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Discussion in 'Off Gassing' started by Cybes, Mar 10, 2016.

  1. Cybes

    Cybes Active Member

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    Not a great deal to do last night. Got a bit bored. How bored? 107 pages of bored.

    At some point I'm going to sort myself out a set of twin 12's. I'm also thinking of exchanging my single 15 for twin 7's so that I only need one set of doubles regs for both sets. This would mean I'm always diving the same config with everything in the same place.

    So I started looking through the Dive Equipment sub-forum for some mention of twin 7's. Not a lot there, all pointers towards twin 12's.

    I ended up going through all 107 pages, admittedly skimming most of the last 70%, cherry picking articles to read. What caught my interest was the ebb and flow of names, the way different people got into and developed their interest, you got a feel for how their diving was changing with their equipment.

    It shouldn't be a surprise that other people have already walked the same path and made many of the same choices, but it does give you a sense of familiarity. Made me smile.

    You could also get a feel for how what is normal and accepted as standard has changed over the years.


    Tonight I am going to give myself a slap and take myself off to the pub.
     
    Sidemount_Stu, Wibble and Doomanic like this.
  2. hawk

    hawk Doing It Rong
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    Personally I've never understood twin 7s. As you are able to manage twin 12s, why not just dive them and come to the surface with enough gas for a second dive?
     
  3. clique

    clique Active Member

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    I loved mine, the purpose of the twin 7s... shore dives and rib diving. Twin 12s are unnecessary for Chesil Cove and the like, given my regular buddy was sidemounting 7s for those dives anyway I could have gotten away with a single 15 but didn't want to have two separate sets of equipment.

    This way I had the same regulators, same wing, same harness, same fit irrespective of the sort of diving I was doing. If I only had twin 12s I wouldn't have bothered with either rib or shore diving.
     
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  4. hawk

    hawk Doing It Rong
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    Each to their own - I do agree that lugging twin 12s down the beach is hard going but I've never had any problems diving 12s off a rib and just can't see the advantage of a bit of saved weight against the cost of having to keep an extra twinset in tests (different story if you have a second twinset for expensive mixes).

    Out of curiosity, what is the weight difference between a twin 7s rig and a twin 12 rig (inc lead).
     
  5. JohnL

    JohnL Well-Known Member

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    For the benefit of a newbie, how do you manage weight with twin 12s? There must be quite a difference between full and empty - three or four kg? Do you have to start with greater overall weight than with, say, a single 12?
     
  6. jb2cool

    jb2cool Moderator
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    Weight yourself accordingly with near empty cylinders but you will be more negative at the start when cylinders are full
     
  7. clique

    clique Active Member

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    The cost of keeping the extra twinset in test is lower than the cost of keeping a dedicated regulator in service, pretty good point about it being the second twinset to use when there's a Gucci mix in the 12s. Meant I didn't waste my 18/40 mix on the Kyarra!

    I can't put numbers onto the weight difference between the two, but I can say that the twin 7 set up was light enough for me to manhandle onto my back fairly easily. Whereas I wasn't strong enough to do the same with twin 12s.
    Just found the 7s easier when diving off the rib - I had a set for shore / shallow diving, so decided I wouldn't make my life hard and just use the same size cylinder as my buddy :p

    There's a fair amount of weight built into the kit; stainless steel backplates (vs aluminium) and the cylinders themselves being inherently -ve buoyant means you end up with a small weight belt. I didn't need any extra weight when diving in freshwater, and had a 4kg weight belt when diving in the sea. I've heard numbers around 3 or 4kg of gas used banded about, so yep I suppose you do start the dive more 'overweighted' than you would with a single 12. I can't say that I've ever really noticed it though!

    I'm sure someone will be along soon to say that I've got it all wrong and can explain it better!
     
  8. Cybes

    Cybes Active Member

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    Exactly my thinking. Keep it all the same, everything simple.

    My only concern is whether I can use the same wing for the 7's as the 12's? Would the40lb wing be too big and baggy for the 7's?
     
  9. Wibble

    Wibble Fish don't talk
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    I've both twin 7s x 300 bar and twin 12s 232 bar. As the 7s are denser -- they need to have thicker walls to withstand the greater pressure -- they are heavier than the equivalent 232 bar 7 litre cylinders, but a bit lighter than the twin 12s.

    Diving with the 12s, I use a 3mm stainless backplate with 4kg 'V' weights for the sea and 1kg for fresh water. This keeps me properly weighted when they're empty. The 2 x 12 x 232 = 5,568 litres of 'air' weighs 6.6kg.
    (A litre of air weighs about 1.286 grams apparently - according to NASA)

    The twin 7s are heavier in the water and need no additional weight when diving in the sea. This makes them too heavy when diving in freshwater, so I'll use an aluminium backplate. They hold 2 x 7 x 270 = 3,780 litres -- you can't get a 300 bar fill without a lot of waiting; the best you'll normally get is 270.

    Why 7s over the 12s? They're definitely easier to handle in a RIB. If I'm diving with a bunch of recreational divers, they'll more closely match my buddy's available gas whilst giving me the additional security of redundancy and resilience. I get two first stages, two second stages, two cylinders and enough gas to get both of us out safely. Twin 7s are nicely balanced in the water.

    When doing deeper or more standard dives, the 12s are better. More gas, more reserve, more for the second dive.

    Both need the same 40lb wing. The only thing to change is the tins.
     
  10. Cybes

    Cybes Active Member

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    I really like the twin 12's but don't expect to do a lot of diving at home this year that needs them. Over here I'm renting them as I go.

    Which is why I'm thinking of a smaller set more suitable for use on a rhib.

    Twin 7's sets me up for 12's.

    232's won't have the weight of the 300's but they are the same size, so if Wibs has no problem with the 40lb wing then all is good. Sort the wing and regs, dive the 7's and add the 12's when I get the funds to do so.

    All is good.
     
  11. hawk

    hawk Doing It Rong
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    Does it really need a dedicated reg set though? How hard is it to take a 1st stage (that you already own) reg and add another reg / hose to it? All you need is an allen key to remove the blank and a spanner?
     
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  12. clique

    clique Active Member

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    I take your point that I wouldn't need a dedicated regulator set for single tank diving. Just the idea of frequent assembly / disassembly of the regs doesn't sit too comfortably with me. Probably no issue, but I opted for two twinsets rather than a twinset and a single set.

    Possibly a little bit of wires crossed, there are two different scenarios here - My twin 7s as a second set. And Cybes looking at twin 7s as a first set. As with all things, its just a tool for a job. If you're not going to do the diving which requires twin 12s, then why make life hard for yourself?

    Possibly worth saying that when my housemate learnt to dive, after finishing his Ocean Diver he went straight onto the twin 7s with a backplate, harness and wing. Means he's got a very easy transition up through the diver grades, as by that point he'd be much more familiar with diving a twinset.
     
  13. Wibble

    Wibble Fish don't talk
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    But you do need different lengths of hose, for example the HP hose to the SPG and the dry suit hose. Also the short hose might be a different length.

    It's easier to keep a backup setup as a single.
     
  14. Cybes

    Cybes Active Member

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    I think initially to keep costs down while I pay for the wing I'll need to use my single regs +1 DS4. I'll need new hoses but as the 7's I'm looking at have the standard width manifold fitted they will be right for 12's as well.

    I don't really want to pull apart the regs on the singles set but I can always rebuild it should I choose to keep the 15. I like my eclipse and I have a separate Ali backplate for it, so it would make sense to keep this for travelling. Best then, eventually, to have a set of regs to go with it.

    All this talk of twin 7's and I haven't dived a set yet. I've heard they dive really well. The set I'm lining up use Faber 232's
     
  15. Wibble

    Wibble Fish don't talk
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    My two short hoses -- backup reg + wing inflate -- are the same length. I use the reg-hose to BCD nipple adapter for the BCD, so both hoses are the same.

    It's fiddly to get the right length of HP hose; not too short, not too long. Same with drysuit inflation hose which tends to be quite long (also used for inflating the SMB). I think my drysuit inflation hose also uses the adapter which I find easier to connect one handed (essential for the wing inflate!).
     
  16. Zubar

    Zubar Active Member
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    I've had been thinking of getting some twin 7s for a while now.

    Mainly as above, can use my twin regs etc but are easier to lug about than my 12s.

    And as a second set for it a Gucci mix dive is blown out.
     
  17. Zubar

    Zubar Active Member
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    Also I suspect it would be cheaper to maintain as I'd not bother with O2 clean. I'd just use them with air and judge what depth and conditions I'm happy with. I'd keep the 12s O2 clean obvs.
     

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