Stage Cylinder

[SoB]

All,

I'm looking to trade down from two steel 10ltrs and get some stage cylinders - with DIR Tech-1 in mind. The 10's are a hang-over from going twin when I wasn't comfortable with 2x12 and was looking at a possible 2x10 (which I never did in the end). They only ended up being used as pool cylinders.

So, questions I'm not certain about are Steel or Ali, 5 or 7 if steel or 80 if ali?

I've already had some good feedback that 7's or 80's are the way for Tech-1 but I'm still at a loss of the Steel/Ali question. First - I have no real preference.

I understand that Ali's are actually heavier out of the water than steel but that due to the lighter metal requiring thicker/bigger build the displacement means that they are naturally more bouyant in-water. However is this so good a benefit when trying to hold bouyancy at the end of a dive.

If an Ali7 is 8.5kgs and a Steel is 7.5kg's but in the water the Ali regains 1.5kg bouyancy then in effect the Ali is only 7kg so only 0.5 kgs lighter - they must both gain the same amount of bouyancy as the gas content is reduced therefore my initial feeling is that I would prefer steel.

However if the suggestion is to go for an Ali80 then as I already have steel 10's am I not just spending money on an Ali version of the thing I have?

All comments gratefully received.

Thank you,
Simon

[Mal Bridgeman]

Doesn't this depend on whether you are ocean diving or cave diving? and whether it's additional bottom gas or deco stages. I am sure the DIR qualified will give the definitive answer but I recall Aluminium for additional bottom gas, aluminium for deco bottles in the sea and steel in caves assuming you are "staging" the tanks.

As I say a more definitive answer will no doubt come from others.
HTH
Mal

[PBrown]

For a stage cylinder you want Aluminium.

Take a peek at Greg Roach's excellent bouancy calculator here:
http://www.subaqua.co.uk/cgi-bin/cyl...i?units=metric

Originally Posted by Mal Bridgeman

Doesn't this depend on whether you are ocean diving or cave diving? and whether it's additional bottom gas or deco stages. I am sure the DIR qualified will give the definitive answer but I recall Aluminium for additional bottom gas, aluminium for deco bottles in the sea and steel in caves assuming you are "staging" the tanks.

The only time a steel cylinder would be used for stage/deco is in a cave at the final 6m stop, even then its a case of wanting to re-use old cylinders, rather than the preferred solution.

cheers,
Paul

[SoB]

Thanks so far guys.

I already had the link to the calculater and for a steel 7ltr ali 7ltre we have the following;


Using Archimedes principle, buoyancy is equal to the weight of the displaced water minus the weight of the cylinder.
The cylinder is made of 7.5 kg of steel, (which has a density of 7.8 kg/litre).
The steel therefore has a volume of 7.5 / 7.8 = 0.96 litres, so the total volume is 7 + 0.96 = 7.96 litres.
The density of salt water is 1.03 kg/litre, giving 7.96 x 1.03 = 8.2 kg
Air has a density of 0.0012 kg/litre.
The air in a full cylinder weighs 0.0012 x 232 bar x 7 litres = 1.9 kg.
The cylinder when empty has a buoyancy of 8.2 kg - 7.5 kg = 0.7 kg
The cylinder when full has a buoyancy of 8.2 kg - 7.5 kg - 1.9 kg = -1.1 kg
7.0 litre 232 bar 8.8 kg (Greg Roach)
The cylinder is made of 8.8 kg of aluminium, (which has a density of 2.7 kg/litre).
The aluminium therefore has a volume of 8.8 / 2.7 = 3.26 litres, so the total volume is 7 + 3.26 = 10.26 litres.
The density of salt water is 1.03 kg/litre, giving 10.26 x 1.03 = 10.6 kg
Air has a density of 0.0012 kg/litre.
The air in a full cylinder weighs 0.0012 x 232 bar x 7 litres = 1.9 kg.
The cylinder when empty has a buoyancy of 10.6 kg - 8.8 kg = 1.8 kg
The cylinder when full has a buoyancy of 10.6 kg - 8.8 kg - 1.9 kg = 0 kg

So the Ali is neutral when full but positively bouyant at all other times which suggests that I need to carry extra weight to counter the +ve bouyancy at the end of a dive - where-as the steel is only just bouyant if completely emptied and just -ve when full suggesting that I could even get rid of some weight (well maybe not)?

So I'm still confused. Taking the points constructively though.

Why would you only use Ali for stages though?

Thank you,
Simon

[PBrown]

Originally Posted by SoB

Why would you only use Ali for stages though?

don't forget that A) you will never completely empty a cylinder, and B) the weight of the reg's.

what I can tell you (from personal experience) is that even in salt water both Ali 7's & Ali 80's are still negative, (although only just with the Ali 80's), when down to 5 bar (with reg's on), so they don't need any extra weight to be carried. The other great thing is the trim of the Ali cylinders, the rear end gets very light and they tuck up under your arm very nicely. I've never seen this with the steels (although I hear there are some lightweight ones around which may do this), which always hang down.

hth

cheers,
Paul

[Kieran Hatton]

These calculations are obviously all correct but you need to add the weight of a pillar valve and reg to the cylinder as well. In my experience [note i've done no calculations for this] a steel 7 is then negative from begining to end and an ally 7 is negative at the begining but neutral when empty. This hopefully meens with the ally you can dive correctly weighted only slightly heavy when the cylinder is full as opposed to being over weight all the time with a steel. In my experience you should be correctly weighted without stage so with a full steel you will quite over weighted at the begining but with an ally only slightly.

Hope this makes sense.

Must be right, someone else got my 10pence worth in in the time it took me to type this.

[Andy Kerslake]

Simon

For stage bottles wherever possible we use Aluminium because of the bouyancy characteristics. They typically swing from being negative at the start of the dive to positive when empty, whereas most steel stage bottle would stay negative.

As for which stage bottles to use we (GUE) normally use either 80's, for wreck and cave diving, Ali 7's, or Ali 40's.

However the newer Ali 7's from Luxfer are heavier than the older style 7's and so their buoyancy is not as good.

For Tech 1 type dives I would use either a 7 or an Ali 80, which would be used as a 21m bottle, later if required you could also use an Ali 80 as a bottom gas stage.

I personally use Ali 7's, 80's , 40's depending on the requirements of the dive.

[Alex]

I used to use two steel 10 ltr stages and they were always destroing my trim. I moved to Ali- I own now two 80's and one 7 ltr (old style 200 Bar)- and find them much better, would never go back to steel . 80's are big however sit nicely under your arm in the water (especially if you dont fill them more than to 160 Bar). In salt water, they become positively boyant at about 30 Bar (with reg on). In emergency, if you breath it out dry and it starts making you too light, you can send it up to the surface (I never tried though).

Alex

[Bob Cooper]

Originally Posted by Alex

80's are big however sit nicely under your arm in the water. In salt water, they become positively boyant at about 30 Bar (with reg on).

Alex, surely you mean 130bar? not 30bar.

Or maybe you have some very heavy stage regs :-)

[Alex]

Originally Posted by Bob Cooper

Alex, surely you mean 130bar? not 30bar.

Or maybe you have some very heavy stage regs :-)

Bob

I have Apex 40 on DST first stages. I meant that at 30 Bar they would really go to the surface if released (well, perhaps at 50 Bar in the Red Sea...)

Alex