Ally 11's for stages

[hotelearith]

I am trying to decide on the best Allys to use as stages. I have just been using 10.7 litres in the red sea, and some 12's at one point and they all worked well. Have decided to use the same strategy in the uk.

I can't decide between the luxfer 207 bar or 232 bar. Has anyone got a view on the comparative buoyancy of the 207 vs the 232.

I figured I would get 230 fills in the 207 most of the time as the fill stations tend to miss this..

Anyone got a view or helpful experience??

[GLOC]

Do a search on here, there are loads of threads about this subject and it does get heated, especially when Chasey gets involved

[Mark Chase]

Originally Posted by hotelearith

I am trying to decide on the best Allys to use as stages. I have just been using 10.7 litres in the red sea, and some 12's at one point and they all worked well. Have decided to use the same strategy in the uk.

I can't decide between the luxfer 207 bar or 232 bar. Has anyone got a view on the comparative buoyancy of the 207 vs the 232.

I figured I would get 230 fills in the 207 most of the time as the fill stations tend to miss this..

Anyone got a view or helpful experience??

I can tell you the 207 bar ali with a full load of 50% gas is 3lb negative in fresh water, without a reg

I beleive the 235 bar version is much more negative. Thicker tank walls and more gas.

The 207 bar Ali 80 seems to be the one most used. I use one for my 50%

I use a steel 10 for my deep trimix bailout of 18/45 because that is only 1lb negative in fresh water with a 250 bar fill of mix. This only works with mix with nitrox it will be a lot heavier. Which is why I dont use two steel tanks.


ATB

Mark Chase

[colinicky]

I know there is this :- UK Scuba - Cylinder Buoyancy Calculation for cylinder buoyancy but is there one anywhere for working out the buoyancy with various mixed gas in ? nitrox & trimix .

[GLOC]

Joe Hesketh across on DIRx created a .pdf which loads of information regarding buoyancy characteristics of different mixes which can be downloaded from here

[wilbo]

Originally Posted by GLOC

Joe Hesketh across on DIRx created a .pdf which loads of information regarding buoyancy characteristics of different mixes which can be downloaded from here

or rather here: Clicky..

[Mark Chase]

Originally Posted by colinicky

I know there is this :- UK Scuba - Cylinder Buoyancy Calculation for cylinder buoyancy but is there one anywhere for working out the buoyancy with various mixed gas in ? nitrox & trimix .

But it dosent work in real life.

Using Archimedes principle, buoyancy is equal to the weight of the displaced water minus the weight of the cylinder.

The cylinder is made of 14.2 kg of aluminium, (which has a density of 2.7 kg/litre).
The aluminium therefore has a volume of 14.2 / 2.7 = 5.26 litres, so the total volume is 11 + 5.26 = 16.26 litres.
The density of fresh water is 1 kg/litre, giving 16.26 x 1 = 16.3 kg

Air has a density of 0.0012 kg/litre.
The air in a full cylinder weighs 0.0012 x 207 bar x 11 litres = 2.7 kg.

The cylinder when empty has a buoyancy of 16.3 kg - 14.2 kg = 2.1 kg
The cylinder when full has a buoyancy of 16.3 kg - 14.2 kg - 2.7 kg = -0.5 kg


So why is the cylinder 3lb or 1.4Kg negative?




First time Janos handed me a full Ali 80 in Vobster I expected it to weigh nothing. We were at 20m but dispite this he went up, i went down . I was shocked. So i decided to carry out some real world tests. The Ali 80 full of 50% was 3lb neg without a regulator 5 1/2 lb neg with a Ocianic First stage reg and suit hose on it. Tank is rigged DIR with two bolt snaps and a single J clip but it does have an additional D ring on it.


Note how the Ali 80 (left of the pic) floats up under my arm and the steel 10right of the pic dosent ??

Oh hang on thats bollocks they both hang down the same



ATB

Mark Chase

[Reikimaster]

Originally Posted by hotelearith

I figured I would get 230 fills in the 207 most of the time as the fill stations tend to miss this..

i have been told thats a great way to get it to fail the neck crack test, or damage the neck theads, after a couple of years (or test cycles)

[wilbo]

Originally Posted by Reikimaster

i have been told thats a great way to get it to fail the neck crack test, or damage the neck theads, after a couple of years (or test cycles)

Yep - you really need to make the gas monkey/technician aware that it is a 207 bar cylinder - either verbally or via a pre-fill sticker..

its really quite annoying - in the States these cylinders are so cheap ($130-$140) that after a couple years - you just bin the old ones and get new ones..

[wreckweasel]

Originally Posted by hotelearith

I figured I would get 230 fills in the 207 most of the time as the fill stations tend to miss this..

Anyone got a view or helpful experience??

Yep. Al's physical properties don't cope well with over-pressuring, dont do it.

If you're lucky it will end its life when it fails its test early, if you're unlucky it will be during your unscheduled testing (ie when you overblow it).

Get a big black marker pen and fil in the 207 bar marks on the tank top.... or write them in big letters yourself.