The Rule of Thirds.

[Uncle Pug]

This thread isn't about DIR....

It is about borrowing those good gas management practices from the technical realm that can be legitimately used by recreational divers to improve their diving safety.

It is also about dispelling the growing myth that somehow the rule of thirds applies to recreational divers... paradoxically the only time it applies... is when it is not enough!

 

[Aquamaniac]

It just dont matter what the subject topic is does it, Always come back to the same silly argument.
This WAS a great thread, I enjoyed it immensly, very educating.

.........................now where's that damned unsubscribe button.

Dave

 

[Uncle Pug]

Aquamaniac once bubbled...
It just dont matter what the subject topic is does it, Always come back to the same silly argument.

I wish people wouldn't argue.

And yes again... any topic can be turned to a discussion of the finer points of DIR.... just watch my lunch thread

 

[MikeFerrara]

Not to complain..but if someone wants to talk gas management lets do it.

It is a very interesting subject because though there are rules they are minimums. As one pushes past the"race for the surface when the gauge says so" mentality we find that having enough gas is good but having way too much is just more to carry. Knowing how much is enough requires the understandin of not only basic air management rules but how to apply them to cover the risks of a given dive.

I thought puting a thread like this out was a great idea. And then...someone has to ruin it with this DIR horse ***T.

DIRF grads are not the only ones who can dive and this I can prove. If you want to teach then do it without beating folks to death with acronyms. We don't want to scare folks away from learning gas management do we?

 

[Spectre]

Spectre once bubbled...

[rule of thirds is not enough] If you're a hoover on air consumption

If you are a hoover, that's already taken into account with your rule of thirds. [your 1/3s are just different].

[...]

Of course, if your buddy is a hoover, your remaining 1/3 isn't going to get them out!

I'd like to discuss a little more on the rule of thirds and how it relates to different SAC rates, as the joke I left in my post about it implies something that is basically wrong...

Although there is plenty of fine print, the rule of thirds will work just fine with a buddy team where the two buddies have much different SAC rates. The bold print is providing they have the same sized tank.

Rule of thirds: 1/3 in, 1/3 out, 1/3 reserve. When thinking it out, it is common to come to the conclusion that if you have a better breathing rate, your reserve 1/3 won't be enough for your buddy to get out. This is not really the case. You must take into account that the point where you are at your turn pressure is a different time than when your buddy is at their turn pressure.

Providing you both dive the plan, and turn with 2/3 of your gas remaining, and you have practiced air sharing drills enough that you are comfortable [e.g. no increased SAC rate], you will always have enough to get your hoover buddy back [providing the rule of thirds was appropriate for the dive in the first place].

For example: You both have AL 80s. Your 1/3 is 1000 psi. Both your turn pressures are 2000 psi. Lets say your SAC rate is 80% of your buddies... So when your buddy hits his 2000 psi, you will have 2200 psi remaining. Subtract the 800 you'll need to return [since the rule of thirds was appropriate], that leaves you with 1600 psi in reserve... more than enough to cover the 1000 psi your buddy requires.

The larger the gradient between two buddies SAC rates, the more buffer you have for unforseen problems above and beyond the catastrophic failure. e.g. 40% your buddy's SAC rate: 400 out, 400 back = 800 leaving 2200. 1000 for your buddy to return, you still have 1200 to account for further problems.

The closer your SAC rate is to your buddy, the more important it is to be prepared to handle OOA situations calmly, as the 'buffer' for increased SAC rates in emergency situations declines: 95% of buddy SAC rate: 950 out, 950 back, 1000 reserve for buddy = 2900 psi. Only 100 psi extra.

Now this all implies the same sized tank [and can be extrapolated out to larger tanks on the diver with a better SAC rate].

Now if you have different tanks, what would be the appropriate action plan? For example: You have a better SAC rate than your buddy. You carry a 90 cuft tank, your buddy a 120. So their 1/3 reserve is 40 cuft, plenty to cover your 30 cuft need. However your 30 cuft reserve does not cover their 40 cuft need.

So, does the 120 cuft diver plan on 1/3s [40 cuft in] and the 90 cuft diver plan on 25 cuft in [25 + 25 + 40 = 90], or does the 120 cuft diver reduce their plan and dive 30 cuft 3rds [leaving 30 cuft unplanned]. Or do you compromise: 120cuft uses 35 cuft, allowing the diver with the 90 to plan the turn at 27.5 cuft.

Obviously 120 turning at 40 is the least conservative, and 120 turning at 30 is the most conservative. However what is correct for planning rule of thirds with different sized air supplies?

 

[Uncle Pug]

MikeFerrara once bubbled...
I thought puting a thread like this out was a great idea. And then...someone has to ruin it with this DIR horse ***T.

The whole reason for this thread was to get some of the concepts that I learned back there in DIRf out onto this board so that recreational divers could benefit from some of the ideas ....

It isn't about DIR, and it isn't about arguing... it is about reasoning,

And it isn't swearing in asterisks... it is about patiently explaining w/humor,

And it isn't about technical diving protocols... it is about only taking from technical diving what can really be beneficial to the recreational diver and leaving behind the stuff that isn't.

And this thread was a good idea but it wasn't being done until I did it.

And everyone (almost) was a happy camper until you found out where the ideas came from... like that was a surprise.

 

[MikeFerrara]

UP,
I don't care where the idea came from. You seem to be a good on-line teacher/mentor. I think you have a real talent. Maybe I should apologize but I find the DIR/non-DIR argument that comes up in every thread distastfull. About now Roak will point out that it's not the dir crowd who makes it an argument. Ok. Maybe they don't start it but the insult others (ok, me) by insinuating that only GUE trained divers have a clue.

I guess that my one request would be more meat and less politics in otherwise good threads.

This is my opinion.

 

[Uncle Pug]

Spectre once bubbled...
The bold print is providing they have the same sized tank.

Different tank sizes are accomodated in the gas planning stage of the dive.

But please.... this is a topic for the tech forum.... the contention of this thread is that recreational divers (basic scuba) don't need the rule of thirds.... but far too many are enamored by all things tech and are using with thirds when it is really not in their best interest.

 

[MikeFerrara]

I agree that the 1/3 third rule is not what is called for in recreational diving. Planning to begin the ascent at a predetermined amount of gas (rock bottom) makes sense. I think, though, that if that amount of gas is specified in presure and different tank sizes are used it would need to be accounted for. If the gas needed for ascent is determined by volume it must be converted to presure for each tank which takes care of different size tanks.

I may have missed it in the thread but has the subject of determining sac or rmv been covered? Is gas matching only required for "tech" divers or is it required for any team who wishes to calculate a turn or ascent presure (based on actual needs)?

 

[Soggy]

Uncle Pug once bubbled...
the contention of this thread is that recreational divers (basic scuba) don't need the rule of thirds.... but far to many are enamored by all things tech and are using with thirds when it is really not in there best interest.

I disagree...there are circumstances when the rule of 3rds may not be "needed" for safety purposes, but definately adds to the dive.

The best example is a shore dive. Swimming on the surface back to shore, in a word, sucks. Use ~1/3 out to the furthest point, 1/3 coming back in and 1/3 for reserve. In this case, the thirds are not as strict as in tech diving (stretching the first third is okay because you don't need an entire third for reserve, as the surface is usually a short sprint away), but the principle is beneficial.