Duncan

I've got 300 bar and 232 bar sets of 7s - the latter get used much more often

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Is that because of the type of cave diving that you prefer or because of the usefulness of 300bar 7's?

Hope you guys dont mind but this excellent debate was straying from the original topic, 'Slinging Stage Cylinders', and I dont want it to be burried so I have split it out from the original thread

Johnny said:

Duncan

I've got 300 bar and 232 bar sets of 7s - the latter get used much more often

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Is that because of the type of cave diving that you prefer or because of the usefulness of 300bar 7's?

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In short probably the sort of diving I do.  The 232 bar 7's are much lighter - in fact they are lighter than usual LP 7's.  Much easier to carry long distances.

In full - however, I also own a compressor that will overblow them a bit.  Using my own pump to fill to 300 bar is not good for the seals.  I don't have ready access to a shop that will fill my HP 7's to 300 bar easily.  Hence the HP bottles used for special occasions and often filled with something exotic.  The extra weight of the HP 7's and more gas they can carry is useful in not having to lug lead.  But you still have to carry the weight somehow.  The last outing my HP 7's had was a trip to Wookey 24 filled with Trimix in the Summer of 2004 they got left there until May 2005 when I used them to dive to 70 m in Chamber 25 to check that the gravel squeeze was still open.  They've be topped up with air since and still contain enough helium to make them useful for a deep dive - Ilam soon maybe.

When I started cave diving I was advised to get cylinders in multiples of two upwards in size.  In old money this is 28 or 21 cu ft (4 or 3 l 232 bar) then 50 cu ft (7 l 232 bar) then 100 cu ft (12 l 232 bar).  The 300 bar 7's (about 75 ish cu ft) sit between the two and often I either don't need so much gas or its not enough.  My 12's weigh almost the same as my 300 bar 7's and there aren't many places that I can't fit them as easily as 7's.

To ramble on this further, you will find that cave divers end up with an assortment of cylinders used for different purposes.  If you do any mixed gas diving then you'll find that some of your tanks contain gas mixtures more suitable for some dives rather than others and if you are tight (like me) then you'll save them for particular projects. Nowadays I've got cylinders in 2, 3, 4, 5, 7, 10, 12 and 15 litre sizes.  Half of them are underground.

300 bar cylinders seemed a good idea when they first came out but the additional weight outweighs (no pun intended) the extra capacity that they afford and they can be difficult to fill to working pressure.  Most modern DIN regulator are rated to 300 bar and this is not the issue.  A lot of cave divers I know have 300 bar sets but don't use them as much as 232 bar cylinders.

Cheers Duncan, all makes sense.

I found my 300bar 7's useful the other day in the Truffe, the water was low and climbing out of sump two would have been almost impossible with a couple of 12's but 7's were fine, 300bar 7's would get you further here.
Whilst I have been learning the ropes I have found the extra margin that they give comforting and although they are heavy I can still carry a pair a good distance. I can quite happily attach a pair of 300 7's at the TSG and walk to Buxton Water have even gone all the way to Ink Sump like this but that was a bit of an effort!
They have been useful for working underwater, lining, lugging cylinders & scaff through sumps as they give you loads of air.

I think its 'horses for courses' as, of course, the kit you need depends on what kind of diving your doing.

Hiya Johnny, long time no see.  For what it's worth, I don't have 300 bar cylinders because:

* it's often not easy to fill them properly, so you sometimes lug round weight unnecessarily.

* On long carries you aren't transporting separate lead and tanks, thus it's not possible to distribute the total weight among more people, making each person's load more manageable (= they might actually help in future!).

* when heavier cylinders fall there is more momentum (so more chance of cylinder taps being deformed on impact and rendered unusable).

* having to put up with more than one style of cylinder valve means some tanks and regs are incompatible (i.e. reducing interoperability in an emergency).

* using larger 232 bar tanks (instead of higher pressure smaller ones) gives the equivalent amount of gas with no significant weight gain (although I do accept that there may be certain situations where the more streamlined 7 litre HP tanks might have advantages over say 10 litre 7 inch diameter 232 bar tanks in sumps which are a bit snug; Ilam Rising springs to mind here).

You lads are doing some cracking stuff in Peak and, on your self carries via Buxton Water to Doom's retreat, I suspect that using high pressure tanks is no disadvantage.  However, over the whole spectrum of cave diving situations, I find them more trouble than they are worth.  So I'd concur with everything Duncan has offered - he is a very experienced bloke and his points are well worth noting. 

Mind how you go with that choke!

When I was diving I had pairs of 4,7,7.7 (207 bar) and 12 litre (232 bar) tanks. I never really considered HP tanks, for three reasons:

a) The difficulty of filling them to anything like full.
b) When doing a project with repeated dives in the same underground site, carrying HP tanks is more weight/unit air. OK, you need less lead with HP tanks, but you only need to carry the lead in once and it then gets dumped at the sump pool.
c) Availability of cheap second-hand tanks was heavily tilted towards lower-pressure tanks. Maybe that's changed now.

My 7.7 tanks were rather heavy, had a parallel thread fitting and a very high test pressure (over 350 bar if I remember right). Why they had such a low working pressure I don't know. I was tempted to overblow them to say, 250 bar, thus fitting in about halfway in capacity between my 7s and my 12s. I wouldn't recommend such a practice on a public forum of course ... :ang:

Mark

Girls, girls, girls,

Whats all this negative attitude about 300b 7's. They only weigh about 1kg more. As for long carries, I used to take mine down to swildons 12 on an almost weekly basis. When I eventually passed that irksome squeeze, (I believe only John Parker had at the time got through it) at -6m and the bank of gravel collapsed, blocking my way out, my high pressure 7 was literally, a life saver. In the absolute zero vis, unable to read pressure gauge, I could feel my breathing rate rise as I knew I had maybe 1 hr to dig my way back through what had taken several monthes to initially open. The fact that I had a hp 7 helped keep me calm at a rather tense time.

As for filling, it's not a problem. Remar diving in Caerphilly fill wet to 310. In the hot weather, I've been taking large blocks of ice to make sure of a good fill. The somerset compressor does 300, Nautique in Paignton Harbour used to fill to 360b, he was a real star, except the day he inadvertantly also filled a 200b 7 to 360. Maybe I'm not so good at maths, but I also like the simple thirds. I always forget, what's say thirds of a 220 b fill, where does the needle have to point at turn around time? This may sound daft, but with the newer stile small contents gauge, with red sections and green blocks and micro writing. Log live the hp 7. Sod the hp 12's though, they are heavy!

As for the krabs I use, they are Petzel Spirit's. I've also used 'smooth' designs in my climbing days, but, in my opinion, these are the best I've come across.

mike barnes said:

Nautique in Paignton Harbour used to fill to 360b, he was a real star, except the day he inadvertantly also filled a 200b 7 to 360.

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That sounds ominous...

I note the use of the past tense ("used to fill,,,", he was a real star..".)

Did the experience prove fatal?

mike barnes said:

Long live the hp 7.

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mike barnes said:

Girls, girls, girls,

...

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I used to use my 7's a lot more when I got fills as Aquasport in Birmingham.  Like Mike I used to chill my tanks in ice before pumping and they would put in more than 300 bar.  My HP's 7's have been to Maytime and back.  I also used them in Ogof Clogwyn - only went to 40 m but you couldn't read your guages most of the time.

However I must correct Mike in that thirds on a 300 bar fill actually occurs at 180 bar due to non-ideal gas laws.  If you turn at 200 bar you are erring on the side of caution.  Which is good especially as the thirds rule is a bit of a nonsense really.

The thirds rule is most definitely not a "bit of a nonsense" Duncan.  It's kept a lot of people alive over many decades.  But like all rules it's best treated as guidance for the (proverbial) wise, not merely for the obedience of fools.  I know you have penned some thoughts on this subject lately but it still makes sense for trainee cave divers to have it rammed into them that they must stick to this rule as a bare minimum.

It is the "rule" bit of the "thirds rule" that causes the problem.  The "thirds" bit is fine as a concept but, as Pitlamp says, the thirds rule should be used as guidance and it is not suitable to be rammed into beginners as an absolute rule that will confer any degree of safety. 

Most dives should be turned at a point that is anything but thirds.

Anybody mind if the topic of the Thirds Rule gets split into a separate thread?

Pitlamp said:

The thirds rule is most definitely not a "bit of a nonsense" Duncan.  It's kept a lot of people alive over many decades.  But like all rules it's best treated as guidance for the (proverbial) wise, not merely for the obedience of fools.  I know you have penned some thoughts on this subject lately but it still makes sense for trainee cave divers to have it rammed into them that they must stick to this rule as a bare minimum.

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I agree completely.  As a bare minimum it is better to surface with too much air than not come back at all.  It is very easy to overlook that the "1/3" of your supply that you are reserving for an emergency does not in fact give you much margin of error and that a more conservative approach should be advocated.  More "creative" gas management strategies can be applied under special circumstances but at the end of the day a cock up in a sump can leave you looking silly, dead or both.

Mike Boon (I believe) recommended that 1/4 of one's supply should be used for the inwards dive leaving a wider safety factor. NSS-CDS and NACD limit the penetration of novice cave divers to 1/6 of their air margin.

By all means split this into another thread Johnny but I think the consensus is one of gradual progression and its not as simple as saying one third in, one third out and one third for deco.

Looks like we're all agreed on erring on the side of safety so maybe we can usefully leave that one there (rather than having another topic).  I only picked up on that comment of Duncan's as it could be taken to mean it's OK to dive LESS conservatively than the thirds rule (which clearly none of us would want to happen).

A colleague of mine has dug this article up on the internet, its extremely informative.
http://www.babilim.co.uk/pages/gas_laws.html

I can quite happily attach a pair of 300 7's at the TSG and walk to Buxton Water have even gone all the way to Ink Sump like this but that was a bit of an effort!

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Oh yes, that does put muscles on your back!  :blink:

Cheers for the link Johnny, was informative if not a bit geeky.. Dunno about the cave dive side of 300bar cylinders much but I know for wreck divers like me they are not favoured. Proper fill troubles and large negative bouyancy/out of the water weight are problermatic and the what I've just read about gas laws just finishes them off for me.
John 

Johnny said:

A colleague of mine has dug this article up on the internet, its extremely informative.
http://www.babilim.co.uk/pages/gas_laws.html

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Johnny,

Thank you very much for this extremely useful link.

I knew that gas mixing was harder for 300 bar bottles, but didn't understand why.

I can remember just enough  physics to make sense of this explanation.

Many thanks,
Peter

In case it helps, I seem to remember a very useful discourse on the strange way gases behave at very high pressure in an issue of the CDG Newsletter.  It was written by Andy Goddard, an industrial chemist and experienced cave diver.  It's probably available online (to CDG members).  If anyone is particularly interested and can't find it I'll dig out my copy and supply the correct reference.

I also recall Mr G as being very good at blowing things up, either intentionally or not !