First stage cylinder - '80' or 7?

[edward]

As per the title really - I'm looking at buying my first stage cylinder. Will be used for (at the moment) diving up to 45m, twin 12's for back-gas with deco probably on 50%.

We used 232bar 7l stages on the course, and they were fine, but I've been playing with some gas calcs and with my SAC (high - plan on about 23 on dive, and possibly bit less - 20ish - on deco) some of the time it is the deco gas volume that is the limiting factor using a 7.

So, would I be better off with an ali 80? that would give me about an extra 700l of gas, even allowing for being 207 not 232 bar.

I understand the bouyancy of an 80 is not much different from a 7 in normal use, possibly a bit more positive, so not really a problem, I imagine both of them would sit up under your arm so wouldn't be in the way anyhow. However, with a reg on, how negative is an empty 80 compared to a 7? the 'tits up' situation I guess, but good to know about before it happens.

Another concern is the 80 seems to be much bigger out of the water.

So, what do you use? would and 80 be overkill?

All views appreciated. Then all I have to do is sort out the regs. Another world of questions........

[milldog]

I have a number of cylinders that i use and dependent on the dive determines the cylinder for me, that said, look at your SAC rate and that will determine the size of cylinder you can use.

but it's always better to have more gas than you need as I'd rather get out of the water with gas to spare than sending up a yellow blob for more gas!!

the sizes are

Al80 (207 bar) = 2295
Ali 7 (232 bar) = 1624
Ali 7 (207 bar) = 1449

as for the buoyancy the Ali 80 is very buoyant when you get down to the last 50 bar or so as are all the Ali cylinders thats why i like to be a little overweight rather than spot on.

if you are down the south give me a shout and I'll bring down a few for you to try before you go and get one, but i think you'd get the Al 80 in the end more uses

Graham

[nigelH]

Ali 80s are very trendy in some circles but 7s are more compact.

How much deco are you planning to do?
A steel 7 will do three quarters of an hour at 6m even on the breathing rates you propose and in reality, once the boredom of extended deco sets in, your breathing rate goes down and down. You end up tapping the gauge to see if it's jammed.

I have a couple of Ali stages out of test and two steel 7s I use for CCR bailout regularly. The Alis need some fancy eddy current check and I can't be bothered as they're a bitch in the water.

[Mark Powell]

The 'best' deco cylinder depends to a large extent on the gas planning rules you use, the type of diving you do and also the deco gas you choose.

EAN70 and EAN80 used to be the gas of choice when using dissolved gas (Buhlmann/Haldane) tables. This is because the stops would typically be starting at 12m or shallower (for 40-60m diving). As your stops start at 12m then switching to a deco gas before this didn't give you much of an advantage. A richer deco mix gave you more effective deco profiles.

If you are switching onto your deco gas at 12m/9m then the gas usage means that your twins 12 will start to become the limit before your deco gas and so a 7l deco cylinder will be fine.

With the advent of bubble models, deep stops, etc it is much more common to use EAN50. These models start your stops much deeper and it starts to be come more advantageous to use EAN50. However now you are using much more of your deco gas (and less back gas). In twin 12s the balance now shifts to the 7l being the limiting factor.

So larger deco cylinders have started to become more common. Hence the popularity of ali 80s.

Despite all the above, my advice is the same as for most equipment queries. Try to borrow both, have a try with each and see which you prefer.

[Juz]

I have a 7l steel you can try and in the next couple of weeks I'll have an AL80 you could try.

Mark is spot on, it's best to try different tanks if you can.

My interpretation:
7l steel - nice on land, compact in the water, more negative in teh water than Ally, which means you must be dive heavy in case you have to hand off the tank. Cost: Cheap, especially second hand.
7l Ally - never used one, but as it is bigger and holds less gas than the steel, I do struggle to see the advantage
AL80 - PITA on land, huge and heavy but hangs very nicely in the water. Cost: not many secxond hand, so really looking at new - £135 - £150

Again, Mark is correct - what diving are you doing now? what diving are you planning to do in the next 2 years.

Juz

[Mal Bridgeman]

Had a similar conversation yesterday with "my instructor" and concluded that there should be more to the deco cylinder choice than just the amount of gas it contains.

Essentially it's a balance between cylinder portability, the amount of deco time you are prepared to build up, what your approach is to minimum gas, what you would do if you lost your deco gas.

If the tendency to take lots of deco gas, there is a tendency to perhaps rack up more deco by staying longer at depth using up more back gas, so if there is a "SHTF" incident there may not be enough gas to get you and your buddy up safely.

Rgrds
Mal

[Foggy]

Originally Posted by Juz

7l Ally - .......and holds less gas than the steel, I do struggle to see the advantage

Erm Juz, if the water capacity of both the steel and ali is 7lt and the WP of both is 232 bar, why does the Ali hold less gas?


Cheers, Paul

[Juz]

Originally Posted by Foggy

Erm Juz, if the water capacity of both the steel and ali is 7lt and the WP of both is 232 bar, why does the Ali hold less gas?


Cheers, Paul

Typically Steel will have a working pressure of 232bar whilst the ally typically would be 207bar. If they were both 232, then they would hold the same gas.

This is not always the case and you do need to check individual cylinders, especially when buying second hand - I have a 207bar steel for example.

Juz

[gwilson]

Originally Posted by Foggy

Erm Juz, if the water capacity of both the steel and ali is 7lt and the WP of both is 232 bar, why does the Ali hold less gas?


Cheers, Paul

Is it that the external dimensions are bigger? Due to having thicker walls than the steel cylinders?

[Janos]

Originally Posted by gwilson

Is it that the external dimensions are bigger? Due to having thicker walls than the steel cylinders?

I have a 7l steel, and old Ali 7, and a new Ali 7. I might sell the new Ali 7. Once I've got the old one tested. Or perhaps the steel 7. Hmmm.

Anyway. All are 7l and have a WP of 232 bar. So hold the same amount of gas. The steel is most compact, followed by the old-style and new-style in quick succession.

However, and Ali7 isn't as nice as an Al80 in the water. Al80's are massive on land, but in the water they just bob behind you, well out of the way. They're brilliant. So I would get one of them.

Janos