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Kate, I'm also wondering about twins 7's, 10's or 12's.
Something I was advised of is that to surface with 7's you need to think of what press you start to come up on, generally it looks like 80 bar (80 x 7 = 560ltrs, about the same as a pony 3l) as you need to assume you'll be on one cylinder if it really goes belly up.
At least that's the way I saw it, no doubt I'll be corrected if wrong.
Please dont hijack this thread to answer/debate my question as I'd like answers to Kate's question about twins too.
If needs be can we start another thread.

Matt



hi matt, yes i've been thinking as well, and the result is that i'll be justified doing exactly the same sort of diving (but no more) with twin sevens as i was with 12+pony...... you've got almost the same amount of air, divided between 2 cylinders, as long as you've got about 600 litres in each as a reserve (200x3l in a pony, 50x12l in your main, so 85x7l in each of your sevens) then it's exactly the same thing really.

the danger i think is thinking you can do more cos you've got a twinset, when really it isn't any different....
 

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Kate
assuming 232 cylinders (or 300 that you dont get filled to max. which would only give the equiv of about 260 bar anyway), the dive ends with 85 bar so we'd have 220 - 85 to do the dive on = 135 x 14l = 1890.
a 12 would give ((220 - 50) x 12) = 2040.

10's ((220 - 70) x 20) = 3000

By my reckoning you have more with a single 12 than 7's, so maybe 10's would be better. Plus 7's x 300 what weight? as opposed to a pair of 10's.

Then we get to the manifold question.

Is it worth it
 

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Although you actually have less available gas for the dive using twin 7's compared with a 12 and pony.

12x200 = 2400
12x50 = 600

Gas available for dive = 1800 litres

2(7x200) = 2800
2(7x85) = 1190

Gas available for dive = 1610 litres

That's 10% less, which is actual lost dive time on thise long shallow dives where gas is the limiting factor.  

Lou

EDIT:  Bloody hell Matt....great minds or what?  I was even going to add the bit about deciding to go for 10's as well but couldn't be arsed!

Apologies to everyone else for the doubling up!  
 

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Lou
get Steve W to fill your tanks, then you'll get 220 bar not 200


Job lot of 10's  - DiversWharehouse are doing them manifolded for about £300 ? Group discount perhaps?

So thats a set for you and C, one for Kate and one for me, anyone else?
Matt
 

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so annoyed, just wrote a post and deleted it with a careless key-stroke!

anyway, here's the gist....

whatever pressure you start with, you've got 15 litres in 12+pony, and 14 in twin sevens. so assuming you leave the same amount of air as a reserve (divided between the two), you've got a litre more with 12+pony. so better than twin sevens gas-wise, but from my point of view i'm moving to sevens for comfort rather than to get more gas.

i like the idea of being able to manipulate my reserve more with twin sevens. if i'm doing a 10 metre potter-about it seems pointless to have 600 litres of air sort of untouchable in a pony. with twin sevens i can just breathe down each tank a bit more, have a longer dive and still have a reasonable reserve.

your comment about tens - yes they would be better gas-wise but i am very small (under five foot!) and i think they'd look a bit ridiculous!

hmmm, just seen all the posts since i've been writing this.... must type quicker!
 

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Well these are just my thoughts on 7's, 10's and 12's...

If you don't think 7's are sufficient I would consider 12's.  

I dive with twin 10's and two reasonable dives would be borderline for me and I do have a reasonable SAC.  The difference in weight between 10's and 12's is not a great deal.

My reasons for 10's was purely logistics (I had 2 of them already!).

I think that depending on the types of dives then 7's or 12's is more suited (The 10's just seem to fall inbetween as a halfway house).

That's not to say I won't be looking at complementing my 10's at the show with twin 7 300bar.


Daz
 

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I just happen to have two 12l tanks, Hmm might try and dive them as Indies as I've already got the kit, well Adam has.
Couple of twinning cam bands and I'm in business. Bit over the top for a 6m dive though Daz?

Kate,
I understand what you are thinking but 7's would empty real quick and at least a pony is full when the smelly stuff hits the whirring thing. Just a thought, I know Dom dives 7's (232's I think) so do others. How about dumpy 10's.
Not trying to say you are wrong just thinking of all the other options so you dont fall into the trap we all do of buying something and wishing you'd bought something else instead.

Matt
 

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[b said:
Quote[/b] (MATTBIN @ Nov. 05 2003,12:42)]I just happen to have two 12l tanks, Hmm might try and dive them as Indies as I've already got the kit, well Adam has.
Couple of twinning cam bands and I'm in business. Bit over the top for a 6m dive though Daz?
Matt

Definitely worth a try..  I use my 10's for everything from 6m pootles to 30m dives.  I have got used to them and find my trim is so much better....

But that's where the 7's come in,  I am looking at the 7's mainly for shore and shallow stuff, why 300's? well more gas and overall about the same weight as 232's when the reduction from the belt is taken into consideration.

Definitely overkill for 6m dives but then most of my 6m dives are with students or similar and in the event of a problem it could make the difference between me scaring the shit out of the student by nicking their octopus or having to head for the surface and leave the instructor with the students on his own.

Daz
 

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Kate, your logic isn't quite right.

The reserve of air has to be the same, you will still need the gas volume to surface in an emergency.

In a 12 and pony your pony is untouched.  That's yours to keep.  Your 12 litre you need say 50b of, which is approx 20-25% of what you started with *for the dive*.

In twin 7s you still need the 600l, but for all poo-type situations you need 600l in each tank.  Now 600l is, as you said, about 85b, which is about 40% of what you started with.  So although the 14l @ 200b is more thatn the 12l @ 200b, overall you have *less* available gas than with a 12 and pony.

If the size is an issue, rather than weight then why not look at the 300b 7's as suggested above.  That would sort the problem.

Lou
 

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<font color='#0000FF'>Kate

Just to say I dive twin 7's have been doing since 98.  I have never had a problem with being short of air.  They are stable unlike a 12 plus pony.  

I do a variety of depths from shallow - 40 metre range with them and as I don't like doing masses of deco I find that time is the limit not the amount of air.  

If I am doing shallow dives I have even got two dives out of them, although there was carfull monitoring for the second one.

Fiona
 

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I'd have to disagree on some thoughts within this thread I'm afraid...

I personally feel that including the contents of a pony bottle in discussions about gas planning is not really the best thing to do.

A pony should not be counted at all as for all intents and purposes the pony should never ever be breathed... it's only there as a redundant air supply and it's only carried because of the inherent problems of trying to isolate a single cylinder or for complete gas loss of single cylinder so why include it in discussions on usable gas amounts.. you are not comparing like for like.

In reality your usable gas on a dive would be..

A 12ltr @ 232 bar will give 2784 litres of usable air
(A redundant air supply is required for emergency use only)

Manifolded 2 x 7ltrs @ 232 bar will give 3248 litres (and being manifolded they include the required redundancy)

So you have more air and the ability to fix a myriad of problems.

I think it dangerous to include pony contents as available air as some might start to think of it as extra gas and stay down longer, really it shouldn't exist in any calculations whatsoever, just make sure it's alway full before you go diving ;-)

Anyway. those are my thoughts on it, I don't expect everyone to agree..

Kate in my opinion twin sevens are more than enough for your current requirements.

You'll get the balance and redundancy you seek and you'll also have more available air

HTH
Regards Dave.  
 

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[b said:
Quote[/b] (Dave Williamson @ Nov. 05 2003,13:39)]Anyway. those are my thoughts on it, I don't expect everyone to agree..

Kate in my opinion twin sevens are more than enough for your current requirements.

You'll get the balance and redundancy you seek and you'll also have more available air

HTH
Regards Dave.  
<font color='#0000FF'>Dave, I agree with you maybe I didn't say that in my previous reply when I mentioned 12 + 3, I was only commenting on the stability not the air availble, When I went from a 12 to 12+3 it was always understood this was emergency use only.

I was advocating 7 over 10 or 12's more than enough air to be going on with.  

Also Kate I don't know what sort or build you are but twin 10 are blinking heavy  


Anyway 7's are still my choice.

FB
 

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Dave, quite agree with the pony statement.
But surely if you dive twin anything you need to plan for an emergency situation based on one cylinder contents just in case?
So the 7's dont have quitethat much gas 'available' in all situations. Or am I being overly cautious?

Matt
 

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[b said:
Quote[/b] (Lou @ Nov. 05 2003,13:23)]In twin 7s you still need the 600l, but for all poo-type situations you need 600l in each tank.  Now 600l is, as you said, about 85b, which is about 40% of what you started with.  So although the 14l @ 200b is more thatn the 12l @ 200b, overall you have *less* available gas than with a 12 and pony.
that's exactly what i said!

my logic is right, you just didn't read my post  


to re-cap -

- with a 12+pony, i have 600 litres reserve in each tank.

- with twin sevens, i have 600 litres reserve in each tank.

assuming that everything else is equal (i.e all filled to the same pressure), that means i have more gas in 12+pony. BUT as i said, i'm not going for sevens for gas, i'm going for stability.

i agree 300 bar sevens would be better - but i'm buying my 232s (unused) for £80 the pair - show me a deal like that for 300s and i'll take it!


dave - i agree with you in principle, but i think when  comparing independent sevens with a 12+pony it is valid to include the pony. once you have breathed from one of your sevens, leaving it with 85 bar (600l), it in essence becomes a big pony. then you breathe down your next tank, surfacing with 85 bar - i.e. the same as surfacing with 50 bar in a twelve, plus 600 litres in your other tank (like a pony).

if it all goes tits-up in while breathing from the first tank, you've got a full seven on the other side to get to the surface. if it all goes tits-up while breathing from the second tank, you've got your 'pony' - the unfinished first tank with 600 litres of air.
 

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[b said:
Quote[/b] (FionaB @ Nov. 05 2003,13:46)]Also Kate I don't know what sort or build you are but twin 10 are blinking heavy  
i'm very small!!
 

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[b said:
Quote[/b] ]agree 300 bar sevens would be better - but i'm buying my 232s (unused) for £80 the pair - show me a deal like that for 300s and i'll take it!
I take it all back!! Go for the 7's!!  


Dave,

I don't think that anyone was actually talking about the pony as available air at all.  Quite the opposite.  Most categorically the pony is only there for redundancy, as is the last 85b in either of the two 7 litre tanks, or the 50b in the 12 litre.  That *is* comparing like with like, even with manifolds.  Even with a manifold you decide what is your minimum required pressure to reach the surface based on losing one tank completely, don't you?

The point with the sevens is that, unlike using twins of a larger size, you actually have to have a higher pressure remaining to give you the volume, and hence you have less available air.  That obviously doesn't apply when doing the maths to change from a single 12 to twin 12's, and matters little changing from a single 12 to twin 10's.

Fiona,
I agree that on deep dives the deco limits you, but on a shallow dive, on a single and pony it *is* gas that limits me, hence my viewpoint on the matter, and why I suggested 300b or larger tanks.  The other minus for me was the need to have two sets for a full day's hardboat diving.  What do you do here?  Do you carry a single too?  I haven't bought any yet as i am still umm-ing and ahh-ing over all these points and more!!

Cheers all

Lou
 

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[b said:
Quote[/b] (Lou @ Nov. 05 2003,12:21)]EDIT:  Bloody hell Matt....great minds or what?  I was even going to add the bit about deciding to go for 10's as well but couldn't be arsed!
Guys,

Without going through gas planning etc.....

There are some other things to consider when getting twins sorted out that can get overlooked.

Twin sevens are great. They are easily manouverable topside and are well balanced. Great for shallow dives or very shortish 25mtr ranges. What you'll find is though that you will possibly need two sets of them. Seeing as most usual UK day boats go out all day and you do two dives (deeper first, shallower / drift second). If you try to get two dives out of them you'll be cutting one of the dives short (or not doing one of the dives at all).

Twin 10's - Better for gas, can even get two dives out of them. More difficult to handle out of the water. Can be a real pain to use when kitting up, especially if there's a delay in getting in the water. The 10's are shorter than the 12's but not much lighter. This leaves them a bit short on the back, hence it's difficult to sit down and rest the rig on your seating position. This means that you've holding the whole weight of the unit on your back whilst your kitted up. In rough weather this becomes a real pain and can be a dangerous hazard. You become a bit unstable on deck trying to hold / balance the rig.

Twin twelves: This would be my choice. They are more difficult to manouvere topside. But this is where one of the cheap diy trolleys comes in very handy for moving them around. Mine even stay on the trolley whilst they're being filled now (Saves the diveshop person from straining their back) They give you enough gas for two 'average' UK boat dives. You've got loads of gas for the first dive (which is usually the most interesting). Saves having to change cylinders between dives on a rocking boat. When you kit up the cylinder is such a length that you can leave it seated and 'climb into it' and kit up around it, the seating takes all the weight until you're ready to get in the water. The only problem you might find is climbing the boat ladder. It helps if you use a backplate as the weight is spread all over the back and not confined to one area ( a bit of weight training can also help with this! - not a bad thing for diving)

Hope this helps

Ian.

Ps. I don't do Rib's, need a bit of comfort with this diving lark.
 

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Sorry for interfering here, and if i've mis-understood you Kate then apologies, but...
[b said:
Quote[/b] ]
dave - i agree with you in principle, but i think when  comparing independent sevens with a 12+pony it is valid to include the pony. once you have breathed from one of your sevens, leaving it with 85 bar (600l), it in essence becomes a big pony. then you breathe down your next tank, surfacing with 85 bar - i.e. the same as surfacing with 50 bar in a twelve, plus 600 litres in your other tank (like a pony).

if it all goes tits-up in while breathing from the first tank, you've got a full seven on the other side to get to the surface. if it all goes tits-up while breathing from the second tank, you've got your 'pony' - the unfinished first tank with 600 litres of air.
Anyone diving independants will surely breath alternately from each, therefore surfacing with roughly the same amount of air in each?  If you breath off one cylinder for the duration of the dive, the have a freeflow from the 'full' cylinder, what reserve do you have?  What Dave's point (I think) is that the pony is totally redundant (i.e. you only breathe from it when you have total gas loss or you donate to buddy), whereas with indies, you breathe off both alternatively (50 bar, swap, 50 bar, swap...) so you have to include both in your calcs.
Hope that makes sense
Martin
 

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[b said:
Quote[/b] (Lou @ Nov. 05 2003,15:05)]Fiona,
The other minus for me was the need to have two sets for a full day's hardboat diving.  What do you do here?  Do you carry a single too?  I haven't bought any yet as i am still umm-ing and ahh-ing over all these points and more!!

Cheers all

Lou
<font color='#0000FF'>Lou

I have two sets of twin 7 232's.  

FB
 
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