Aliminium or steel tank Which one to buy???

[Bob3]

... AL tanks have to be visually inspected every year?

In addition to the "normal" annual visual inspection, some shops require an extra test be done on Al tanks, an eddy current test.
Here's a little more reading material:
http://www.luxfercylinders.com/news/releases/20000717.shtml
http://www.keylargodiving.com/veddy.html
http://www.undercurrent.org/UCnow/articles/AlTanks200008.shtml

Luxfer, the leading manufacturer of Al tanks in the world, has a slightly different take on the use of eddy testing, as some of their brand new tanks failed the test.
http://www.luxfercylinders.com/news/
(scroll down to "1999 releases"

Prices for the eddy test vary considerably, from free to over $50.
Some shops even flat out rip you off by requiring steel tanks be eddy tested.

Rust isn't a problem with steel tanks as long as they're properly treated & maintained.

The "K" valve designation is just used to differentiate it from the "J" valve (reserve). It's a holdover from the 1953 US Divers catalog where the "J" and "K" were the item names.

You're probably thinking "scuba" vs "DIN" valves.
Yoke valves are LP, rated to 200 bar, a bar being 1 atmosphere or 14.7 psi
DIN valves come in to flavors, the HP/ 300 bar & the LP/ 200 bar.
A yoke will not fit over the 300 bar valves, they intentionally make them thicker so ya can't get the yoke over or screw in an adaptor. The threads are deeper on the HP, so the 200 bar adaptor won't seat.

300 bar/ HP DIN is good to 4500 psi.

Confoosed yet?

 

[SeaJay]

Not at all.

I've dived most of the mainstream configurations that most people talk about, so I can picture each of these in my head, helping to keep things clear.

What I didn't know was that regular yoke valves were rated at 3000 psi or less.

PST puts them on their 3300 psi aluminum tanks... Maybe this isn't as exact a science as I thought it might be.

 

[yknot]

What are you using to document the rated pressures? Is it the valves themselves or the usage that limits the pressure? As an example I have one AL80 that must be charged to 3000psi in order to be at 80 c.f. I was told that it is the yoke style regulator that is limiting me to pressures below 3500psi. I don't know at what pressure the burst disc would blow for this tank but as far as I can see this tank could be charged to, say, 4000 psi. According to what I have been told I would NOT want to connect my yoke style reg to a tank at that pressure, only a DIN style. Example #2 is for an OMS (Faber) L.P. steel tank. OMS rates this tank at 4000 psi for 10,000 fill cycles. The OMS valve has a center threaded insert- all I have to do to switch between DIN and yoke is unscrew this bushing and again, stay below 3500 psi when using my yoke style regs. What the big attraction is in my area to these OMS steel tanks is the ability to overfill and still remain within safe pressures. As all local diving is fresh water there is no issue with salt driven corrosion. If I filled my AL80 to 3200 psi I should have about 85 cf of air. If I filled a LP steel 80 to the same 3200 psi I should have about 97 cf of air. Using this criteria, would it be fair to compare 80 cf steel tanks with 96 cf aluminums?

 

[SeaJay]

yknot once bubbled...
...OMS rates this tank at 4000 psi for 10,000 fill cycles.

If I filled a LP steel 80 to the same 3200 psi I should have about 97 cf of air. Using this criteria, would it be fair to compare 80 cf steel tanks with 96 cf aluminums?

You're talking about taking a tank normally rated to 2600 psi and pumping it up to 4000 psi? How, then, is that a LP tank?

My impression of "the big picture" is this... The bigger the tank, the more air it will hold, all other things being equal. Note, however, that AL tanks require much thicker walls than do steels, because of the strength in the material. So we're not talking external size here, we're talking internal size. With that in mind, all other things being equal, the bigger interior-sized tank will hold more air.

Unfortunately, all other things AREN'T equal. Some tanks can only hold 2600 psi, while others being sold on the market can hold as much as 3500. For two equal-sized tanks, obviously the one that can sustain more pressure will hold more air. While this seems like a good idea, not all fill stations will fill to 3500 psi. Thus, there's going to be times when you're going to be faced with sticking to low pressures. But not always.

In the diving world, we all want the smallest and most neutral tanks that hold the most air... While on the surface, it's always nice to have a lighter tank.

...Have I got all of that right?

DIR says that tropical divers in a 3/2 mil are best off with AL80's, but I've been eyeballin' the new E-series PST HP120. It's almost neutral empty, so there's only 3-4 lbs difference between each's buoyancy. I could make a few changes (ie: Go with an AL bakcplate instead of a steel one) and do this change, I believe. The only issue that I think I'd have maybe is that full, this thing weighs like 11.5 pounds in freshwater, which is a lot to swim up in the event of a bladder failure. Also, the tank would put all of it's weight much further from my CG, which perhaps could cause somewhat of a "turtle" effect.

The only problem with all of this is the fact that my air consumption is already the best in my group... And improving. Dunno what I wanna do now. ;-0

 

[Mo2vation]

4 plus years ago, when I first got into diving I bought wifie a PST HP Steel 80 for all the reasons above (stubby, lighter, surf entry, higher capacity, favorable buoyancy profile, etc.) and I got me the PST HP100.

At that time, she sipped gas like a chick with 40+ dives, and I hoovered it like a new diver with 36 pounds on his belt. We would both go in at 3400 or so, and both come out even.

Fast forward 5 seasons - I've got 60 some dives, I'm in a Drysuit with 16 pounds on the belt and I sip gas like a chick. We'll do gauge checks and I'll be at 1700 and she'll be at 1050, you know? She hits topside with 500 and I'm still at 1100ish.

The domestic issues this causes aside ,I'm finding the same thing with dive-boat strokes I get paired up with.

Its frustrating (not that I need to sip it dry like SOME people... ahem-UP-ahem) but I've sort of turned it around and now appreciate the safety factor. If anything should go wrong near the end of a dive, God forbid, it gives great peace of mind that I'm the one with the gas supply and will be there for my buddy, my wife or anyone else in my little group.

Its sad to effectively cut a dive short, but the safety trade off is better in the long run.

Of course, try telling me that when I gotta leave my eyeball-to-eyeball with a BSB off Catalina because my buddy hoovered through their gas...I'll be singing a different song. :banging:


K

 

[Spectre]

BTW: Since the original poster didn't provide any information to be able to accurately suggest a type of tank; I've closed the poll. The poll; with no accompaning information; is worthless IMHO. If you disagree; please state your reasons in a PM to myself.

 

[Wreck/Tec]

Personally, I prefer steel. I dive fresh water, I hate wearing lead on my waist. Using steel helps reduce belt weight. You can fill them to higher pressure and have the tank survive. While they are more expensive, and require more care, I am willing to endure the fuss. The only time I wear aluminum tanks would be as a Pony or in the dead of winter crawling out of the ice.
Wreck/Tec

 

[jepuskar]

Did you flunk the short essay class in high school?

 

[SeaJay]

...And it's "SeaJay," not "Sea Jay." Just like "Madonna" is "Madonna," not "Mad Donna."

"Mr. SeaJay" to you.

 

[Genesis]

I own three AL80s. Why did I buy them? Everyone had them, they're cheap, easy to get full fills, and a reasonable compromise.

I refused to buy the old PST HP100s because they had a funny neck thread, thus making them usable ONLY with the valves they come with and a handful of others. If you decide you want to double them, the only people who made a manifold that would work at the time was Genesis, and I HATE their DIN manifold (the angled connections just don't do it for me.)

So, I bought AL80s.

NOW, having the new E-series about to come online, I have ordered two E-series HP100s. These are the same volume and buoyancy-wise as the old HP100s, BUT they have a convertable DIN/Yoke valve on them AND a standard 3/4" neck thread, so they can be doubled with the Sea Elite manifold I already own if I choose to in the future.

Part of the reason why I did this is that I have decided to put in my own compressor and nitrox blending system as well at the house, so I simply won't care what a dive-shop weenie thinks any more. Now its MY call, not theirs.

The shorter, near-neutral tank means I can remove almost all of my weight diving wet in the summer, and cut back on the weight diving dry in the winter. Its ALSO lighter than an AL80 straight-up! So I win twice - my mass is lower besides the physical size being smaller, which means less mass to move, and should translate into a better SAC.

If I only want to (or can) fill it to 3000psi, I still have an 80 cube tank..... there are dives where that will make sense, in that on deeper stuff I tend to run out of NDL before I run out of gas....