Small Steel Doubles vs. Single Tank?

[kidspot]

my math won't be very precise, but remember european tanks are listed by water volume, not gas volume under pressure - so it's a 7 litre empty volume - divide by 28 (number of liters in cubic foot) and multiply by the bar (200 etc..) my guess is it's what we would call a 50cf tank.

Someone is welcome to correct me if my assumptions are wrong

Aloha, Tim

 

[Jimmer]

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?

I don't use mine so much for gas redundancy as regulator redundancy. I like the fact that I have a fully independant backup first and second stage reg, that still has access to the full volume in the 2 tanks in the event of a freeze up or freeflow.

 

[Nemrod]

If you go back to the 50s and 60s you would find that many divers chose double 38s/45s/50s as their rig of choice. The small twin steel tanks had excellent bouycany characteristics for pre-BC diving. They also had good trim and balance. They were not interested in redundancy or any of that, it was strictly for their good diving characteristics.

The high cost of true vintage doubles has me sitting out on most ebay sells but I do have a set of Double 50s using aluminum 50 cf tanks. I enjoy using them.

N

 

[CuriousRambler]

Here are a couple of the links I dug up in my history. It's not the initial website I saw the idea, that one seems to be hiding from me, but both of these were found in the process of researching the idea. I don't understand what a "dumpy" tank is, either..

152mm 7L ALUMINIUM FLAT BOTTOMED cylinder boot wanted - Page 2 - scubish.com - Scuba Diving Forum
Which cylinder size/type? - BSAC Scuba Diving Forums

 

[Lee Taylor]

If you go back to the 50s and 60s you would find that many divers chose double 38s/45s/50s as their rig of choice. The small twin steel tanks had excellent bouycany characteristics for pre-BC diving. They also had good trim and balance. They were not interested in redundancy or any of that, it was strictly for their good diving characteristics.

The high cost of true vintage doubles has me sitting out on most ebay sells but I do have a set of Double 50s using aluminum 50 cf tanks. I enjoy using them.

N

Killer Tanks!

Reminds me of the first time I experienced love at first sight, which was a hopped up 1964 Chevy with a "Four in the Floor" shifter!

 

[pescador775]

Hy-Mark makes a 6.9 inch aluminum cylinder rated 50 cf.

I'm a little surprised at the questions concerning small doubles. Basically, they can be set up to take advantage of all the redundant systems touted by cave divers and the like. If you want all the bells and whistles, a set of doubles is the best platform to hook everything up. In the past, I have wondered why someone would opt for a klutzy pony bottle when they could sling on a neat little set of doubles with twin regs and triple shut off valves. Some people claim better balance for a long single as opposed to a shorter set of doubles. I don't know about that but have come to accept that there are divers that are unusually sensitive to a couple degrees of attitude up or down, or maybe they have hollow legs or something. I just dive what I have on hand and don't worry about imaginary (to me) issues.

In the day, the double hose guys liked twins because the regulator could be centered low on the back for better breathing and less head knock. Also, surplus 38 cf bottles could be purchased for about $20 ea.

 

[oldmossback]

I've 4 sets of the short doubles, 2 of the twin 38's, 1 twin 45 and 1 set of the aluminums like Nemrods, not as pretty tho........I prefer these over the single as I can sit down easily on shore or in a boat, move around easier underwater and they don't roll around in my truck when traveling like my two singles do. I hve used them with a single hose reg. in the past, but now I dive exclusively with doublehose regs and these fit nicely together. And like it was stated above, the reg sits low on the tank valve and on your back.
Probably not good reasons for some people but I like them.

I prefer the twin 38's....that's about 76cu/in with a 2000 psi. fill...these are rated at 1880. They are simple single valve with no o-rings in the manifolds. They are like Mike Nelsons as used in the tv show Sea Hunt.

The twin 45's are heavy as hell, but hold 90 cu/in on a 2400 psi fill. (I have my own compressor) The twin al. 50's are nice and really short. They come with a single band and back pack and this I do not like as much as the commerical/military type harness setup on my other sets.

The lakes are only about 100 to 150 feet deep, but I rarely go that deep now, so these are perfect for diving here in North Texas.

 

[DA Aquamaster]

I used to dive twin 45's (2215 psi service pressure) as well as a set of what I think were also twin 45's (with an 1800 psi service pressure). The latter set was slightly taller. The 45''s were heavy for the gas carried, but they felt great in the water and were the perfect starter doubles.

It was not uncommon for both sets to be mistaken for 3000 psi aluminum 50's and to consequently get filled to 3000 psi - the test pressure on the 1800 psi tanks - resulting in about 150 cu ft total in the 1800 psi tanks and 122 cu ft in the 2215 psi tanks. In that regard I was getting "cave fills" long before they were popular even in cave country.

 

[Couv]

Here is a thread does not address the OP's question, but useful information when considering doubles over singles. See Divin' Hoosier's, TSandM and post like their's.

http://www.scubaboard.com/forums/tanks-valves-bands/228847-doubles-recreational-diving.html

 

[GrumpyOldGuy]

I used to dive twin 45's (2215 psi service pressure) as well as a set of what I think were also twin 45's (with an 1800 psi service pressure). The latter set was slightly taller. The 45''s were heavy for the gas carried, but they felt great in the water and were the perfect starter doubles.

I had a set of the low pressure 45s also. The reason I had them was back in the late 70's, compressors on boats often could only fill to about 2,000lbs. The 45s rated at 1800 with a + would be full at 2000, the guys with steel tanks would be underfilled 10-20%. Of couse anyone stupid enough to use the new AL80's not only would be grossly underfilled, but brain damaged from breathing air from an AL tank (according to my local Scuba Pro dealer, times have changed).