twin tanks

[beche de mer]

What's best? Independent doubles or manifold-connected doubles?

Are there any really compelling reasons for selecting one configuration over another, or is it just a matter of preference?

 

[Scubaroo]

Independant tanks give you the ability to access gas in the afflicted tank in the event of a regulator failure - you close the isolator to isolate the unaffected tank from the compromised tank, switch regs (if necessary), close down the post with the failed regulator, and reopen the isolator.

With independant tanks, if a regulator fails, the gas in that tank is either inaccessable if you close it down, or lost rapidly.

There are also trim issues with independant tanks - my understanding is (never having dived them) that if you breathed one tank down fully before switching, then you're going to be several kilograms lighter on that side, and be inclined to roll the other way. Independant doubles divers claim that if you can't switch regs periodically to counteract this, then you shouldn't be diving doubles, but with a manifold, switching regs is eliminated entirely.

Independant doubles still might pop up in your configuration though if you want to dive doubles at some vacation destinations, and you twin two rental tanks.

 

[MikeFerrara]

There are many issues to consider.

Independents...

Gas management issues - You need to breath them down evenly. Ít wouldn't do you any good to have a failure on the only tank with gas in it. LOL. To do this you'll need a SPG on each tank which leads us to equipment configuration issues - As already noted you need two pressure gages. Where do you put them. I like only having one (on a short hose and clipped to my left hip. Which reg will you donate. I donate the one from my mouth and have the backup neatly tucked under my chin on a short hose. With independents, if you want to donate a long hose and if you donate the one from your mouth hose, both hoses will need to be long because you don't know which one will be in your mouth. Regardless of your prefered air sharing method both hoses will need to be long enough to sare. How do you secure the one you're not using? IMO, those silly octo holders (that never seem to hold) have no place in any dive requiring doubles. You see where I'm going with this? I think independents lead to a sloppy set up as far as streamlining and procedures.

If you do need to shut a valve down because of a failuer, you no longer have access to the gas in that tank.

Manifolded doubles...

You avoid all the problems from above by always breathing one reg. You know which will be donated and which needs the long hose. You only need one SPG. If a reg or hose fails (the most common failure) you shut down the valve and still have access to the gas in both tanks. An isolator provides a way to respond to a valve/oring or burst disc failure but, in theory, there are failure modes that could cause a total loss of gas. If it has ever happened, though, it sure hasen't been often. I've heard of one and that was a scooter crash that took out a manifold.

Independents are popular in some places so maybe some one will argue the points I've made. Let them.

 

[MikeFerrara]

BTW, what are your diving interests? While doubles can be used anyplace, they're often used in "technical diving". Should this thread be in the tech section?

 

[beche de mer]

Thanks for your thoughts. Yes, I'll post it again in the tech section.

 

[WaterDawg]

What nobody dives doubles recriationanly???

 

[Genesis]

I like Double 72s or HP100s for a day of recreational diving.

I don't have to screw with switching tanks over - I can get 2 dives out of the 72s most of the time, and three out of the 100s.

I trim out nicely, and have redundancy - so I can dive solo if I want (intentionally or not) and be "ok" with it.

They ARE heavier out of the water. But correctly weighted, in the water this is not a problem.

Just be sure you can swim up your kit from the bottom, with a full load of gas, and NO GAS in the wing. If you cannot, then you need to reconsider what you're doing, because you can kill yourself being overweighted, gas or no gas.

 

[DA Aquamaster]

Why not doubles for rereational diving? Why change your configuration to a single tank just because the dive profile is within recreational depth limits and NDL's?

If it's a boat dive and you don't have to pack the extra weight, it's just as easy to dive doubles.

 

[Snowbear]

I do. I like 'em better than a single for the trim/balance. Sometimes I get laughed at, other times people are jealous .

 

[MikeFerrara]

WaterDawg once bubbled...
What nobody dives doubles recriationanly???

Of course they do, which is why I asked what the original posters interests were.