Rule of thirds for redundant air supply

[Foxfish]

The SDI solo course book recommends using the rule of thirds as a way of determining the requirements for the redundant air supply when diving solo. This is the same recommendation for the reserve supply when diving with a buddy. In one case you are using the rule of thirds for a single diver and in the other for two divers. Please explain.

 

[KD8NPB]

I would assume they mean:
If you're using an AL80, your ideal redundant air supply would be a 26.4 cf tank...which doesn't exist, so you would carry an AL30.

For the question with two divers, I'm not sure.

 

[uncfnp]

The pony is your "buddy" and as such needs to carry the reserve for you that your buddy would otherwise be carrying. Per SDI, it is not to be used in calculations of gas needs for your dive plan. It is for emergency use only. Using the "rule of thirds" is a simple way to calculate the amount needed in your pony or redundant gas source.

Another way to calculate is to determine your SAC rate adjusted for the dive factor of your dive (or an average somewhat stressed RMV for you) your max depth and estimated time for ascent. Plug it all in and you arrive at a minimal volume for your pony. This is of course for dives that allow direct ascent to the surface in case of a catastrophic gas loss. There are several interesting SB threads that discuss.

I primarily drift dive so this was the formula I used to select the cf for my pony bottle.

 

[NAM001]

This is the same recommendation for the reserve supply when diving with a buddy. In one case you are using the rule of thirds for a single diver and in the other for two divers. Please explain.

As i am reading this.. It is trying to say that when diving with a buddy and you are both using a 90 cuft tanks then you use rule of 1/3 to deterine turn point. if at that point you have a loss of gas (your remaining 2/3 is not available to you) the buddies remaining 2/3 is enough to get you both back. Now with the solo dive you still use the 1/3 rule you turn at 2/3 and in event of a loss of gas (you loose your remaining 2/3) you switch to the pony that has 1/3 in it. the calculations are the same but the applicaton is different.

 

[Foxfish]

Rule of thirds is defined in the course books as having 1/3 of your original air supply available in your tank after completing the safety stop. The course books I mentioned above state you need the same volume of reserve air when diving solo that you'd need say deep diving with a buddy. In fact if you are diving with a buddy you will need twice the amount of reserve air that you'd need if diving solo.

 

[GrumpyOldGuy]

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.

 

[uncfnp]

I think I may understand your confusion. You are trying to apply the gas management/planning concept "rule of thirds" to your redundant gas supply. You are mixing 2 different dive applications. "Rule of thirds" as you know it, applies to gas management and dive planning to your primary gas or "back gas." You can use this rule in solo diving as well. But for solo diving, your pony is not calculated as part of your dive plan. It is for emergency use only per the SDI manual. I believe the manual references thirds only as a way to grossly estimate your pony bottle size. As I and others have said, a better estimate is to actually do the calculation. This is mentioned in a later chapter of the SDI manual.

 

[lowviz]

The SDI solo course book recommends using the rule of thirds as a way of determining the requirements for the redundant air supply when diving solo. This is the same recommendation for the reserve supply when diving with a buddy. In one case you are using the rule of thirds for a single diver and in the other for two divers. Please explain.

Reserve air supply (when buddy diving) is gas in your primary tank that you set aside for your buddy should it be needed.
Redundant air supply is gas that cannot be compromised by a total failure of your primary gas supply, in this case, a pony is the redundant supply.

Assume a dive where you have to exit at the same point you entered, like a boat that is tied into a wreck:

1) You and your buddy go straight out from the tie-in until the first of you hits "thirds" and you both turn. This is the most dangerous point in the dive. Your buddy loses all of his gas, including your third. You have two thirds to get two divers back out with nothing to spare.

2) You are solo diving, same dive, and lose all of your backgas at exactly the same point. Your pony holds one third, enough to get you out with nothing to spare.

-same.

Note that this only *just* works if both of your SAC (RMV) rates stay the same and you haven't violated NDL's.

 

[Foxfish]

I think I may understand your confusion. You are trying to apply the gas management/planning concept "rule of thirds" to your redundant gas supply. You are mixing 2 different dive applications. "Rule of thirds" as you know it, applies to gas management and dive planning to your primary gas or "back gas." You can use this rule in solo diving as well. But for solo diving, your pony is not calculated as part of your dive plan. It is for emergency use only per the SDI manual. I believe the manual references thirds only as a way to grossly estimate your pony bottle size. As I and others have said, a better estimate is to actually do the calculation. This is mentioned in a later chapter of the SDI manual.

In both cases the redundant or reserve air supply is just that. It should not be factored in as part of the air supply. The only difference is that for solo diving you keep this air in a separate tank.

The SDI course book backs up the statement regarding the rule of thirds with a calculation. This is actually the first time I've come across the calculation method in written form in a diving course. One of the main reasons for taking the SDI course over the PADI independent diver course was the course book provided by SDI and the perceived additional rigor in the course. So far that is proving correct. The calculation method was not included in the PADI deep diver course book. Like you I agree it is a worthwhile approach in these circumstances though frankly I can't see myself doing a 40 m solo dive.

I chose the SDI course over the PADI course because I hoped this calculation would be provided in the course book so I was pleased to see the calculation included, even if only briefly.

I've attached a copy of the calculation. It is pretty much identical to the results obtained in the SDI book. You need twice that amount of air for a buddy dive.

I also checked the results for a buddy dive and found the rule of thirds was inadequate for a dive to 40 m which does highlight the need for the calculation when progressing to deeper dives.

 

[Doc Harry]

1. "Rule of Thirds" does not apply to open water buddy diving. It is dangerous to do so, and will not give you enough gas to get out of an emergency in many situations.

2. "Rule of Thirds" applies only to buddy diving in an overhead environment. (EDIT: and tech/deco diving)

3. "Rule of Thirds" also has no logical basis in OW solo diving. Your gas reserve for solo diving is based upon your calculated needs, not some arbitrary "Rule of Thirds."

===================================

If you want a more detailed explanation, here it is:

"Rule of Thirds" applies only to buddy diving in an overhead environment. The basis for "Rule of Thirds" is that your turn around when you've used 1/3 of your gas (so 2/3 of your gas remains). If you use 1/3 to penetrate then you'll need 1/3 to exit, and that leaves 1/3 in reserve.

If you buddy suffers catastrophic gas loss at the point of maximum penetration, then you will still have 2/3 of your gas - 1/3 to get you out and 1/3 to share with your buddy for the exit. For a team of two, "Rule of Thirds" in an overhead environment is cutting it dangerously close and consideration should be given to using "fourths" instead of "thirds," or carrying stage cylinders for more gas reserves.

"Rule of Thirds" is dangerous when applied to open-water buddy diving. That is because in OW diving you must also account for the gas required to ascend. So for OW diving you must use a "Modified Rule of Thirds" in which you must subtract your "rock bottom" gas from your total gas before applying "Rule of Thirds."

For example:
OW Buddy Diving on the wreck of the Spiegel Grove
Total Gas = AL80 at 3,000 PSI
"Rock Bottom" Gas = 1,200 PSI
Subtract "Rock Bottom" from total gas: 3,000 - 1,200 = 1,800 PSI
"Modified Thirds" = 1,800 divided by 3 = 600 PSI (NOT 3,000 PSI divided by 3 = 1,000 PSI - this is wrong!)
Therefore your "Turn Pressure" is 3,000 - 600 - 2,400 PSI

If you were to use "strict" Rule of Thirds (3,000/3), and you used a "Turn Pressure" of 3,000 mius one-third = 3,000-1,000 = 2,000 PSI, then you would arrive back at the mooring line at a depth of 100 feet with only 1,000 PSI remaining, which is 200 PSI below your "Rock Bottom" gas!

"Rule of Thirds" has no logical basis for OW solo diving. You must calculate your reserves based upon the dive scenario. You cannot arbitrarily use a "Rule of Thirds." For example, in the above scenario on the wreck of the Spiegel Grove: worst-case scenario is a catastrophic gas failure at your maximum distance from the mooring line. So you have to decide whether you will use your gas reserves to do an emergency drifting ascent from the wreck, or if you will make your way back to the mooring line to ascend, and you must calculate the gas requirement for your chosen contigency plan. "Rule of Thirds" is irrelevant.