18L v Twins ...& a J-Valve

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18L v Twins ...& a J-Valve

Postby Jason » Thu Jan 15, 2009 12:01 pm

Dive buddy (and now boss) uses an 18L tank to dive with and having tried his spare 18L I found it quite comfortable underwater and of added luxury for bottom time. The down side of an 18L is the extra pull around my shoulders and neck upon long shore dive walks (thinking of using small trolleys upon flat walks though).
Am wondering though if it is better and more cost-efficient to invest in a Twin-setup?
If so, what then would be best: a Twin-10L or a Twin 12L or a Twin 15L (though I think a Twin-15L might be too cumbersome?)

I also have been given a 12L J-valve tank but I've been told that they work differently compared to modern tanks. Does anyone have advice upon how they work?

cheers
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Re: 18L v Twins ...& a J-Valve

Postby NEMES1S » Thu Jan 15, 2009 12:37 pm

J valves should be in the bin...!! :shock:
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Re: 18L v Twins ...& a J-Valve

Postby tara » Thu Jan 15, 2009 1:27 pm

the twin 10s to twin 15s is a huge range - and quite a different weight too.
have you thought about the diving you will be doing and the rate at which you use your air?

no sense in trucking round about 6000litres of gas if all you are ever going to use is around 3000.
plus are you carrying a pony bottle?
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Re: 18L v Twins ...& a J-Valve

Postby Jason » Thu Jan 15, 2009 7:37 pm

Well the J-valve was a free gift and still in good (tested) order though I've forgotten the advice upon its use. something about I switch a valve when I get to such a bar or something. I don't mind the novelty of using old or historic diving stuff as it arouses a sense of nostalga and belonging of where i've been and where I'm going to.
Oh damn! Just remembered that I forgot to take my father-in-law to pick up a new suit from a Dive Shop. It was a 'reward' for raising the alert that the Dive Shop was losing a lot of gear and tanks to some thieves. He won't mind my forgeting as he still prefers his ratty and holy 2-peice mothball suit. I don't get a kick outta that sort of old though. :lol:

I never use a Pony Bottle until I start Solo Diving (I think I took Andy's advice upon this) so where does a Pony come into play with Twins?
I take your point about using Twins when they aren't justified upon a dive that can be done on just a single...just that I've seen a number of photos upon this Forum where people have Twin-setups in use.

What Twin-Setups do you guys use? I think 15L would be a too cumbersome Twin-setup though maybe 12L Twins might be the go for airtime and weight.

...should I just save my Pennies and put it towards a Rebreather eventually :?: :?
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Re: 18L v Twins ...& a J-Valve

Postby Andy » Thu Jan 15, 2009 8:50 pm

I've got a set of twin 10s and a set of twin 12s - both work quite well, no problems with trim etc. Though I don't really like lugging either around for shore dives - did a shore dive at the eastern side of Jones Bay recently with the 10s, a couple of hundred yards walk to get from the lagoon carpark to the beach was a long way.

Twin 15s is a lot of gas....short tanks, not readily available (in NZ at least) bands and manifolds. I've never really contemplated twinning up my two 15s!!

Even a single 18 is a lot of gas - what sort of diving are you wanting that much gas for???
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Re: 18L v Twins ...& a J-Valve

Postby Jason » Thu Jan 15, 2009 9:49 pm

Just your normal 30max Shore & Boat Diving. Its just that the 18L for myself is do-able for size and not too heavy (saves me $300 at the gym) though I don't think a tramp over sharp rock and loose stone for a few hundred yards either would be 'relaxing'. People are amazed when they see such a tank with my Buddy - but I guess I'm seeking luxury time underwater if its offered. Only one Dive Shop mentioned something about charging extra for the 18L fill, but that club ended up loosing a small fortune due to such poor customer service ( :twisted: ).

Maybe a 12L Twin is the go as it gives me the 'extra' option without being too heavy with the manifold.
Do LDS charge extra for filling Twins though?
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Re: 18L v Twins ...& a J-Valve

Postby Andy » Thu Jan 15, 2009 10:05 pm

Most shops will charge for two fills with twins, it is two tanks after all!

For dives in the 30m range, more gas = more likely to exceed no-decompression limits. If it's a fixed depth, e.g. a wreck in flat sand, you really will be hitting NDL before you run out of gas on a single 12 (or 15) unless you are sucking it back faster than an Electrolux!!

If it's somewhere more of a wall dive where you can multi-level up the wall to keep with in NDLs, well.... you'd probably get a good hour out of a single 12 or 15 anddo you really want to be doing hour plus dives??

Before you start shelling out $$$$s for equipment that you may not need, I'd suggest measuring your breathing rate on a few dives. Even if it's "average", you should still be able to get an hour out of a single 12. If you are sucking more than an Electrolux, you'd probably get more benefit from working on impriving your gas consumption rather than looking for an equipment solution to a problem that can be solved much more cheaply and efficiently.
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Re: 18L v Twins ...& a J-Valve

Postby Jason » Thu Jan 15, 2009 10:39 pm

I think you're right Andy, totally forgot about my NDL, especially when I've nver passed it in nearly 90 dives.
I guess a 15L single is my tank size without overstepping the mark.
Why then do you use 12L Twins for then?
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Re: 18L v Twins ...& a J-Valve

Postby Andy » Fri Jan 16, 2009 11:08 am

Vampire Squid wrote:Why then do you use 12L Twins for then?


The main advantage is redundancy - flexibility in dealing with equipment failures and still having access to the whole gas supply. This is most use when doing deco dives - where you can't ascend directly to the surface as you can when in recreational limits.
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Re: 18L v Twins ...& a J-Valve

Postby Jason » Fri Jan 16, 2009 11:28 am

Ok, 15L seems my 'single' choice although this may dent my Buddy's 18L ego :lol:

12L might be my go if I took up Twins and started doing 'deep' dives ( a little more edumecation is needed also)

...would it be a better investment to save for a Rebreather though?
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Re: 18L v Twins ...& a J-Valve

Postby Andy » Fri Jan 16, 2009 2:03 pm

Vampire Squid wrote:Ok, 15L seems my 'single' choice although this may dent my Buddy's 18L ego :lol:



Ah, even better is the 12L and make it last longer than your buddy makes the 18L last!! 8)
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Re: 18L v Twins ...& a J-Valve

Postby Jason » Sat Jan 17, 2009 12:02 am

Ah, wise point from wise man and point taken.
You realise I will now have to scratch number 5 off my list for 2009.

...something lost is something gained.
cheers :)
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Re: 18L v Twins ...& a J-Valve

Postby Hoppy » Fri Jan 30, 2009 1:17 am

If you want some 15's, I have a couple here... and bands if you want to twin them as well :D
Twin15's arent light, especially when full of gas.

Twins are more about redundancy and flexibility than anything else.

Twin 12's should do you for about 2hrs diving if you are doing normal recreational diving. Ive done 150 min runtimes on a set of 12's in the lake before.

The 12's trim out nicer than the 15's, and the 15's provide noticeably more drag as well.

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Re: 18L v Twins ...& a J-Valve

Postby Hoppy » Fri Jan 30, 2009 1:21 am

And the way you work j-valves is that they are up regularly (from memory from my padi training), and then are flipped down when you run out of gas... about that point, you head for the surface!!!!
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Re: 18L v Twins ...& a J-Valve

Postby NEMES1S » Fri Jan 30, 2009 8:04 am

Hoppy wrote:And the way you work j-valves is that they are up regularly (from memory from my padi training), and then are flipped down when you run out of gas... about that point, you head for the surface!!!!

Yep they have a lever in the up position you have reserve,when you start to feel your breathing restricted you would pull on the cord and put the lever in the down position and thats your reserve,problem was of course that the levers often got bumped or knocked to the ON/DOWN position and when the breathing became difficult it wasnt because you were ready to pull for your reserve air it was because you were completely out of air!!!

..they were designed before the SPG....now SPGs are commonplace...the J-Valves should be shelved
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