daviddem(Offline)

Hi,

I am contemplating getting equipped with larger back gas for deeper dives.

1) What cylinders are the preferred ones among the DIR crowd in the twin 18-20 liters size? It seems people more often go for twin 18. What is the reason why the extra capacity of the 20's is not so appealing? Trim issues? What brand cylinders would you recommend?

2) Which wing are you using with such a setup? I don't believe my Evolve 40lbs would provide adequate lift with this back gas + multiple stages.

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Cyrille (Online)

Hi David

The Evolve 40lbs is designed for doubles up to 2x12L. You should consider a larger model: the Evolve 60lbs (max. 2x20L).

Kind regards.

Cyrille

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reinoud(Offline)

What kind of dives? In your profile I read you are DIR-F certified. so twin 12 liters on Nitrox should get you a comfortable half hour on 30 metres..

(and once you will do Tech 1 you might have learned that a stage might be a better idea)

The only people I see in DIR with twin 18 or 20 litres are the exploration cave divers with rebreathers...

Cyrille (Online)

Hi,

If you're Trimix certified (and provided your SAC is not excessive), a 2x12L twinset is sufficient for dives down to 70-80m. Consider larger twin-sets only if you want to dive deeper or if your SAC is higher than average or if you plan extremely long dives.

Kind regards.

Cyrille

PS: for the double 20L you need a crane ... to gear up and to get out of the water

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David Martin(Offline)

Double 18s are useful in cave even without an RB80 between them. It's a set up that can be used from C1 upwards. D18s are very similar to the D104s/120s used in America. I think the main reason we don't see more D18s in cave diving in Europe is that D12s are the standard ocean diving cylinder and most people have multiple sets of D12s. Adding an(other) AL80 is an easy alternative with D12s for most dives.

Cylinders of this size also have their place for ocean diving. At the point when you'd need a bottom stage plus 3 deco gasses with D12s then larger backgas cylinders start to make sense.

David

reinoud(Offline)

OK, but that is still far beyond someone with only DIR-F ";-)

lizardland(Offline)

I'm surprised that twin 20's aren't more popular than 18's. Not a massive difference in weight on land and underwater my 20's were very nice and neutral. I loved them for cave diving, the two big downside with 18's & 20's, obviously weight but secondly length. They make negotiating some restrictions quite difficult because they limit the angles that you can get into. My only experience with 18's has been carrying them as stages as support for another diver and they felt an awful lot more negative than the Aircon 20's I was using. I don't know that I would want to use either for open water diving, I couldn't get out the water at some cave pools while wearing them, I'd hate to try getting up a boat ladder in them. I'd rather add stages to a set of 12's.

For cave diving with stages I was using a 45lb wing. I wouldn't particularly like to use such a small wing with such a heavy set if I was going to end up floating on the surface waiting for a boat to pick me up though. If you aren't cave diving and they are your only twinset then I'll bet you'll get bored very quickly with them.

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NIMROD CHEN(Offline)

i love my D18 they sit great on my back , trim is great - no problems at all . as for the weight issue - there is a 6 KG difference from my D80's i when i dive the 80's i add about 4KG and use a SS plate so the total weight is the same .
as for the wing i use them with my Evolve 40 and its great , the D18 allows me to do the deeper stuff with enough gas reserves .


here you can see the evolve 40 with the D18 +a bottom stage+ 2 deco tanks



Nimrod

Seydlitz(Offline)

I Keep a set of D18s With A Brokk Bands in France ....I have dived the Evolve 40 With them no probs .However its OK to float down the Cele in them...Not sure if You could get away with such a small wing in UK coastal waters though...Med, Middle east etc with generally calmer waters they might be OK ..never done it

They are with out doubt the nicest cylinders I have used in the water .. trim out just perfect ..The only tip I would give ..You going use them ..consider going the gym and improve your body strength.. it could be so easy to pull a muscle or injure yourself with a full rig there is alot of weight

graham_hk(Offline)

In ocean dives, for me it all comes down to minimum gas. If you can carry enough gas in twin 80s for minimum gas then there is very little point in buying new cylinders for your current dives. Eventually you will reach a point where minimum gas becomes a limiting factor for your dives - you could add an extra deco gas to reduce minimum gas or consider bigger tanks.

With a normal SAC you can do reasonable times at 80 m with a single stage, 50 and 100 and twin 80s (12s). Anything longer or deeper than this and you are probably looking at bigger back tanks. I have used 15s, 104s and 18s. I like the 15s least can live with 104s and love the 18s - that is until I start climbing out of the water - however on a dive where you really need them you probably should have someone else to lift them out the water for you

They are quite heavy full of nitrox but then on a dive where you need them they'll be full of trimix anyway thus I have been quite comfortable in a 40lb wing and 4 stages even got lost at sea (for a few hours like this)


Of course you'll need a buddy with the same tanks

HTH

Graham

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