Rule of thirds for redundant air supply

[markmud]

You asked what size pony bottle you need for a certain dive. I'd start another thread and have your query discussed in some detail.

Hey Foxfish,

No, you did not read the post or I did not make my point (most probably the latter, and I apologize for that).

Bret Gilliam, Mitch Skaggs, and David Sipperly started TDI/SDI twenty years ago.

In publications, Bret Gilliam rails against institutions and divers who adhere to "rules of thumb" as if they are religious dogma. The founders of TDI/SDI were leaders in bringing Nitrox (Voodoo gas), solo diving, tech diving protocols, computers as a training emphasis, and many other logic-based procedures for diving to the mainstream.

Bret also railed against a 90s phenomenon that seems to survive here on some SCUBA board threads. Here is a direct quote:

"The mainline recreational diver was not the problem now, but rather the advanced or technical diver had emerged looking like some mutated version of a Navy SEAL crossed with the Sharper Image Catalog."

Bret also writes about dive trips where techies were fully geared-up with argon for insulation, doubles, deco gas (the whole get-up) for a dive to between 60 and 90 feet. They were in the tropics on a calm day. The recreational divers with a single tank and no redundancy, except for their buddies, were not equipment stressed, they accomplished more, and they enjoyed their dives far more than the overburdened techies.

At 50 fsw with my SAC rate of .48, I can breath off a:
--6 cf pony for 7.5 minutes (sucked dry)
--13 cf pony for 17 minutes
--19 cf pony for 25 minutes
--30 cf pony for 50 minutes
--50 cf pony for 83 minutes.

Which pony rig would be sufficient for my solo dive that I profiled above? My answer is: The 13 cf pony or the 19 cf pony rig. Too bad I am violating the "rule of thirds."

As my Solo instructor stated: "check your computer, and if you are not close to NDLs, go ahead and skip your safety stop if you are having issues such as buoyancy or other issues while on your pony rig, as the surface is your primary goal." "Don't swim to the boat on your pony; surface swim back to the boat."

As Bret writes regarding taking too much gear, "A tuxedo is not required for a backyard barbecue."

Tailoring your dive package for the dive's profile is sometimes more important than following a rule of thumb that was created for bigger and better dives.

I still think my point is germane.

I am sorry if you disagree; however, most people disagree with me.

markm

 

[Foxfish]

So you obviously don't need any help on this one. For what it is worth I think you make the point eloquently and agree and the post was an interesting read.

Just looking at the gas times you've listed for breathing off pony bottles I assume the 0.48 cfm would be your SAC. That would then need to be factored to get the time available in an emergency. I use a 2.7L/19cf pony. It is okay for most dives I do. For deeper dives like 30-40m I'd go for the 5.7L/40cf.

I like what your solo instructor said here:

As my Solo instructor stated: "check your computer, and if you are not close to NDLs, go ahead and skip your safety stop if you are having issues such as buoyancy or other issues while on your pony rig, as the surface is your primary goal."

 

[mick allein III]

I just read an article by Doppler-Steve Lewis and he recommends treating each dive as a pinnacle dive. Be prepared for an emergency at 130ft the rec limit and your covered for everything shallower. Simply put- more air good. I had the same question regarding being overburdened by too much equipment and started a thread minimalist solo diving. It's interesting to hear that TDI/SDI people advocate pony's smaller that 40cf. The solo manual doesn't read that way. Other thoughts?


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[Doc Harry]

Thanks Devon for your clarifications, I agree with most everything you say. I guess our differences are mostly a matter of word definitions and terminology but I think we're generally on the same wavelength. I think something is still getting lost in the translation, though, I apologize for any confusion but I don't how I can get you to see what you're missing.

"Thirds" is not always calculated as 1.5x primary, all depends on your goals. Sometimes you just penetrate until you've used 1/3 of you gas and then you turn around. That's also called "Thirds." If I am trying to reach a specific room before turning around, then 1.5x would be appropriate.

I see too many rec divers try to apply that same rule, but they don't understand the implications and a rec diver cannot simply keep going until they hit thirds in ALL situations. It doesn't work.

I agree with markmud, "Rule of Thirds" for soloing is just a rule of thumb and you need to make some real gas calculations, not just blindly follow a rule of thumb.

After you've made the calculations and you understand what you're doing, then you can intelligently follow a rule of thumb because you understand the implications and consequences of your choices. Obviously the OP does not fully understand solo gas planning, hence their question. To instruct someone like that to blindly follow a rule of thumb, without a full understanding of why they are following a rule of thumb, is bad advice.

I think Devon did a good job of explaining a lot of things.

Foxfish, I used the same pony bottles as you if I am doing solo rec dives. The 19 is my primary and I generally don't go below 100 feet when rec solo.

Thanks

 

[Foxfish]

I just read an article by Doppler-Steve Lewis and he recommends treating each dive as a pinnacle dive. Be prepared for an emergency at 130ft the rec limit and your covered for everything shallower. Simply put- more air good. I had the same question regarding being overburdened by too much equipment and started a thread minimalist solo diving. It's interesting to hear that TDI/SDI people advocate pony's smaller that 40cf. The solo manual doesn't read that way. Other thoughts?


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Actually I think you'll find Steve Lewis recommends the 40 cf tank from what I've seen of his comments on other threads. I'd guess that is because it is suitable for all depths down to 40 m. I'll let him speak for himself on that one.

 

[MXGratefulDiver]

I cannot answer for SDI

I do many solo dives of a boat with 30 other dives in the same ocean, so my reserves include enough to handle running into someone in trouble even though they are not my buddy.

IMHO, Rules of thumb are a starting point, an approximation for those too lazy/stupid/rushed to calculate.

I don't understand why anyone would teach them as any more than a general reference, as rock bottom can give you an accurate value for reserve air (with or without a buddy) and be logically applied to you your situation. If your diving solos, working with rock bottom should be a well established practice.

... he doesn't believe in rock bottom ... we've established that already in another thread ...

... Bob (Grateful Diver)

 

[Foxfish]

... he doesn't believe in rock bottom ... we've established that already in another thread ...

... Bob (Grateful Diver)

It would be more accurate to say I don't see the merit of people diving OW depths being forced to learn "rock bottom" calculations. Back the the topic at hand.

 

[GrumpyOldGuy]

It would be more accurate to say I don't see the merit of people diving OW depths being forced to learn "rock bottom" calculations. Back the the topic at hand.

Sorry I missed the other thread. I think you have the bar set pretty low if calculating rock bottom is a roadblock.

 

[Foxfish]

It's not. Do you have any comment regarding the question raised in the OP?

 

[GrumpyOldGuy]

It's not. Do you have any comment regarding the question raised in the OP?

My comment was in post #6, Diving 1/3rds is a crude rule of thumb, thus I don't really look for sound logic from an agency who preaches it.