120 Rule

[Mike Boswell]

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?

Hi GCBrian,

I have no problems with "rules of thumb", when they work, but I don't think this rule works: Not even close. This rule says your NDL at 80 feet is (120-80=) 40 minutes, when the DSAT table says it is 30 minutes.

Is the correct answer 30, or is it 40? To me, that extra 10 minutes at 80 feet is a big difference. The DSAT 30-minute answer is based on our best science and many years of experience. The 40-minute answer was derived because someone with a straight-edge and a pencil drew a straight line through a curve to try to best fit the data. Which approach will you bet your life on?

Even worse, though, are all of the exceptions and explanations surrounding this so-called "easy" rule. (For more on that read: Precision Diving | NDL)

I have a computer and I have the tables, and I'll continue to use those, and if, in the unlikely event that I don't have either, I will use my buddy's computer or call the dive.

 

[dumpsterDiver]

Hi GCBrian,

I have no problems with "rules of thumb", when they work, but I don't think this rule works: Not even close. This rule says your NDL at 80 feet is (120-80=) 40 minutes, when the DSAT table says it is 30 minutes.

Is the correct answer 30, or is it 40? To me, that extra 10 minutes at 80 feet is a big difference. The DSAT 30-minute answer is based on our best science and many years of experience. The 40-minute answer was derived because someone with a straight-edge and a pencil drew a straight line through a curve to try to best fit the data. Which approach will you bet your life on?

Even worse, though, are all of the exceptions and explanations surrounding this so-called "easy" rule. (For more on that read: Precision Diving | NDL)

I have a computer and I have the tables, and I'll continue to use those, and if, in the unlikely event that I don't have either, I will use my buddy's computer or call the dive.

Maybe you don't get it? The 80/40 and 60/60 etc.. (the 120 Rule) are the US NAVY DIVE TABLES! That is what was taught by PADI for many years. I have not looked up the navy tables in a long time, but we did many dives without getting bent diving the navy tables and without doing any stops and probably some fast ascent too. Maybe the Navy has changes their tables?

 

[Rainer]

And the fact the NAUI table gives 35 minutes at 80', and the Navy 39 minutes? I guess their science and many years of experience don't match DSAT's. Too funny. Remember, the recreational dive tables have to cover the most obese, most out of shape, heaviest smoker, oldest, least well-hydrated, fastest ascending, tiredest divers out there and basically "guarantee" they won't take a hit. If that description applies to you, then by all means, stick with 30 minutes at 80' Or better yet, be real conservative and keep your BT to 20 (or 10 or 5) minutes (shoot, maybe just stay dry all together). I'm quite happy doing 35-40 minutes at 80 average. I'll certainly pad my shallow stops a bit, but still, the 120 rule *for me* is a reasonable approximation of dives I'm willing to do (well, with a depth credit applied to 32%).

Is the correct answer 30, or is it 40? To me, that extra 10 minutes at 80 feet is a big difference. The DSAT 30-minute answer is based on our best science and many years of experience. The 40-minute answer was derived because someone with a straight-edge and a pencil drew a straight line through a curve to try to best fit the data. Which approach will you bet your life on?

 

[Rainer]

Maybe the Navy has changes their tables?

LOL, the latest adjustment for 80' went to 39 minutes. Big whoop.

 

[dumpsterDiver]

LOL, the latest adjustment for 80' went to 39 minutes. Big whoop.

Damn, there goes the entire "rule".

 

[Blackwood]

Damn, there goes the entire "rule".

119 rule.

 

[Kevrumbo]

Using deco plan it graphs your profile for you so you start to see the shape of your curve.

But given that this stuff isn't rocket science let me demonstrate how you calcualte "on the fly" the 120 and 20%, it's pretty basic stuff..

120, based upon Navy tables, means that depth and time always add to 120. In other words an 80' dive means 40 minutes NDL, a 90' means 30 minutes NDL a 100' dive means 20 minutes NDL and so on.. So all one needs to do is take their depth and [subtract from] 120 [to get the NDL time]. If they can't do that then they shouldn't be diving. 20% means that if you standardize you mix [ie; 32%] rather then go through the silly formulas that you spent all that money in your nitrox class to learn, all you need to know is that your EAD advantage is 20%.. So if you do a 100' dive your EAD is 80'.. So an 80' dive using the 120 rule means you have 40 minutes NDL.. This stuff is simple and doesn't require a degree in math and can be done in seconds "on the fly".. Those that tell you it's too complicated more then likely don't know it themselves or are trying to sell you a computer or a class.. Now if you wanted to be conservative you could use your max depth for the 120', but if you're turning your brain on underwater you can take 5 minute snap shots so for example if I did 10 minutes at 100' and 10 minutes at 80, I could simply average it as a 90' dive, therefore 120 - 90 equals 30 minutes NDL.. If you want to be more conservative use 115, the number doesn't matter to me, the idea is that it's simple and isn't impossible as many in the dive industry will tell you and it saves you $400 for your computer.. BTW, you can use that same money to take a DIR-F class because this is but one nugget of information we teach in the class ;-).. The shape is what is important, but mathematical models can only plot in depth and time...

Hope that helps..

http://www.scubaboard.com/forums/381802-post28.html

 

[Mike Boswell]

And the fact the NAUI table gives 35 minutes at 80', and the Navy 39 minutes? I guess their science and many years of experience don't match DSAT's. Too funny. Remember, the recreational dive tables have to cover the most obese, most out of shape, heaviest smoker, oldest, least well-hydrated, fastest ascending, tiredest divers out there and basically "guarantee" they won't take a hit. If that description applies to you, then by all means, stick with 30 minutes at 80' Or better yet, be real conservative and keep your BT to 20 (or 10 or 5) minutes (shoot, maybe just stay dry all together). I'm quite happy doing 35-40 minutes at 80 average. I'll certainly pad my shallow stops a bit, but still, the 120 rule *for me* is a reasonable approximation of dives I'm willing to do (well, with a depth credit applied to 32%).

Hi Rainier. Not to be argumentative, but information about the differences between tables is easily obtained and there is a lot of it. For example: PADI's clock stops when you leave the bottom, Naui stops the clock when you hit 15', thus the time difference is less than the 5 minutes you think. There is much more....

As to the Navy, most of us are not Navy divers, we are not doing Navy dives, we don't have Navy support ships, emergency Navy equipment and trained Navy support crews, and we mostly consider ourselves to be less expendable than Navy divers when it comes to mission/attrition calculations. Navy tables are not designed for use by recreational divers and they are not the best guide for recreational diving.

When it comes to tables it makes more sense IMO for recreational divers to use the tables that were developed for recreational divers. And since PADI has trained more recreational divers than all other agencies combined, it makes more sense to me to use the PADI tables.

 

[Rainer]

Miche-

Feel free to use whatever tables YOU want. Those of us using slightly more aggressive tables (with respect to BT, but not ascents and gas choice) will continue to do so with no ill effects. I'm sure since PADI teaches more divers than any other agency, they are definitely the best resource for you and everyone else.

Hi Rainier. Not to be argumentative, but information about the differences between tables is easily obtained and there is a lot of it. For example: PADI's clock stops when you leave the bottom, Naui stops the clock when you hit 15', thus the time difference is less than the 5 minutes you think. There is much more....

As to the Navy, most of us are not Navy divers, we are not doing Navy dives, we don't have Navy support ships, emergency Navy equipment and trained Navy support crews, and we mostly consider ourselves to be less expendable than Navy divers when it comes to mission/attrition calculations. Navy tables are not designed for use by recreational divers and they are not the best guide for recreational diving.

When it comes to tables it makes more sense IMO for recreational divers to use the tables that were developed for recreational divers. And since PADI has trained more recreational divers than all other agencies combined, it makes more sense to me to use the PADI tables.

 

[Rainer]

Pretty much how I've planed my dives for several years (as do many other DIR divers). Of course, this approach does assume a particular ascent strategy and rules about surface intervals / repetitive dives (plus gas choices). A slightly more conservative rule for 32% that I know Sherwood teaches (and which I've used the last six months with my newly certified wife) is a 130 rule (no depth credit adjustment).

http://www.scubaboard.com/forums/381802-post28.html