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At first glance, it doesnít seem like thereís too much decision-making involved in buying a scuba tank (or if youíre really a mossback, a cylinder or bottle). And while itís not complicated, itís not quite as simple as pointing to the first one you see and saying, "Iíll take it - fill Ďer up."

WHY BUY A TANK - Not every diver is a candidate for a tank purchase. If the bulk of your diving involves getting on an airplane to go somewhere, donít waste your money on a tank because youíll never take it with you. But if you do a lot of diving in your local area (or even just a short drive away) then buying a tank is something that makes sense. But whether you own your own or rent from a dive shop, there are still some things you should know.

STEEL OR ALUMINUM - The first, and perhaps most significant, decision youíll make is what type of tank you get. The two choices are steel and aluminum. (Sorry, no oneís yet invented a lightweight ABS plastic tank - but when they do, theyíll make a fortune.)

Steel has been used as a tank material since the dawn of scuba diving. Over the years, it has proved extremely durable and useful as a tank material. However, steel rusts. So if you get a steel tank, assume that over the life of the tank youíll be paying for a few tank tumbles (to remove interior rust). And youíll also be paying more for a steel tank than an aluminum one, sometimes twice as much. But steel will last (almost) forever if you take care of it. There are steel tanks still in service today that were originally manufactured in the 50s.

Aluminum tanks came into the vogue in the 70s. Because itís a softer material, the walls of aluminum tanks are thicker, which also make them a little larger than steel tanks of the same capacity. The big advantage of aluminum is that it doesnít rust and itís a less-expensive metal than steel. So you can find aluminum tanks for $100-150 and you wonít have to factor in as many tumbles over the life of the tank.

But some serious questions have been raised about the safety of aluminum tanks over the past few years. The most serious concern is that hairline neck cracks have been found in some older aluminum tanks. (This is a problem that has not been seen in steel tanks.) Obviously, this raises the possibility of catastrophic failure of the tank (probably while being filled). And the juryís still out on whether this defect was a manufacturing problem with a large number of tanks, or is an inherent problem with aluminum. And while aluminum is still considered an acceptable material, you should be aware that these concerns exist and they may effect the projected life of the tank.

BUOYANCY CHARACTERISTICS - The other big difference between aluminum and steel is their buoyancy. Out of the water, aluminum and steel tanks of equal capacity weigh roughly the same, around 32-34 pounds (empty) for an 80cf tank. But underwater, itís a different story.

Steel tanks (empty) will be anywhere from neutrally buoyant to three pounds negative. Aluminum tanks (empty) will be 2-4 pounds positively buoyant. So it means you may need more weight with the aluminum tank than with the steel. Conversely, you can take off a few pounds when using a steel tank.

(PHYSICS MADE EASY - The reason for the difference is that with tanks of equal capacity, say 80cf, the aluminum tank will be slightly larger than the steel but weigh perhaps a pound less. Because itís larger, the aluminum tank displaces more water than the steel, which means it has a slightly greater buoyant force pushing up on it. And because itís dry weight is a little less than the steel, the aluminum tank has slightly less mass to counteract the buoyant "push" of the water. More force pushing up on the aluminum tank with less force pushing down makes for a tank that is slightly more buoyant than its steel cousin.)

SIZE MATTERS - Once youíve weighed all the pros and cons of steel vs. aluminum, youíll need to decide how big a tank you want. When we talk of tank size, we refer not to the physical dimension of the tank but the capacity of the tank expressed in cubic feet of air (cf). Although you can probably find tanks of almost any capacity, the most commonly-encountered sizes in todayís market will be 65, 72, 80, and 95cf. And while the dry weight of similar-capacity tanks is pretty much equal, there is a definite weight difference when you start changing capacities, with smaller-capacity tanks being significantly lighter (dry weight) than larger-capacity tanks.

Youíll need to examine your own diving history to decide what size tank is right for you. If youíve been renting a tank, ask your shop what size it is. If youíve been using an 80cf tank but come back with a lot of air, you can probably go to a smaller size. But if youíve been running out of air before youíve been running out of bottom time, then you might want to go to a larger-capacity tank.

I personally switched a few years ago from an aluminum 80cf to a steel 66cf. Not only is the new tank smaller and lighter on land, but in the water itís actually more negative than my old tank, so I was able to remove about four pounds from my weightbelt. And, for the type of diving I do in California, it provides me with plenty of bottom time as well as leaving me a comfortable reserve of air.

SIZE MATTERS (AGAIN) - Once youíve decided the capacity that works for you, the actual size of the tank becomes a factor. If youíre a smaller person but you need a larger-capacity tank, you can give some thought to a High Pressure (HP) tank. These tanks fills to 3500psi and, because of the higher fill pressure, are physically smaller than a standard tank of the same capacity. But remember that not all compressors (especially ones on boats) can fill to 3500. And when you use this type of a tank, you also need to convert to DIN fittings for the valve and your regulator.

HYDRO DATES - The final thing to note when you purchase a tank is the hydro/manufacture date thatís stamped on the tank. Like buying milk, you want to get the "freshest" tank available. A hydro date thatís 4-8 months old is pretty much the norm. But hydro dates over a year old means the tank has been sitting in the store for a while (which may also give you some leverage to negotiate a discount). And make sure the store puts a current VIP sticker on the tank before you walk out the door with it.

A tank is probably the last piece of equipment youíll need to buy but, unless you can hold you breath for a REALLY long time, itís something youíll probably need eventually.

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