Beyond blind rote air management

[MXGratefulDiver]

The key thing is knowing how to determine how much gas you are likely to use for the dive you are planning to do. This is especially true for solo diving ... keeping in mind that your bailout is NOT part of your gas plan.

Perhaps there are some concepts in this article that you'll find helpful ... Understanding Gas Management

... Bob (Grateful Diver)

 

[DivemasterDennis]

The 1/3rd reserve rule of thumb is a good general rule, but clearly Kharon is right about variations thereon. Depth is a reasonable variation, unless the hazard for which you want a reserve is entrapment or entanglement, in whihc case you want all the air you can get. Tank size is a reasonable variable to use in deviating from the rule of thirds too, unless of course, you use a bigger tank because you use more air than others. 1/3 of a 100 may be the same time for the high rate user at 1/3 of 80 is for the more moderate consumer. The pointis that gereral rules are about as good as one can do, because general exceptions aren't much better. Now INDIVIDUAL rules work, and each diver can create them based upon logging a slew of dives, analyzing their air consumption under different conditions, depths, and with different equipmnet, and chart out their rules. Debbie and I have done this, to a degree, based upon hundreds of logged dive data. ( A reason to log dives, don't you know) However, for the casual recreational scuba diver who does10 or less dives per year, I recommend following the general rules as a nice safety margin. As for the intrepid diovers of great experience, I see no problem in deviation from the generalel rules which is consistent with your diving experience and track record. BTW, neither Debbie or I have ever ( and that's over 10000 dives, run out of air or come close, at least not too close.
DivemasterDennis

 

[TMHeimer]

I agree with all who say it's not a strict law. I don't even recall hearing it mentioned when I took OW. I guess I first heard of it was during either the Deep or Wreck course--maybe AOW? Obviously thinking is the answer. And as pointed out, you can make decisions based on environment and a bunch of your past dives to various depths. A good rule I recall learning is keep 100 PSI in reserve for every 10 feet of depth. So you figure out what your time is out and back from that. Obviously if diving 30' or less you don't need much reserve--only for rare emergencies or so you don't have so little left as to risk water entering the tank. I have figured that for an 80' dive on an AL80 for me it's almost always 10 mins. out and 10 mins. back. I can now use that as a reference point for differing depths. If you know your SAC rate and the conditions of the dive (current?) you can figure it out for either PSI or time, keeping track of both during the dive. It's just using your head.

 

[boulderjohn]

The rule of thirds is one system of gas management that is used in some circumstances. It is not commonly used in normal open water diving, though. When I wrote my Dive Planning course, PADI at first argued that it had no role in recreational diving and did not need to be included in the various options, but they finally accepted that under certain circumstances it is a good choice.

What about the rule of halfs?

We were taught to turn around and ascend to half the depth when the first person got down to half their tank or you reached half of the allowed dive time.

This was for caribbean boat dives.

This is still in use in Bonaire as of last week.

A thinking diver uses whatever system works for the dive being done. The rule of halves is a good choice under certain circumstances, too. When I teach it, I use Bonaire as my example of a dive site where the rule of halves makes perfect sense.

 

[DaleC]

I have never encountered thirds in rec diving beyond scubaboard. Initially I was taught to surface with 500psi minimum (wah?). Then I learned about rock bottom or what some call minimum gas. I would not follow anyone's plan suggesting thirds.

 

[TMHeimer]

What about the rule of halfs?

We were taught to turn around and ascend to half the depth when the first person got down to half their tank or you reached half of the allowed dive time.

This was for caribbean boat dives.

This is still in use in Bonaire as of last week.

This obviously doesn't work in places where there is nothing to see half way up the water column (ie. one-depth diving to the bottom-- square profile). I would guess a good % of the world's dive spots.

 

[boulderjohn]

Like the rule of thirds, the rule of halves is actually more of a generic term for a set up dive profiles. The rule of thirds describes a set of dive profiles for situations in which it is essential that a diver return to the starting point before beginning the ascent. The rule of halves works for situations in which it is nice to get back to the starting point, but not essential.

If you are using the rule of thirds and are heading out with the current going with you, you will definitely want to turn the dive before you reach thirds. If you are diving the rule of halves and the dive is gong to be all at the same depth, you will want to turn before you reach halves. Once again, you have to do some thinking. If you are just blindly using thirds or halves without adjusting to the reality of you dive, you can potentially make a serious mistake.

 

[kwinter]

I like the fact that the OP is thinking and challenging a commonly heard term. But I also think he is confused or incompletely educated about some of the finer points. Reserving one third of your gas supply isn't just for an 80 cf dive with a buddy on a 60-90 foot dive. It came out of cave diving, where there was only one way in and you had to return the same way. And it isn't just for "ourselves and a buddy to get back to the surface." While that would be the most common scenario, your own respiratory rate can skyrocket due to stress or CO2. You might find that the 2/3 you have at the end of your planned bottom time might barely be enough for you alone on a solo dive when the smelly stuff hits the spinny thing. It all depends on the specific situation you might encounter on the dive.

And if you're using a 100 or 120 or carrying a pony bottle, it might make sense to just reserve the same 27cf you calculated based on an 80 if you're dive is only extending bottom time in the same place. Yes, you are correct when you think about a simple dive where you can head to the surface from wherever you are, nice and calmly. But there are plenty of dives other than cave dives where you really need to get back to the same starting point and therefore need to reserve enough gas to do that. So again, the amount of reserve depends on the specifics of the dive.

It seems to me that the rule of thirds is a necessary evil to make the OW classes less complicated. However, once we’ve got our skills down, I believe we need to start thinking, rather than blindly following an overly simplistic generalization.

The rule of thirds is not even taught in many OW classes because of its limited application for those simple dives. And you are correct that as you progress in your diving you need to start thinking rather than blindly following a rule. But that thinking requires an understanding of the reasons that guidelines developed. The smart diver asks questions about it. The arrogant ones assume they know the reasons and post things online that criticize a well-established safety protocol and demonstrate their own ignorance.

 

[Peter Guy]

I'm teaching my first PADI Open Water class using the new curriculum which includes some basic gas management dive planning. The video and the book discuss setting up a reserve and then using halves of your "available gas" which I like a lot. It was funny, however, when one of the students said he was confused by a "test question" that used thirds. He just didn't understand it.

OK -- query -- in recreational diving, should you EVER do a "thirds" dive? My answer is "No" -- but it really depends on one's definition of "thirds" or "halves" or "all available air.". As someone mentioned, "thirds" come from overhead environment diving -- that is, when you MUST get back to a point certain (upline for example). IF you must get back to a point certain, how can you be doing a "recreational dive" which has as one of its controlling parts that the surface is always an option?

The OP is correct that the "reserve" is for getting you and your buddy safely back to the surface and that is really totally independent of the size of the tank. OTOH, the "available gas" for your dive (descent and wandering around) is totally dependent on the size of the tank (minus, of course, your needed reserve). Fun stuff, huh?

 

[boulderjohn]

OK -- query -- in recreational diving, should you EVER do a "thirds" dive? My answer is "No" -- but it really depends on one's definition of "thirds" or "halves" or "all available air.". As someone mentioned, "thirds" come from overhead environment diving -- that is, when you MUST get back to a point certain (upline for example). IF you must get back to a point certain, how can you be doing a "recreational dive" which has as one of its controlling parts that the surface is always an option?

I agree with you in general about thirds, but I think there may be situations in which it is advisable to think along those lines, even though it may technically not be true. For example, if you are visiting a wreck with an upline and a strong current, you might be advised to plan your dive to thirds because you would rather not do your ascent off the line in current. That might being you to the surface away from your boat and bring more complications than you would like.