tank selection...al verses st. hp or lp. help!

[downunder]

I am a newby open water with a question about tank selection
and pro's and cons of the different types of tanks.
can anyone help me?

 

[Rick Murchison]

The only reason to buy a standard aluminum 80 is cost. If you can afford it, you will love the buoyancy characteristics of steel, and LP tanks are much easier to get filled than HP anywhere you go. My personal favorite open water tank is still the original steel 72, a little smaller and lighter than the 80, but I can dive it with no weight at all in warm water - and as I'm usually diving with others it is a rare thing indeed that my tank pressure is the one that calls the dive. If your air consumption is average, then my recommendation for the absolute best tank is the OMS LP 85. You will *love* it - it's essentially the same size as the aluminum 80, (it's actually a little smaller and lighter, but as steel tanks have round bottoms and you'll probably use a boot, it'll appear the same size) but you can shed eight pounds from what you have to carry with an aluminum. If you really need the air, a steel 95 will let you shed 12 pounds, but now we're getting into a heavier, bigger tank.
I personally don't like HP tanks because of extra wear and tear on the high pressure components of the regulator, and the combination DIN fitting and high pressure makes a fill unavailable in lots of places. (Yeah, folks routinely put a yoke adapter on there and fill to 3500+ with a yoke, but it ain't smart since yokes aren't rated for it)
Rick

 

[Mario S Caner]

Downunder, There's only one single benefit to owning an Aluminum cylinder, and that's cost. A steel cylinder, especially HP cylinders are considerably smaller that the Al equivalent. The reason is simple, steel is a much stronger material and therefore they can get away with having thinner walls, especially in the boot and neck area.

The downside to steel is price. If that's not an issue for you then we can move into HP vs LP. It's true that the vast majority of technical divers use LP cylinders. The reason for that is because LP cylinders like OMS have a 10000 cycle Hydro life. Most usually fill their cylinders beyond the 10+ overfill allowance to cheat some more bottom time out of it. LP is heavier for the same size as HP. LP is gentler on your regulator.

It's difficult to get a truely good fill on a HP cylinder because dive shop personnel don't like placing the cylinder in water, filling it, waiting for it to cool off, and then top it off. It's time consuming. It's not that the compressor doesn't have the capability, it's a time thing. In fact if a shop tells you they aren't configured to fill HP, walk away and find a new shop. One with upto date equipement. HP is costlier than LP.

Here's a :nono: I stopped by a shop to have my cylinders filled on the way to a Lobster dive a couple of years ago. I usually fill my own cylinders but I had to stop by the registers to get a new Fishing Lisence, so I let the dive shop manager fill it. The dope filled my 2400 psi rated cylinders to 3700 psi. Hehe, nothing happened but the 133cuft of air didn't hurt.

So in conclusion, I use LP because it fits my large frame, and the type of diving I choose to take part in. Your needs may be different.

Mario

 

[Kevin]

My reply is not as technical as the other two but my advice would be to buy steel rather than aluminium. I dive in England and some dive shops are refusing to fill aluminium cylinders. A new test has been introduced where the cylinder has to be ultrasonically tested for cracks around the neck thread. I am told that the equipment to carry out this test is very expensive and so it is also becoming more difficult to find somewhere to have it tested.

 

[Mario S Caner]

Kevin,
The test you're referring to is done with a VIP plus machine. There is a device that has an electronic sensor on the end of it that you screw into the neck of the cylinder. The sensor detect's pits, valleys and even neck cracks that the human eye can't pick up. The machine costs right around $700 US, but who knows what the prices will inflate to by the time it makes it's way out there huh...

Anyway to make a long story short, out of the hundreds of VIP's I wound up doing every year, I've removed 6 or 7 that have had cracks, 3 of which would have passed without the benefit of the machine. As a person who trained other's in the hazards of filling a bad cylinder I know the risks well enough to be nervous about the process when a cylinder hasn't been 'plus'ed' via the 'machine'.

Here's a scary thought: A 72cuft cylinder filled to 2400 psi has the explosive force equivalent to a Cadillac hitting a brick wall head on at 500mph. Now just imagine what your 80cuft cylinder filled to 3300psi has instore for you! Can you say Ouch! For something you strap to your back, would you want to take the chance?

Mario

 

[Rick Murchison]

Mario, you do come up with the most interesting numbers! The "explosive force" of a 72cf cylinder is great, but could propel a caddie no more than about 50 fps (about 35 mph) *if* you could channel all the energy properly... Now that's no small amount of energy, and enough to do considerable damage.. (enough to blow it about 20' straight up in the air) but it's a long shot from 500mph.
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As for the neck cracks, only tanks manufactured from an alloy (6351-T6) commonly used prior to 1989 are subject to the cracking in question, called "sustained load cracking." If shops are refusing to fill more recently manufactured tanks that are otherwise in hydro and VIP they are doing so out of ignorance of the real problem.
Rick

 

[Iguana Don]

Regardless of the numbers, whether it be 5, 50 or 500 the damage will still be the same, the pieces just won't be as scattered. There is an interesting article in Diverlink about a tank explosion.

About HP vs LP steel tanks, I asked the same question when I was new to this board and rainreg "the resident equipment guru" said there is a measureable difference in useable air in the HP tank compared to the LP tank. It's in the U.S. navy Dive Manual. I'm checking out the Volume,Chapter & Page & will post it when I find it.
As for HP being hard or equipment. Any reg that won't stand up to a 3500 fill needs to be trashed IMHO. Most all of my fills are around 3200 psi anyway, on a standard 80 cu ft. tank. (Which I hate)

Gotta go and shovel more snow

Don

 

[Rick Murchison]

Not quite sure what you mean by "measurable difference in useable air." For any given physical size or weight, it is generally true that a high pressure tank will hold more air than a low pressure tank. And as the interior volume of a high pressure tank is less than that of a low pressure tank of the same capacity, there is slightly less "dead air" in an "empty" HP tank. Could you be referring to that? HP tanks as a rule have good to excellent buoyancy characteristics as well. The only disadvantages (aside from cost) to using HP tanks are equipment wear and fill availability.
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Wear and tear on equipment - regardless of whether a reg will "stand up to a 3500 fill," a regulator that is routinely subjected to higher pressures will have components wear out (mainly the high pressure seat) sooner than the same regulator that isn't. [You may drive on tires that are rated at 160 MPH; if your average driving speed over the same course is 90, they'll wear out quicker than if your average speed is 60... that doesn't mean they aren't good tires] For me, the best tanks are LP steel. I have buddies who dive HP and love it, but they do put up with more regulator maintenance than I do, and my little compressor I carry to remote sites gets me a full tank while they have to settle for either carrying extra tanks, short fills or long drives for air.
Rick

 

[Mario S Caner]

Hey guys, great numbers from everyone, but I would like to clear something up. I never intended to imply the force I talked about to be used in the context of pushing a Cadillac, but what I was referring to is the amount of energy generated at impact with the wall. When compared to the clinder rupturing that is... Either way :boom: is not cool.

Mario

 

[Rick Murchison]

Whether you're talking acceleration or deceleration, energy is energy, and the same amount's involved in going from "X" speed to 0 as from 0 to "X" speed. No way around it - physics. But if I'm interpreting your post correctly, what you're really saying is that when you said a caddie at 500 what you were really meaning to say was "one hell of an explosion" - which is accurate.
I have this thing for scientific reasonableness...
Rick