. For people that are taught in this manner, the 120 rule might give them some basis for understanding what the limits are and a double check on their computer. I would hope that they follow their computer but if they dove a reasonable nitrox mix, I think the old 120 Rule would probably work great for planning a single no-deco dive, if you had no other tools.

 

[Diver0001]

I think the old 120 Rule would probably work great for planning a single no-deco dive, if you had no other tools.

I wouldn't dispute that.

I would ask how you *got* yourself in a position where you needed to plan a dive with "no other tools".

R..

 

[Peter Guy]

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 don't believe anyone is advocating the "rule of 120" is anything other than a simple way of planing -- just as "Don't dive below the cubic feet of your tank" is a simple way of doing "gas management." Just because these use imperial measurements doesn't make them invalid -- after all, what reasonable people use a system other than feet, inches and PSI?

 

[Bob DBF]

Back when I finally had to get my C card, the 120 "rule" was conservative.

NDL off the PADI tables of the day:

140' 10 min
130' 10 min
120' 15 min
110' 20 min
100' 25 min
90' 30 min
80' 40 min
70' 50 min
60' 60 min
50' 100 min
40' 200 min

The ascent rate was 60' per minute and the safety stop was not invented yet.

Also the tables gave decompression information for your mandatory deco 10' stop should you overstay the NDL.

As an aside the repetitive groups were A to N and the copyright date on the card is 1978.

I dove the tables and 120 rule 'till I got a computer several years back. With the computer I still run the numbers in my head, old habits.... Problem now is the Tables and Procedures have changed and the numbers in my head have not.

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