Twin Set

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Honkeye

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Hi all i am quit new to diving and am looking around at some of the goodies available like twin sets. How long would you say is a good length of time tto wait before going onto twins, also what is the optimum cylinder size. I have 15ltr Steel at the moment is it a bad idea to make this into a twin. I also use a seaquest spectrum 4 is this cabable of holiding twins?.

Your advice welcome.

Twins will not be used for tech diving just to allow a bit more bottom time.
 
I currently dive in a dry suit and as i am using a 15ltr dont think i can go any bigger 18ltr would be to big.

what does it mater any way dry suit or wet suit

cheers
nathan
 
I just upgraded to doubles. I got a set of 121cuft Fabers. These I find boarder line to big. I am 6'2" tall. I like the volume but would not go to 131 which I think would be way to big. 104's seem to bee a good size. Sorry everything is in cuft rather than liters but in canada we get everything from the US.

Dai-Sho
 
Well, what's going to happen to you if you're wearing twin steel tanks with a wetsuit and your BC fails at depth? How do you plan on getting up?
Don't bother making something up :). Ultimately, what you're after is a balanced rig, which basically means that you should be able to swim up your full tanks with no aid from your BC. It ain't gonna happen with double steel tanks and a wetsuit.

Double steels and wetsuits are a bad combo. The preferred set up is double AL80's if using a wetsuit. With this combo (cheaper, BTW), you can ditch a weightbelt and swim up with full tanks. There won't be any reason for double bladders or whatever (also keeping things less expensive).

If you're diving dry, use whatever you want -- just weight yourself correctly -- a balanced rig.

Take care.

Mike

I dive dry and use LP95's (PST) for boat diving. I like 'em. If I need more air than that, I'll probably freeze :D. Really, I would just bring a stage of bottom mix. A lot of tech guys like the 104's, and the 120's are really heavy and most likely unneeded for anything but the most extreme dives.
 
Mike,

I agree with your basic premise. You need to be able to swim up in the event of a BC failure. OTOH, you are incorrect when you say, "It ain't gonna happen with double steel tanks and a wetsuit." I can easily swim up from depth with NO air in my BC even without a wetsuit with my twin slim steel 72's. I know because it's something I practice. Admittedly, these are pretty small tanks.

DSSW,

WWW™
 
Yooper, buoyant tanks or neutrally buoyant steel tanks were in vogue 40 odd years ago because it was thought that such tanks, when empty, would assist the diver back to the surface. There were no BCD's. NOW, that BCD's are available, I would have thought that the buoyant tank would go the way of the dino and BCD's would allow more flexibility. Yet, even with this modern answer to the buoyancy control problem, we're hearing "don't trust it". There is such a thing as being too cautious.

BTW, Walter is right. Twin 70's are not particularly heavy. They are neutral to slightly negative. Note that they were in use everyday BEFORE there were ANY BCD's at all. My bud has a set of these dated 1954. I used twin 70's with a wet suit and often with nothing but swim trunks, mask and fins.

There is no single rule about tanks, including steel tanks of which there are a great variety. As far as our English poster's question, be aware that twin tanks will SLOW you down, be prepared to increase air consumption if you do any swimming. Perching on a wreck is the best place for twins. Make those low pressure steel tanks, please. The HP Fabers will sink you like a rock, thereby vindicating Yooper. Twin aluminum will float like a cork making you want to strangle Yooper.

Seriously, I suggest twin 12L Faber. Fill them to 200 bar. This setup should provide enough air for two non-decompression dives to normal depths. It will also allow a single deco dive with a short stop, 15 minutes or so with reserve air. If you are a weight lifter or Iron Man competitor, and fairly tall, you might consider a slightly larger set. I am assumming you will encounter swells and various weather conditions on your trips. Moving around wet decks and climbing ladders can be hazardous if you are overloaded. Use common sense.

There is no time scale for purchase of a set of twin tanks. However, you should have 20 or so OW dives logged before doing deco dives.

I don't know the answer to your BCD question. A dive shop pro should know whether a kit is available. If anyone advises you to use a "backplate", make sure you figure its weight in your buoyancy estimate.
 
As long as you can swim up, then go ahead. I thought we were talking about bigger steel tanks...oops.

Mike
 
Originally posted by Walter
I can easily swim up from depth with NO air in my BC even without a wetsuit with my twin slim steel 72's.
Actually Walter, no exposure protection is the absolutely optimal case, not worse case, since you body's buoyancy does not change with depth (if you're equalizing). You need to look at worst case scenarios. In all examples below I'm going to assume correct weighting.

Twin 72s have a swing of about 11 pounds. If you're not wearing a wetsuit, at the start of a dive (worst case) that's what you'll have to swim up, 11 pounds.

However, if you're wearing a wetsuit and the weight required to sink it, things change.

I'll pull numbers out of thin air for an example.

Let's say you have to wear 20 pounds to sink your 7mm wetsuit. As you dive to 100 feet the pressure will crush the bubbles in the wetsuit, it will displace less water and therefore will lose buoyancy. Let’s say going from 1ATM (surface) to 4 ATM (100 feet) it looses half of its buoyancy (it's not as flexible as a balloon). So that means you're 10 pounds more negative at 100 feet than you were at the surface. Add the 11 pound swing from your cylinders and you're faced with swimming up 21 pounds.

No way.

Of course, if you're wearing a weight belt, you have ditchable weight and you're all set.

Roak
 
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