120/20 rule or a 130 rule?

[JessH]

I have been using the 120/20 rule for a while now and it has served me well, but recently I saw a thread where someone said that they learned a method where you start with 100ft and 20 minutes and then add or subtract 5 minutes as you move 10ft in either direction. Well for 100ft and shallower the 120 rule seemed to be more "accurate", but it did make me start taking a close look at various tables.

While doing this I realized that while simple, there is a far simpler way to calculate NDL when diving EAN32 than by using the 20% rule to calculate the EAD. Why not just use 130 or 140 instead of 120? 130 actually resembles the NOAA/NAUI EAN32 tables perfectly between 70ft and 100ft, which is just as accurate as the 120 rule ever was with the Navy tables. Just like the 120 rule outside of this range it will err on the conservative side. Of course if you do the EAD calculations a 140 rule would resemble the 120/20 rule better, but 130 matches the NOAA/NAUI tables as well as the Navy EAD tables so I think 130 is a better number to work with.

This can also be extended to EAN36. I know it is not a DIR mix, and I honestly don't have much use for it myself, but if someone wanted to they could use 140 to remember the NDL times between 80 and 110 feet.

Any thoughts?

~Jess

 

[texdiveguy]

Jess, in what application are you using this 'rule' and why....curious?

 

[Ben_ca]

This can also be extended to EAN36. I know it is not a DIR mix, and I honestly don't have much use for it myself, but if someone wanted to they could use 140 to remember the NDL times between 80 and 110 feet.

Any thoughts?

~Jess

For simplified Scuba math 32 give you 20 VS 36 giving you 25... plus the GUE standards mixes are simple 32 top offs...


As for your other questions I would (and I know you guys hate this) have to say.. Take the class... There's the bigger picture that cannot be learned from here.. but write down these questions and ask your instructor.

 

[JessH]

Jess, in what application are you using this 'rule' and why....curious?

I use this rule for "NDL" diving on Air and Nitrox respectively. I find it useful to not have to refer to a table every time that I want to know how long I can spend at a particular depth, etc...

The 120 rule is a simple way of remembering the Navy air tables for NDL diving. To use it you simply subtract your current depth from 120, and the result is the maximum NDL for a given dive according to the Navy tables. It doesn't work for all depths, but it is a good start and with a few additions can be used to remember the Navy tables for a single dive.

This simple method of remembering the tables has been around for a long time. EAN32 gives you rougly a 20% shallower EAD(Equivelant Air Depth) so some people calculate their EAD in their heads and then use the 120 rule. While this is very simple to do I came to the realization that you can skip a step and come up with more "accurate" results by using either 130 or 140 to begin with and not having to do any EAD calculations. While 140 does give closer results to using 120 with a 20% EAD than 130, I personally chose the more conservative 130 for EAN32 because I feel slightly uncomfortable using a method that results in a table that is even less conservative than what the navy is using.

~Jess

 

[JessH]

For simplified Scuba math 32 give you 20 VS 36 giving you 25... plus the GUE standards mixes are simple 32 top offs...

These numbers are useful and I do think that doing this math in your head is simple and useful. That being said, I think the simplicity of bypassing this step when calculating NDL is useful.

As for your other questions I would (and I know you guys hate this) have to say.. Take the class... There's the bigger picture that cannot be learned from here.. but write down these questions and ask your instructor.

I would like to take the class at some point, and while I would bring this method up with the instructor I don't seem to have any questions here that require taking the class. For that matter I don't really have any questions about this particular subject, I simply wanted to share my realization and if anyone has any comments on it I would like to hear them. My modification of the 120 rule, and the 120 rule itself isn't even DIR specific. I simply chose to post this here as this is where this type of discussion most commonly comes up since GUE is the only(?) organization that teaches these types of methods.

~Jess

 

[Kevrumbo]

MHK posted about the 120/20 rule a while back:

http://www.scubaboard.com/showpost.php?p=381802&postcount=29

For Repetitive Dives:
1.5 hour SIT minimum, then use 110 rule for Dive#2;
1.5 hour SIT minimum, then use 100 rule for Dive#3;
1.5 hour SIT, use 90 rule for Dive#4
1.5 hour SIT, use 80 rule for Dive#5
and so on. . .

 

[JessH]

MHK posted about the 120/20 rule a while back:
http://www.scubaboard.com/showpost.php?p=381802&postcount=29
For Repetitive Dives:
1.5 hour SIT minimum, then use 110 rule for Dive#2;
1.5 hour SIT minimum, then use 100 rule for Dive#3;
1.5 hour SIT, use 90 rule for Dive#4
1.5 hour SIT, use 80 rule for Dive#5
and so on. . .

This method is a very aggressive method of doing repetative dives and as such should only be done with minimum deco.

~Jess

 

[Kevrumbo]

Then be more conservative: for example, use 2hr SIT and then 100 rule for Dive#2; 2hr SIT and then 80 rule for Dive#3 etc.

 

[JessH]

Then be more conservative: for example, use 2hr SIT and then 100 rule for Dive#2; 2hr SIT and then 80 rule for Dive#3 etc.

I totally agree. Just felt the need to point this out incase someone decides to try it without actually comparing it to tables. If you compare it to most tables with a 1.5hr SIT you should actually use something more like 90 for the second dive. But you also have to remember that these tables were designed to try to keep you from getting bent even if you did a straight 60fpm ascent to the surface. So in practice the number you should use for the second dive could range anywhere between 70 and 110 (or even lower or higher). Which number you pick should depend on how much you are pushing things, how your ascent was,are you well hydrated, tired, out of shape, etc..... You just have to use your brain and be honest with yourself.

And this of course also works with Nitrox if you start with 130 for the first dive, etc...

~Jess

 

[Archangel]

Jess, methinks what Kev and MHK are advocating is that a diver THINK, THINK, THINK, rather than rely on a ridiculous padi table advocating 240 minutes at 30 fsw. Be a thinking diver rather than relying on a 'puter. Be conservative as is comfortable for you.