Small Steel Doubles vs. Single Tank?

[CuriousRambler]

I was surfing around the net and saw some British dive sites that talked about using smaller steel doubles in place of using a single tank, saying they've got numerous advantages over single tanks. I'm interested in the idea, I can see definite benefits based on trim, buoyancy, and the ability to practice valve drills, etc. The one thing that threw me off was that obviously, all the measurements were metric. There was a lot of talk about twinned steel 7L tanks. I did the simple math to convert L³ to cuft and came out somewhere ridiculously low, like less than 1cuft. I'm curious what size tanks that would be to us Western folk? My only thought is that 7L would be the dry displacement and not the compressed gas capacity?

 

[OldNSalty]

This will be an interesting thread I hope. I once knew an instructor/ex-seal that did all his dives with twin AL 60's and thought it was the perfect setup especially for student checkout dives. I never understood why he felt that the twin 60's were better then a single 120 as it sounded like the only advantage he could note was that he didn't have to change tanks and it stood up balanced on its on....

Anyway, I hope someone can give us some insight. To the OP-do you have a link you can post?

 

[micklock]

You've never heard the old saying- don't put all your eggs in one basket

 

[SteveAD]

The european sizing standard is to take the actual volume of the tank (in liters) multiply by the pressure (in bar) and that gives the air volume in surface liters.

 

[Jimmer]

http://www.scubaboard.com/forums/tanks-valves-bands/221077-my-new-doubles.html

This is the second time I've posted this link today. I have double Faber lp45's, which I believe are around the same size as the 7 litre tanks. Filled to their rated capacity you have more than an al80, and they do balance better, since they are low to your back and the weight is distributed across the back. I like em, I just wish I had more medium depth shore dives to take advantage of using them. Most of my dives are Lake Erie wrecks in the 100+ range, so I dont use them as often as I'd like.

 

[micklock]

There is a thread going on in solo divers also.

 

[OldNSalty]

Well, OK micklock. In my case I don't recall the guy having 2 separate regs on the system. In fact, I am pretty sure he just had the one. I can see the point of redundancy but I don't think that is what is going on according to the OP-maybe it is.

You need about 100 cubic feet of air for a dive say, the way I read the post was that the divers were jumping in with 2 60's instead of the large 100-120 tank. If the reason for this was redundancy, well, they really need to re-think because aren't they going to need to switch tanks half way into the dive...and there goes the redundant backup.

 

[micklock]

I've only dove to 45' using two 40s in open water. I breath one down to 1000 psi then switch I ended with both at 500psi. Most of the time I use a 40 and hp80, the 40 is just a backup.

 

[OldNSalty]

Well, OK, I suppose I am wrong then about the false redundancy-thanks for clearing that up micklock and I suppose the balance that Jimmer pointed out could be an issue as well.

Are there any other justifications?

 

[micklock]

Another thing is I have my wing inflator on the left tank and my drysuit inflator on the right tank.