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Twin 7's ?

Discussion in 'Dive Equipment' started by Cybes, Mar 5, 2016.

  1. Cybes

    Cybes Active Member

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    I'm saving up for a set of twin 12's. I've borrowed a set often enough to know I want one. Bit of weight to moving them around on the dry but they balance really nicely in the water.

    Now I know twin 12's are the norm, but I've also heard that twin 7's are good to dive. Last time the weather was kind one of the other divers was an Italian girl diving twin 7's. She came back with more gas than I did. She (sorry, didn't get her name) was GUE so was appreciably better than I was in the water. I was surprised to see a Gooist diving anything other than twin 12's, but she obviously didn't need the gas, the only difference in equipment was the size of the cylinders, and there was a swell running which made getting back into the boat a little interesting.

    My point is if you taylor your rig to suit the diving you do, then twin 7's are an option. Bigger than a 12, smaller than a 15, nicely balanced with some redundancy.

    Are the benefits of the twin 7's worth the added complexity and expense over a single?

    Are they too small to be considered a practical choice?
     
  2. Wibble

    Wibble Fish don't talk
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    The main challenge with diving 2x7 vs 2x12 is the difference in gas volume with your team-mates. This means that you'll need to work out turn pressures based on the smaller volume, which will be you as 2x7 is smaller. You can get 2x7x300, but you're rarely going to get 300 bar and you'll still be smaller.

    Volumes:
    2x12x232 = 5568 litres
    2x7x232 = 3248 litres
    2x7x270 = 3780 litres (the practical pressure)
    2x7x300 = 4200 litres (the theoretical pressure which you'll never get in reality)

    But, of course, this depends on the type of diving you do. If you're diving primarily recreationally then twin 7s are great for the redundancy.

    Twin 7x300 are good and weigh almost the same as twin 12 x 232 bar. However you will need less additional weight, so your rig will be a little lighter.

    Another issue with twin 7's is the valves are closer to your back, so are different, possibly harder, to manipulate than twin 12s.

    Twin 7's are smaller so will be more streamlined and it's also possible to use a single wing (depending on the model).

    I've a pair of twin 7x300s which I'm seriously thinking of selling/swapping for another pair of twin 12s simply because I've only used them once in the last year - twice if you count lending them to someone! Not got as far as working out the price - they're in test but need O2 cleaning - last test was hydro about 18 months ago.
     
  3. MikeyH

    MikeyH Active Member
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    Yes going through this thought process too for the future. I use a 15 and 12 singles both with a 40 cu' RAS - also thinking about twins, but really not sure I need them ...so it has to be about what and how deep - what is the neatest solution and I know some like twindies.......

    I have a little spreasheet that allows me to put a load of different options together for inital comparison......the 8.5s look interesting.....

    Cylinder capacity......Enter BAR for litres......litres

    12x232 ...............................230 .................2760
    15x232................................230..................3450
    2 x (8.5x232).......................230..................3910
    2 x (12x232)........................230..................5520
    8.5 x 230.............................230..................1955
    2 x (7x300)..........................300..................4200
    2 x (7x300)..........................270..................3780
     
  4. Cybes

    Cybes Active Member

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    As you say twin 12's keep things simple and uniform gas calc wise. They're what I'm looking for next.

    I need the whole set, cylinder, manifold, wing and I'd like another set of regs so that I don't need to split the set on my single every time I change set.

    I'm diving a single 15 with an Eclipse wing at the moment. I'm curious as to what twin 7's would give me over the 15. If I were to change the 15 for twin 7's I would only need another DS4 to complete one set of regs for both sets of doubles. The saving I would make on regs and the sale of the 15 would offset some of the cost of the twin 7's.

    I would also be diving the same config all the time which is good for building and re-inforcing routine.

    I'm not sure if the Eclipse would carry twin 7 232's. I've heard the 300b set is close to the same mass as the twin 12's so I would need to use the Evolve 40lb.

    Is the Evolve a good wing to use with twin 7's - not too big or too baggy for use with the smaller set?

    If the Evolve is good for the 7's as well as the 12's then I can also sell the Eclipse to help pay for them.

    Trouble is I like my Eclipse.

    Any thoughts?
     
  5. Wibble

    Wibble Fish don't talk
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    I used to dive the twin 7s with an Apeks WTX3 which is allegedly for a single but worked fine with the twin 7s.

    I've got an Eclipse for use on holiday with a single tank adapter. If nobody else knows the answer, I could see if it fits on the twin 7s. Mind you, I don't have a pool to test it in!

    Weight wise - that's In-Water weight - the twin 7 300s are more dense than twin 12s. I dive in the sea with an additional 4kg on the twin 12s, but I don't need this with the twin 7s. I use a SS backplate with both. If I'm diving freshwater in the winter (more padding), then I use a stainless BP and 1kg for the twin 12s, but an aluminium backplate and no weight for the twin 7s.

    As I try to be fairly neutrally buoyant, then the wing just needs to compensate for the change in gas weight; 6.6kg for the 12s, 5kg for the twin 7s, so a smaller wing would work.

    They feel fairly similar in out-of-water weight, but I suspect the twin 7s will be a bit lighter. The twin 7s are definitely easier to handle on the boat being that bit lighter.
     
  6. MikeyH

    MikeyH Active Member
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    I tried last night :) The eclipse will not work with twin 7x 300s on a SS BP. So I am looking for a twinsize wing now.
     
  7. Wibble

    Wibble Fish don't talk
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    Keep an eye open for an Evolve. Rare, but they do come up. If you see one for £200, grab it. Crazy money for a rubber ring...

    Other alternatives are the standards: ISTC, Frog, Agir...
     
  8. Cybes

    Cybes Active Member

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    I didn't expect the Eclipse to carry the twinset because the shape of both are different, mismatched. I suspect the lift capacity would also be inadequate.

    The question is whether a twinset wing such as the Evolve can operate efficiently with both twin 12's and twin 7's. If we need the 40lb Evolve for the 12's would it be too big and baggy for the 7's? Do you need a different, smaller wing for the 7's?

    I suspect that I am looking at this wrong.
     
  9. Wibble

    Wibble Fish don't talk
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    The Evolve works well with the twin 7's. I've used it a few times. I don't think it's particularly baggy - but I don't have eyes in the back of my head:)

    I'd suggest that you definitely don't need a different wing for twin 7's.
     
  10. Zubar

    Zubar Active Member
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    Yay I'm now the owner of a shiny set of twin 7s!

    Need to work out what wing is best?
    Have a frog midnight which is 50lb or an IST 30lb.

    It is set up wide but have narrow bands and isolator. (Also non isolator mani).
     
  11. Cybes

    Cybes Active Member

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    I got the parts together for my 7's last month. I have a few issues with the need to service old equipment, but nothing unexpected.

    They dive wonderfully well. Very well balanced, very smooth. I am very happy with them.

    I'm using an 30lb Evolve Wing. I was using an 30lb Eclipse with my 15L. As it turns out I'm using the same lead with the 7's as I did with the 15. The Evolve feels like a good fit, not too baggy. I've only had it down to 20m but it didn't seem overly full at that depth.

    I don't think they do the 30lb Evolve anymore, but your IST has the same lift capacity.

    Did I mention they are a pleasure to dive?

    Enjoy
     
  12. Zubar

    Zubar Active Member
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    The IST jt30 is a lovely wing but sculpted in shape so it really hugs a single tin.

    I may look at getting a wtx3.

    Quite fancy trying it ultra narrow also.
     
  13. Tel

    Tel Super Moderator
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    My first twinset was 7lt x 300's and they trimmed out really well and were great on the dive.

    Even with a decent fill though the average two dives a day off a UK boat didnt have enough gas.
    That meant taking another cylinder to decant or even switching to a single (it was actualy easier
    and meant both dives had full cylinder(s). It also wasnt great if you needed to donate, just not
    quite enough gas.

    In the end I sold them and went for twin 12's :)
     
  14. Zubar

    Zubar Active Member
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    Tel you are quite right, and a single 15 or pair of would make more sense but o want my regs to work the same.
     
  15. Doomanic

    Doomanic Dinosaur Wrangler
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    I've got to ask; why?
    Do you want to borrow one?
     
  16. Zubar

    Zubar Active Member
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    I've always wanted a set. Seem to do more shallow diving than deep. And it's light! And shiny.

    Yes please on the lend.

    Just sometimes twin 12s seems a bit much and it's quite liberating to have a small light set.
     
  17. Doomanic

    Doomanic Dinosaur Wrangler
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    I know what you mean, you really need to try twin 3's... :D
    PM me an address to send it.
     
    Willie likes this.
  18. hawk

    hawk Doing It Rong
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    He said light :)
     
  19. Doomanic

    Doomanic Dinosaur Wrangler
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    The standard harness is light, It needs so much lead I thought AP had worked out how to get helium to stay in a cloth state! :D
     
    Sidemount_Stu and hawk like this.
  20. Zubar

    Zubar Active Member
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    Thanks don't need it yet but will let you know.
     
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