Twin 10's or 7's?

[Woz]

If you are diving Scapa then the boats normally come with 12L cyls anyway so I would not bother buying anything and just twin up a pair of indies when you get there. That's what we did. And you don't have to lug them any more than 1/2 a doz steps so you may as well leave them twinned up all week.

[NotDeadYet]

I have a pair of 7's, 10's and 12's. I hate the 10's, they give me really bad backache because they are so short and they are a pain to get on and off.

I treat my 7's as an equivalent to single & pony type diving. Even though they are 300bar, in my experience of using them (only 10yrs), about 90% of the time you will be starting a dive with about 270bar, especially if you are using nitrox. On the other hand, they are the most comfortable bottles I have.

If it was me, I'd buy either 7's or 12's (unless you can get the European long 10's).

Don't forget to consider having to donate air to someone on the second dive if you are looking to get two dives from one set. You might have enough for you to do the dive but it can get a bit tight if you consider air sharing.

[Buoyant Babe]

Originally Posted by NotDeadYet

I
Don't forget to consider having to donate air to someone on the second dive if you are looking to get two dives from one set. You might have enough for you to do the dive but it can get a bit tight if you consider air sharing.

Good point - I'm confident for the depth I do that I wouldn't have a problem with this, but anyone else going a bit deeper would need to take that into account.

[Rob Dobson]

I find that 7's are lovely to dive as are 12's. I find 10's really uncomfortable - they are nearly the same weight as 12's but too short so that they trim badly in the water and give me backache on the boat. I think 10's are a bad setup.

Some people with relatively low SAC rates dive 7's to fairly deep depths - that's fine until you start working out the min gas figures with a panicked diver breathing off of your rig and suddenly the maths don't add up.

Scapa on 7's? Forget it. Get 12's.

My 2p.

[gingerstu]

so would I be considered mad to buy another 15ltr and twin up with these?

[Rob Dobson]

Originally Posted by gingerstu

so would I be considered mad to buy another 15ltr and twin up with these?

Not at all - 15's normally twin up quite nicely (as long as they are identical) and are nice in the water for not too much extra weight on your back - but it does kind of depend upon what the other people you dive with use.

If you are diving in a team that dive twin 12's then your gas planning will always be around the 12's so you are carrying extra weight which you are never planning on using and paying more for fills which you will never use.

If this was the case then I'd buy a set of 12's and keep the existing 15 for single/shallow/shore based stuff or for decanting into the twins to give you that extra 40 or so bar for the 2nd dive.

[gingerstu]

I have no problem lifting them and my buddy is useless with air, a complete wuss who will only go on twin 7s so I think having the extra reserve is a good idea.

Its only an extra £1 for the air fill. My peace of mind is worth a quid!

[EdHornby]

twin 15s are pretty heavy - I know 12s are heavy too but the 15s are another level

personally I'd say to try the 15s buddy banded as indies to see if they suit physically, getting rid of unwanted manifolded 15's could be tricky

[Rob Dobson]

Originally Posted by gingerstu

I have no problem lifting them and my buddy is useless with air, a complete wuss who will only go on twin 7s so I think having the extra reserve is a good idea.

Its only an extra £1 for the air fill. My peace of mind is worth a quid!

Personally I think that there is a bigger issue here than the size of your twinset:
If you lost your gas would you both be able to get up your mates 7's?

Take the highest SAC rate you have between you and double it for 2 divers, then double it again for for an emergency (panic) situation. You should now be looking at something in the order of at least 60LPM.

Now think about your deepest dives and workout the average ATA for that ascent profile. Take a minute to deal with the issue at depth and then think about a 9m per min ascent rate and then the stops you would expect to have to do. How long in minutes would it take you to get up safely from your deepest dives?

Take the 60LPM X times the average ATA X the number of minutes your ascent will take + one minute for the issue. Convert that litre volume into BAR pressure in twin 7's.

That is *min* amount of gas you should be planning to end all of your dives with. Your 15's are not the restricting factor here, it is your buddies rig.

[gingerstu]

fortunatley my buddy doesnt often get up in time to go diving as he is a lazy bastard (so I agree, my buddy is the problem).

When I do dive it is normally its with someone from in2scuba, or Mr Henson (frequents this site).

I havent gone for the extra 15ltr yet but will in the new year. I just think that you cant have enough air and if I can lift 'em, why not.