120/20 rule or a 130 rule?

[MonkSeal]

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

IMHO every dive inside MDL should be done with minimum deco.

 

[Charlie99]

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.

Ignore for a minute that 240min @30' isn't on any PADI table that I've seen.

Why is 240 minutes at 30' more aggressive or ridiculous than
80' for 40', 90 minute SI, followed by 80', 30 minutes?

Please help me THINK, THINK, THINK, as you put it.

Charlie Allen

BTW, if you are going to say "It's OK because of the slow ascent and stops, please tell me what they are so that you find the 80' 40min, 90 SI, 80' 30 minute profile reasonable.

 

[peter_steinhoff]

Just though I'll add something to this discussion.

In my opinion you are better off forgetting all about air diving and learning some "limits" for nitrox 32% instead since that is the preferred gas for 0-100ft/30m. As a DIR diver you should not really dive air if it can be avoided.

So think about this for a second. Why even bother with an "air rule" and calculate EAD (20% off) each time when all you need to do is memorize a couple of depths for nitrox?

For instance from the NOAA EAN32 table (available online and derived from USN table):

• 100ft 30min
• 90ft 40min
• 80ft 50min
• 70ft 60min
• 60ft 100min
• 50ft 200min

Have a nice day!

Peter Steinhoff
http://dir-diver.com

 

[JessH]

So think about this for a second. Why even bother with an "air rule" and calculate EAD (20% off) each time when all you need to do is memorize a couple of depths for nitrox?

Exactly what I was thinking. And a 130 rule seems to be a decent place to start, atleast for me. It gets rid of all of the air rule EAD sillyness. It gives you the numbers between 100ft and 70ft, then it is rather easy to remember the exceptions for outside of that. For contingency you can remember the deeper depths by simply remembering that 110 is actually the same as 100 and then you subtract 5 for every 10 feet after that(the subtracting 5 also works for depths deeper than 100 with the air rule). At 60ft you jump to 100 min and for every 10ft shallower you double it(also works for the air rule starting at 50ft). 20ft and shallower with EAN21 or more you have unlimited bottom time but you can still build of a good amount of residual nitrogen.

Rather simple and now you know the tables for EAN32 and EAN21. EAN36 is also easy to add. You can also come up with simple rules for EAN28 and EAN30, of course out of all these mixes the only one a DIR diver should be diving is EAN32, but there are times when this is not feasible for various reasons so I do feel that it is useful to know the other tables, especially since it is so simple once you know the one for EAN32.

~Jess

p.s. Thank you Peter for the great website and all of the great information!

 

[Ben_ca]

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.

It's the big picture that you are missing... Like Kev and the others said you have to always be thinking.... have that computer between your ears running all the time...

The GUE tables are supposed to be used with deep stops and the other ascent profiles...

Now if you mess all those up and do a direct ascent to the surface will you be bent? Probably not but you might not feel 100%... You have to use all the tools and not piece meal...

 

[JessH]

while I don't disagree that there is a big picture that should be kept in mind, these are NOT GUE specific tables. They are the same tables used by the US Navy, NOAA and even NAUI. It is simply a simple method of remembering them. Since I was trained by NAUI, there should be no problem with me using these tables that I have memorized. When it gets into repetative dives that is a different matter, but luckily I am a THINKING diver. You do not have to be trained by GUE to know how to use your brain. I personally think GUE is great, but I havn't had the oportunity to take DIR-F yet. I don't think that should keep me from using my brain, as long as it doesn't tell me to go and do something stupid ;-)

 

[amascuba]

while I don't disagree that there is a big picture that should be kept in mind, these are NOT GUE specific tables. They are the same tables used by the US Navy, NOAA and even NAUI. It is simply a simple method of remembering them. Since I was trained by NAUI, there should be no problem with me using these tables that I have memorized. When it gets into repetative dives that is a different matter, but luckily I am a THINKING diver. You do not have to be trained by GUE to know how to use your brain. I personally think GUE is great, but I havn't had the oportunity to take DIR-F yet. I don't think that should keep me from using my brain, as long as it doesn't tell me to go and do something stupid ;-)

The difference from a Haldane based deco model (i.e. navy tables, rdp tables, buhlmann tables, etc) and the tables like gue (minimum deco) and naui (rgbm) are now providing is significant.

The Haldane based models are a disolved gas model. Most of those tables using the disolved gas principles say that the inert gases in your body won't start turning into bubbles until you reach a certain pressure on your body, say 10fsw. These tables bring you from depth directly to your 10ft stop so that your body can offgas enough residual inert gases so that the bubbles won't grow into such a size that it will cause dcs.

The tables that are based off of the bubble model, such as the tables that gue and naui provide say that the bubble formation will exist much earlier during the ascent and accounts for that by making multiple deep stops instead of a single stop. This allows your lungs to purge the inert gases from your body more effectively than a disolved gas model.

Some of the disolved gas and bubble models may have similar numbers when it comes to depth/bottom time, but their application of the tables are different. Using a minimum deco table, but using the rules from the navy dive tables aren't what the tables were designed to do. If you want to memorize a table then memorize the tables that you are using and apply them how they were meant to be applied. Correct me if I'm wrong, but it almost sounds as if you are trying to take the features from each table that you like and apply it in a manner that it wasn't meant to be applied.

Look at Peter Steinhoff's website (http://www.dir-diver.com). He has an excellent article on the application of the minimum deco tables along with a very nicely laid out minimum deco table ready to print out. That should provide you with what you want.

 

[Charlie99]

The Minimum Deco tables on www-dir.com were developed using the dissolved gas Buhlmann ZHL16 model (tweaked by 30/85 gradient factor), NOT a bubble model.

In practice, the specific model doesn't have that much effect in minimum deco or NDL diving, other than giving you guidance as to the optimum ascent.

The bubble models, such as VPM, contain dissolved gas compartments within the model. That's what's driving the bubble portion of the model. That's why the GF tweak on dissolved gas models comes up with results similar to dual phase models.

One cannot make the problem of dissolved gas go away simply by ascending a bit slower. This seems to be a common misconception in the DIR world.

Deep stops and slow ascents help by keeping you closer to the optimum profile. By reducing bubble formation, most gas is kept in the dissolved form, which is more quickly offgassed. Deep stops and slow ascents help, but they do NOT eliminate the need to pay attention to dissolved gas.

 

[BarryNL]

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...

Given that this is the DIR forum, shouldn't the general attitude be that you plan your dive and dive your plan?

Personally, I take the attitude that you either do plan your dive and dive your plan or you strap on a computer. Trusting your brain to be able to get the limits right always seems like an accident in waiting.

 

[JessH]

I do plan my dives, I just don't have to look at a table to do it. I don't personally see how I am going to get them wrong when it only requires remembering a single number and then doing very basic subtraction. To quote MHK on the topic: "If they can't do that then they shouldn't be diving." (http://www.scubaboard.com/showpost.php?p=381802&postcount=29)

~Jess