HP, LP, AL? what does any of that mean

[franksrq]

I am new to diving and (until i read some of the threads here) have no idea what any of this stuff means. Could someone give me a quick rundown on the different tank materials and the pros/cons of each? how many diferent metals are tanks made out of and what sizes/pressures/cubic feet of air can each material hold?

[fire diver]

Well, tanks are either AL (aluminum) or AA (steel). Some tankd have an AL inner shell and are wrapped in fiberglass, but these are rarities indiving, and almost never seen.

LP is low pressure. Typically only rated to about 2400 - 2600 PSI
HP is high pressure, and is typically rated for about 3500 - 3600 PSI.
Normal AL tanks are rated for 3000 PSI.

There is only one type of AL currently being used (6061 IIRC), but used to be made from 6351 for a while. 6351 tanks are banned by many dive shops for safety reasons, but many of those tanks are still in use (I have 2 that I fill and use myself).

AL tanks are cheaper than AA, and are what you find in most rental fleets. If you are used to diving the standard AL80 (80 cubic feet of gas, but it's really only holding 77.6 CF) then everywhere you go in the US, or Caribbean or Mex, you will already be used to that type of tank.

AA tanks are more expensive but are considered to be more robust. AA tanks are also heavier (more negatively bouyant in water) so diving with steels, means less lead on your belt.

AL tanks are actually about 3 pounds positively bouyant when near empty.

Capacities are anywhere from 3cf to 130cf.

FD

[reefugee]

Frank,

I am pretty new to tanks, but I will explain what I know. I am sure other members will chime in and correct me if I am wrong.

Material. Tanks are usually made of two types of material - Aluminum (Al) or steel. Most rental tanks are aluminum. A fully filled aluminum tank is negatively buoyant, but at the end of the dive, they tend to be positively bouyant. However, I recently seen aluminum tank advertised as "neutrally buoyant." Nice thing about aluminum is that they don't rust easily.

The other material tanks are made of is steel. Steel rust easier than aluminum. As a result, most steel tank a "galvanized dipped." Steel tank starts out negatively buoyant, and remain negatively buoyant. When switching from aluminum tank to a steel tank, you can probably drop a few pounds off of your weight belt. Most people believe that steel tanks have better buoyancy characteristics than aluminum tanks. Steel tank costs more than aluminum.

HP and LP refers to high pressure and low pressure. I have only seen it applied to steel tanks. A high pressure tank can be filled to a pressure of 3442psi. A LP tank can be filled to a pressure of around 2400psi. To make it even more confusing, many LP tank has a "plus rating" which means that you can fill the tank another 10% - thus making the true capacity of a new tank at 2640psi. If you can fill the same physical size tank with more pressure, then you have more air. The disadvantage of HP is that there is no guarantee every place you go to will be able to fill your tank to that pressure. Most LDS probably can fill a HP tank to 3400psi - but a liveaboard many not be able to.

You will also often seen people refer to tank as AL80, HP100, LP130 and so on. AL means aluminum. 80 is the amount of air (in cubic ft) that the tank will hold. Note that an AL80 really doesn't hold 80 ft^3 of air. I believe it's a little less, but it's close. If you see HP or LP - it means its a steel tank. So a HP100 will hold 100 cubic feet of air if filled to 3442PSI.

With all that in mind, I ended up purchasing four HP100. My reasoning is that most of the time, I will get the tank filled at my LDS - which can fill the tank to about 3400psi reliably. However, the tank has roughly the same dimension as an Al80. So if I am on a liveaboard where they can only fill the tank to 3000psi - I have roughly equivalent to an Al80. The HP100 is a few pounds heavier than the Al80 - but I was about to drop about 6 lbs off my weight belt when I went from Al80 to HP80.

Last - Steel tanks are more expensive than Al. I got a great deal on my HP100, but it still cost $289 per tank. An AL80 would have cost me $139. But considering how long I plan on keeping these tanks, I think it' well worth it.

Minh

[subsur]

nothing to really add to aforementioned posts. you did not say where you live. in any case, steel tanks are very popular in the north where lots of exposure protection is used, so heavier tanks are better to compensate for extra buoyancy of the wetsuit (or drysuit). also, it's irrelevant how much a tank weighs on land as long as you are comfortable carrying it. all you need to know is the buoyancy of a tank when full and empty, in particular when empty.

[franksrq]

thanks for the info. i dont think i will be buying a tank because renting them for the day is pretty cheap, but at least i know what it means. i went to get a tank a few days ago and the guy asked me which kind i wanted... "give me the most tank-iest tank you got." i dont know what i ended up with. next time i will be able to play it cool and not look like such a dork.
thanks

[fire diver]

The financial aspect of tank buying or renting depends on how often you dive. If you dive a lot you can save money pretty quickly buy having your own. Then there's also the convenience factor.

Fd

[ScubaToys Larry]

Originally Posted by franksrq

"give me the most tank-iest tank you got." i dont know what i ended up with. next time i will be able to play it cool and not look like such a dork.
thanks

A line like that! I'd never call you a dork... In fact I think we'll start calling the HP steels the "Tankiest Tanks" on our web page. Nice line!

Oh, and to the other guys... nice descriptions and explanations. Thanks for sharing the knowledge with the new folks!

[ScaredSilly]

Another pressure point is MP - midpressure cylinders. There are some steel cylinders that have fill pressure at 3000psi.

Also most steels come with a plus rating. This is indicated by a "+" next to the last hydro date. This allows for a 10% over fill. Alunimum cylinders never have an overfill.

[franksrq]

larry- glad you like my moment of ineptitude. use it for the greater good.

all others- whouldnt i need to buy more than one tank since its common and more fun to go under multiple times in one day? i'm sure even if i do own a tank, or many tanks, i would still have to end up renting some of the time. so this is my question to tank owners: taking into account the cost of filling and annual tests and hydro tests and everything else, would you buy tanks again or just rent? but owning your own stuff is cool, especially if you can get high quality stuff for really cheap prices, which is common practice when shopping at scubatoys.com
thanks

[CrzyJay456]

Originally Posted by ScubaToys Larry

A line like that! I'd never call you a dork... In fact I think we'll start calling the HP steels the "Tankiest Tanks" on our web page. Nice line!

Oh, and to the other guys... nice descriptions and explanations. Thanks for sharing the knowledge with the new folks!

Larry, Arent LP steels more tankiest than HP? i've seen a few and they did look tankier