Twin tanks: manifold or redundant

Is it okay to dive twins with two redundant rigs or is it better with a manifold? Part of the reason why I like the idea of twin tanks is that you can potentially set them up independently. However, now that I think about it, it seems that I might run into trim problems when there is significantly more air in one tank versus the other. Also, I don't recall seeing anyone dive twins w/o a manifold.

There is drawbacks to both. When backmount I did a manifold. You have a chance to isolate if something goes wrong but you better be proficient at it. I am sidemount now so no isolator but I can see my valves and what is going wrong. If your not comfortable switching regs you might want a manifold for sure and there is more task loading when there is no manifold. However you should be comfortable doing heavy task loads if your diving doubles, more gas does not = safer system if you dont know what you are doing.

I doubt you'd have noticeable trim issues due to air. I've used redundant doubles and never noticed a difference. I suppose if you found you did you could alternate between the two tanks to keep the air volume close to even. Say use a 1000psi from one tank then use a 1000psi from the other...... No big deal but you'd have to keep a closer eye on your air than usual. If you have doubts do a few dives without the manifold before you go and buy one. If it works out and you like the redundant doubles you'll save the $$ on the manifold.

This is an often debated topic in technical diving forums; search for independent doubles vs isolation manifold. In real world settings, the advantages of the manifold outweigh whatever theoretical advantage there might be to independent doubles IMO.

Tech divers using independent doubles do switch tanks periodically on a pre determined schedule, but it's not for trim, it's to ensure a reserve in each tank.

By far the most likely equipment failure that would result in gas loss is regulator and/or hose failure. Tank valves and neck o-rings are FAR less likely to fail during a dive. The manifold allows you access to both tanks in the event of a regulator failure; that's the big advantage. The theoretical disadvantage is that the isolation valve is a potential failure point, although I have never heard of one failing in a dive. In order for the manifold to release gas from both tanks uncontrollably, a total of 6 consecutive o-rings (on modern manifolds) would have to fail, or 3 and the valve itself.

clafrieda:

Is it okay to dive twins with two redundant rigs or is it better with a manifold? Part of the reason why I like the idea of twin tanks is that you can potentially set them up independently. However, now that I think about it, it seems that I might run into trim problems when there is significantly more air in one tank versus the other.

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Yes, it is certainly OK, and a number of people do it.

mattboy:

This is an often debated topic in technical diving forums; search for independent doubles vs isolation manifold. In real world settings, the advantages of the manifold outweigh whatever theoretical advantage there might be to independent doubles IMO. Tech divers using independent doubles do switch tanks periodically on a pre determined schedule, but it's not for trim, it's to ensure a reserve in each tank.

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There is an additional potential situation where diving independent doubles makes sense - you can ocassionally do this when traveling, and renting tanks, where manifolded doubles are simply not available and you want the air supply that a set of doubles may offer. Of course, it may mean traveling with a set of tank bands, which adds to weight. Like mattboy, I haven't had a situation where a failure of the manifold on my backmount doubles has caused a problem, although I regularly practice valve shutdowns, nonetheless.

clafrieda:

Also, I don't recall seeing anyone dive twins w/o a manifold.

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I think you will see more of this in the future, as the interest in sidemount rigs continues to increase. Also, I actually do periodically encounter a few divers who are using independent backmount doubles. Uncommon, but they are out there diving, at least in the NC area.

Kevin Carlisle:

I am sidemount now so no isolator but I can see my valves and what is going wrong. If your not comfortable switching regs you might want a manifold for sure and there is more task loading when there is no manifold. However you should be comfortable doing heavy task loads if your diving doubles, more gas does not = safer system if you dont know what you are doing.

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Well put. It is a practiced skill - switching regularly from one tank to the other - not unlike valve shutdowns for manifolded doubles. Both skills are quite manageable, but neither should be taken for granted. When diving SM / independent doubles, I breath off both tanks initially - first one then the other - to ensure both regs are working and there is not a detectable gas problem. Then, I breath my right tank down 500psi, then the left tank down 1000 psi, then the right tank another 1000, then the left tank another 1000. At that point, I change to 500 psi increments. Possibly more switching than necessary but it helps keep me conscious of where I am in terms of gas supply.

Colliam7:

There is an additional potential advantage of independent doubles - you can ocassionally do this when traveling, and renting tanks, where manifolded doubles are not available. Of course, it may mean traveling with a set of tank bands, which adds to weight. Like mattboy, I haven't had a situation where a failure of the manifold on my backmount doubles has caused a problem, although I regularly practice valve shutdowns nonethless. I think you will see more of this, as the interst in sidemount rigs continues to increase. I also actually do encounter a few divers who are using independent backmount doubles. Uncomm,on, but they are out there diving, at least in the NC area.

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The last guy I saw diving independant backmount was from NC.

Might need to say if you sidemount you will plan your swaps for trim too. It does affect your trim in sidemount.

Last guy I saw dive them was in Texas. I wouldn't dive independent doubles, if I wanted them independent, I'd shut the isolator.

Kevin Carlisle:

The last guy I saw diving independant backmount was from NC.

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Well, I guess I could use this for some self-deprecating humor, something about the intelligence level of us divers from NC, but ...

Kevin Carlisle:

Might need to say if you sidemount you will plan your swaps for trim too. It does affect your trim in sidemount.

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Good point. I don't sense it as much with my steels. With my AL80s, it is annoying to have one tank at a significantly lower pressure than the other.

Colliam7:

Well, I guess I could use this for some self-deprecating humor, something about the intelligence level of us divers from NC, but ...Good point. I don't sense it as much with my steels. With my AL80s, it is annoying to have one tank at a significantly lower pressure than the other.

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Depends on the tanks.. With 95s not too bad but with 130s you notice the difference around 300psi.

No you will not have any problems with the trim. You just compensate the roll with the wing. We often dive with stages and suck them almost down - it's a 5lb swing and it does not affect the trim.