Beyond blind rote air management

[Kharon]

Something has been percolating in my head for a while now. I understand the reason for the rule of thumb – it’s easier to teach, easier to understand, easier to put into practice, and in most instances it’s workable. The problem is that it is simply wrong in a lot of cases.

What I’m talking about is the rule of thirds. It is a more or less reasonable rule of thumb if we’re talking 80 cf tanks when diving with a buddy on reasonably deep dives – say 60 to 90 feet. We are taught to reserve one third of our air supply for emergencies. That should cover ourselves and a buddy to get us both back to the surface.

One third of an 80 cf tank is just shy of 27 cf. If we are diving a 100 cf or a 120 cf tank wouldn’t make better sense to reserve the same 27 cf rather than an arbitrary third? After all the dive parameters haven't changed. What if we are carrying a pony? Does that reduce the amount of main tank air we need to reserve? What if we are diving a 64 cf tank or as I often do a 50 cf? One third there is only 21 cf of a 64 cf or less than 17 cf for my 50 cf. What about a diver using 80 cf doubles? Should they hold back just over 53 cf?

How about the issue of diving solo. There is no buddy to reserve air for. If we have a sufficiently sized pony why should we reserve a rather large amount of main tank air to cover a non-existant buddy?

What about a diver limiting their depth to 20 or 30 feet? Do they need to hold back the same reserve as a diver hitting 90 or 130 feet? Do we need the same reserve in 80°F water as in 50°F water? The same reserve in water with 100 foot visibility as in water with 10 foot visibility?

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.

 

[rivers]

that is why minimum gas is beautiful thing

 

[LiteHedded]

you dont always need a third of your gas in reserve

 

[nimoh]

I wouldn't use thirds on an open water dive.

My reserve would be the minimum amount of gas to get two divers to the surface, which is much less than a third of the gas I am carrying. On a 60' OW dive with an AL80, I would set my minimum gas at 900psi. If I had my double HP130s, I would go with 400psi.

 

[Steve_C]

Thirds is a suggestion. Not a law. What makes you think that nobody else is thinking and everybody else is blindly treating it as a law?

For most of the diving I do the goal is not to get me back to the surface safely. It is to get me and buddy back to the boat safely. Diving a larger tank means that I may be further from the anchor line and thus need more air to get back to the anchor line. So I start out with an idea of thirds and then modify depending on the particular dive. Sometimes it changes during the dive. For example was doing a second dive to 70 ft. There was a heavy thermocline at about 40 ft. Viz above around 70 ft. Viz below maybe 5 ft. Went down to 70 to show a couple divers a couple critters. Then I went back up to 35 ft and just hung around for a while watching the life swim by. At that point I was diving solo and in a 35 ft dive. So I greatly reduced my pressure at which I would go up. Pony was not included in air calculations. It never is.

 

[victorzamora]

As has been said, thirds is really a cave diving rule that has eeked out. However, it DOES have its applications in the OW. For example, an unguided dive along a wall. Swim a third of your tank down in one direction, swim a third of your tank back, and then the third can be re-divided to allow for further diving. The idea of leaving that third there is that if it takes YOU a third to get down, you can assume it takes you a third to get back. The "reserve" third is for your buddy.....so if you're as FAR as possible from the start point (anchor line, shore, etc), you would theoretically have enough gas for you AND your buddy to get back.

MinGas or RockBottom are MUCH better for calculating gas reserves in the OW where ascents and descents must occur in the same place. I've got a MinGas/RB table for different sized tanks that I base my decisions off of in OW. If you want it, PM me an e-mail address and I'll send it your way.

---------- Post added April 11th, 2014 at 10:54 AM ----------

Cool thing about MinGas is that, if the underwater terrain is right, you can continuously push closer to the end of your tank by getting shallower. If you need X cubic feet at 100fsw, you need a LOT less at 30fsw. Ascend, keep piddling, and then eat in to what WERE reserves.

 

[underH2Oman]

Your goal is to always have enough gas to exit the water calmly for you and your buddy. Over the many years of diving I have seen several equipment failures due to freeze up or o-ring failure. Most divers tend to get excited when they have this type of issue under water and calm can happen if divers are prepared for it(most are not) Definitely plan your turn pressure for every dive with your buddy. You both will benefit from planning.
[FONT=Helvetica Neue, Helvetica, Arial, sans-serif]Do you need to use the rule of thirds on every dive-no as long as you have sufficient to exit calmly in the case of total gas loss. Varied environment will play a part as well. If you dive cold water and there is a risk of a reg freezing, and yours does be confident that so will your buddies if both of you are breathing from it.

The 1/3rd rule is designed to determine a turn point to ensure you have sufficient gas to exit safely with your buddy from an overhead environment (cave,deco,ice etc).[/FONT]
[FONT=Helvetica Neue, Helvetica, Arial, sans-serif]One third of the gas supply is planned for the outward journey, one third is for the return journey and one third is a safety reserve.[/FONT]^[1][FONT=Helvetica Neue, Helvetica, Arial, sans-serif] However, when diving with a buddy with a higher breathing rate or a different volume of gas, it may be necessary to set one third of your buddy's gas supply as your remaining 'third'. This means that the turn point to exit is earlier, or that the diver with the lower breathing rate carries a larger volume of gas than he alone requires.[/FONT]

[FONT=Helvetica Neue, Helvetica, Arial, sans-serif]As a note an aluminum 80 has only 77.4cft and the 1/3 should never be planned to use the entire volume lets say we leave 300psi in the tank at the end. that wold be 7.75cft giving us a usable volume of 69.65 cft so using the rule of thirds turn should be when 23.21cft are depleted or at 2100psi. [/FONT]

 

[Bob DBF]

Something has been percolating in my head for a while now. I understand the reason for the rule of thumb – it’s easier to teach, easier to understand, easier to put into practice, and in most instances it’s workable. The problem is that it is simply wrong in a lot of cases.

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.

You have it right, except that it is not to make the OW classes less complicated, it is to make it faster and cheaper to teach. It is not that the students can not understand, but that it takes time to train knowledgeable, thinking divers; one could teach another class instead. What I think is interesting is that the air management concepts are not expanded upon in AOW.




Bob
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That's my point, people, by and large, are not taught that diving can be deadly, they are taught how safe it is, and they are not equipped with the skills, taught and trained to the level required to be useful in an emergency.

 

[KeithG]

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.

 

[Steve_C]

A small pony you carry is more useful than a huge pony back on the boat.

In the same way when they are starting out, a simple rule is more likely to stick with them AND get used, then a complicated bunch of gas planning when they are still trying to remember everything else and still enjoy a dive.

With instabuddies where I am the lead, we agree on ascent pressure. Then I usually say "We will dive thirds except when we get back to vicinity of the boat, if we are above ascent pressure, we will look around a bit." It is easy to follow, no one has every complained, and it gives us relaxing safe dives.

---------- Post added April 11th, 2014 at 10:32 AM ----------

What about the rule of halfs?

I thought that rule was you turn the dive when your better half is cold?