Gas planning question

[Instructor_Tom]

"Rule of thirds" is gas a management rule for overhead environment. Wether that be a cave or a decompression commitment which would make an emergency ascent unadvisable. 1/3 of you gas in from your entry point, 1/3 of your gas out to your exit and 1/3 of you gas should your buddy lose all his gas at the point of maximum penetration.

This also means that you do not plan your gas and dive based on your SAC rate, but based on the highest sac rate and the smallest tank being used.

When not diving in overhead and not hitting a decompression obligation your gas calculation can be based on maximum use. You can develop you own rule based on ascent rate and depth to account for your own personal comfort level. Rule of thirds does not apply since you have no overhead obligation and can abort a dive at any point due to a failure and exit with your buddy sharing gas in the ascent.

My personal rule is 500psi from 50ft or less, and 1000psi from 50ft-100ft. Beyond 100ft the NDL gets very short so i plan this as technical dives with Deco.

With side-mount the trick is to walk the tanks down and keep the within 500psi of each other. I personally treat side-mount tanks as 1system if my left side tank reads 1000psi and my right side tanks reads 1000psi, i have 1000psi in the system which will get used ascending from 100ft dive. These are "My rules". You will eventually make your own based on experience.

Never worry about having extra air in your tanks at the end of a dive.


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[tursiops]

Part of the beauty of sidemount is you are likely to lose less air than you might for a similar failure in backmount, because you can see the valves and get at them easily and quickly and accurately. It is possible to lose ALL or MUCH of the gas from a backmounted system if you don't get to the isolator in time after a neck o-ring failure, for example, but you can't lose more than half your gas from a sidemounted neck o-ring failure, plus or minus a little bit.

Sidemounters diving together are not carrying air for each other, in general, only for themselves. Yes, it can be shared, but since the buddy is not likely to have lost all of his/hers, they can still carry on for the exit.

The problems come from mixed teams....some BM and some SM. Who is carrying what for whom? A very good thing to talk about and work out the gas-matching in advance...

 

[Scubadude_LA]

My two cents and elaborating on the above: Thirds is Thirds. As stated that may or may not be enough reserve depending on the situation. Regardless of what you decide to reserve there is one big difference regarding gas management between BM and SM, as stated previously. With BM you don't have to think about anything but the turn pressure, and not breaching it, the gas in the tanks can be used by both regs if the isolator is open. With SM there is no isolator (unless you are using one of the new turbo complicated manifold solutions that IMO, a humble IMO at that, presents a more failure prone gizmos than a BM manifold setup does, that's just me at this point), you need to make sure your buddy and you can be trusted to balance the tanks during the dive, or you will end up doing "REAL" buddy breathing, sharing a single reg, which sucks in perfect situations and gets progressivly more ugly after that.

I'm no pro, but here is a scenario:
Your diving using 1/3s in two man team both in SM. You and your buddy are diving same size tanks, same fill pressure, same rmv, say double AL80s so 6000psi each diver. You have been doing your reg switches as needed and you are at 2000psi on each tank, dive is turned. Extreme worst case scenario is a total OOA of a diver (You) at this point and it happens. The donor diver slings you the long hose and he gets on his necklace. The donor has not managed his regulator switches properly (easy human error, especially if you don't know how or why to do this). So he has been breathing on his long hose the whole time so, 2000 psi used up in the long hose tank. Your are 2000psi from the surface and you are on a tank with 1000psi in it.

In BM the total available 4000psi would be avail to both regs, so 2000psi each.

I like BM for some reasons like stated above, but the manifold creates failure points too, if the isolator valve or manifold orings go you lose it all. I like SM to reduce the failure points and allow tank handoffs, but more manual input on balancing the tanks, and more clustery when sharing at the end of the gas supply if not properly managed.

Anyway, that fact that you asked the question is good. But, you may not have trained in SM if you are asking, maybe?? Find an good mentor to guide you, or train. There is a difference in BM/SM for sure. To me there is more need to make sure you and your buddy are swapping regs and tank balancing, or you will end up sharing one reg and that could get real ugly.

 

[rongoodman]

I'm no pro, but here is a scenario:
Your diving using 1/3s in two man team both in SM. You and your buddy are diving same size tanks, same fill pressure, same rmv, say double AL80s so 6000psi each diver. You have been doing your reg switches as needed and you are at 2000psi on each tank, dive is turned. Extreme worst case scenario is a total OOA of a diver (You) at this point and it happens. The donor diver slings you the long hose and he gets on his necklace. The donor has not managed his regulator switches properly (easy human error, especially if you don't know how or why to do this). So he has been breathing on his long hose the whole time so, 2000 psi used up in the long hose tank. Your are 2000psi from the surface and you are on a tank with 1000psi in it.

You're postulating the OOA diver loses BOTH tanks simultaneously, either of which individually could get him to the exit?

 

[Ste Wart]

Part of the beauty of sidemount is you are likely to lose less air than you might for a similar failure in backmount, because you can see the valves and get at them easily and quickly and accurately.

Not at all.

If you shut down a tank in sidemount you have lost that tank; effectively you have just lost half your gas. If you shut down a tank in BM you still have all the remaining gas in the system. Yes on BM you may lose more into the water column, but after the shutdown you still retain more for use.

It is possible to lose ALL or MUCH of the gas from a backmounted system if you don't get to the isolator in time after a neck o-ring failure, for example, but you can't lose more than half your gas from a sidemounted neck o-ring failure, plus or minus a little bit.

A rarity at best and not a reason to dive Sidemount in OW

Sidemounters diving together are not carrying air for each other, in general, only for themselves. Yes, it can be shared, but since the buddy is not likely to have lost all of his/hers, they can still carry on for the exit.

Basically a Cave example, which is where SM should only be used.

The problems come from mixed teams....some BM and some SM. Who is carrying what for whom? A very good thing to talk about and work out the gas-matching in advance...

Agreed.


Just don't dive mixed teams. Unless needed for the task don't dive SM. Why would you want to? In an air sharing emergency I would rather be being donated gas by BM.

As an example: 1 diver on BM using twin 12s and one dive on SM using independent 12s
Total gas in each system is 4800L (based on a 200bar fill)
If you set the Minimum gas to 1200L (based on an ascent from 30m with both divers breathing 30lpm) this equates to
50 Bar in the Twinset
50 Bar in each of the SM Tanks

In air sharing emergency which is the better option? BM
Why?
The BM system can compensate for one divers' SAC being higher than the other, a SM system can't. If the OOA diver's SAC spikes beyond 30lpm on the SM tank, then you will not reach the surface. If it spikes on BM then (hopefully) the donating diver is keeping their SAC under control and can deal with the surplus gas use.



Each system has it's benefits and flaws. Personally unless the situation absolutely warrants it i.e. extreme restriction (which includes shoulder injuries, which no doubt someone will mention ), SM is not the tool for the job.

 

[tursiops]

If you shut down a tank in sidemount you have lost that tank; effectively you have just lost half your gas. If you shut down a tank in BM you still have all the remaining gas in the system. Yes on BM you may lose more into the water column, but after the shutdown you still retain more for use.

And in SM you can still get at the gas in the shut-down tank, by feathering the valve. Not readily done in BM.

I agree, SM is not for all situations. But in situations where both SM and BM are applicable, I'll choose SM. I REALLY like having both valves right at my hands.

 

[Ste Wart]

And in SM you can still get at the gas in the shut-down tank, by feathering the valve. Not readily done in BM.

True. But you don't need to in BM. Shutdown and breathe no hassle

I agree, SM is not for all situations. But in situations where both SM and BM are applicable, I'll choose SM. I REALLY like having both valves right at my hands.

Fair enough. I remain unconvinced on SM for anything other than task specific dives.

 

[Scubadude_LA]

You're postulating the OOA diver loses BOTH tanks simultaneously, either of which individually could get him to the exit?

Yes, I said worst case scenario. Just trying to convey my view point, and my example used nice round numbers. Obviously, the event would be a rare one.

 

[PfcAJ]

And in SM you can still get at the gas in the shut-down tank, by feathering the valve. Not readily done in BM.

I agree, SM is not for all situations. But in situations where both SM and BM are applicable, I'll choose SM. I REALLY like having both valves right at my hands.

depends on the failure.

its so weird how some of us have no problem manipulating valves in BM. ive also never wished i could see them. curious indeed.

 

[kwinter]

If you shut down a tank in sidemount you have lost that tank; effectively you have just lost half your gas. If you shut down a tank in BM you still have all the remaining gas in the system. Yes on BM you may lose more into the water column, but after the shutdown you still retain more for use.

Overly simplistic. Lose a tank neck o-ring or burst disk on manifolded backmount doubles and all of your gas will be lost unless you shut the isolator. Now you've lost half your gas just like sidemount. But you should still have enough in the one remaining tank to get you home because the 2/3 of that tank you have left is the same as the 1/3 of both tanks you used getting in.

That's no different with sidemount tanks except for the way you get to use the first third. I don't think this is news to anyone. I don't understand why people think the math is so different just because the two tanks aren't connected to each other. The rule of thirds with manifolded doubles does NOT assume that all gas from both cylinders will be available.

If you're alone, your remaining tank should get you back. If you're with someone, he has a reserve too. And if you're together and both suffer failures, you're screwed. This goes for either manifolded doubles, independent BM twins, or sidemount. However, all of these configurations are better than a single tank with H- valve.

Just the ramblings of my warped, sleep-deprived mind.


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