Consider the humble scuba tank. If all your diving is from resorts and charter boats that supply tanks, you use what they give you, almost always an aluminum "80." But at more sophisticated operations—or if you're in the market to buy a cylinder—you may notice a few oddballs: a couple of smaller tanks, maybe some bigger ones, and tanks of unusual proportions. Who are these tanks for?
Maybe you. The aluminum 80 is so popular not because it's best for everyone but because it's cheap. In fact, choosing a different cylinder—one that fits your size, air consumption, buoyancy and style of diving, can make a world of difference on your next dive.
How to Choose a Tank
- Capacity. More air is better, of course, but you've got to have an intelligent balance between capacity, physical size, weight and buoyancy. Bottom line: there is such a thing as a tank that's too big.
- Size. How big is "too big?" That depends on your body size. Most divers will be bothered by length more than diameter. If you can sit down without the tank pushing the BC over your head, gearing up will be much easier. If you can carry the tank by the neck with your arm fully extended and not drag the tank on the ground, you'll prevent damage to both it and your back.
- Buoyancy. You'll need to adjust your weight belt to overcome a tank's positive buoyancy when it's nearly empty. If you don't add weight you'll have trouble making a safety stop. Likewise, if a tank is negatively buoyant when empty, you can remove weight from your weight belt.
- Weight and "Real Weight." Positive and negative buoyancy characteristics also affect the amount of weight you'll be lugging around topside before and after a dive. If you choose a lighter tank that's also more buoyant, you're not saving weight, you're just shifting it from the tank to your weight belt. To find a tank's "real" weight, add the published dry weight of the tank to its buoyancy when empty.
By those criteria, the standard aluminum 80 is too long at 26 inches for many divers. But there are options: Luxfer makes an 80-cubic-foot aluminum tank that is fatter than normal (8 inches diameter instead of 7.25 inches) but 3 inches shorter. It's also a little heavier. And both Luxfer and Catalina make 63-cubic-foot aluminum tanks that are shorter yet, under 22 inches. A steel tank is another option. Pressed Steel makes a high-pressure 80 that's less than 20 inches long.
For example: a Catalina aluminum 80 is listed at 31.6 pounds with 4.1 pounds positive buoyancy empty. So its real weight is 35.7 pounds. On the other hand, a high-pressure Faber steel 80 that's listed at 32.4 pounds has 7.3 pounds negative buoyancy when empty, for a real weight of 25.1 pounds. Its published weight is more than the aluminum tank, but its "real" weight is 10 pounds less.
For Rent: Tank Tips for Travelers
Special tanks are like special airline meals: You'll never see one unless you ask. (Tip: call ahead. Even if the dive operator doesn't have them, you might get them thinking.) By far the most common rental tanks worldwide are aluminum 80s like the Luxfer S080 or the Catalina S80. They measure 7 1/4 inches by 26 inches and are about 4 pounds buoyant when empty. But there are a growing number of choices. A common alternative is an aluminum 63. It's 4 inches shorter than an 80, lighter and less buoyant—the "real" weight is about 6 pounds less.
Many of the better resorts also have smaller tanks for children and smaller adults, and some have bigger tanks, too. Or a divemaster may have personal tanks he'll lend you. (Don't forget a generous extra tip.)
Regardless of the size of rental tank you use, there are some common things to check out before the first dive.
- Check the valve O-ring. Rental and resort tanks are used and abused. O-rings probably aren't replaced until they actually leak. Give yours a close look for fraying, nicks and cracks. Listen closely for a leak, or splash water on the pressurized connection and look for bubbles. O-rings are cheap and easy to replace (you can even bring your own in a save-a-dive kit) and the dive operator shouldn't hesitate to do so, or to provide a new cylinder.
- Check the valve-to-tank seal. A leak there suggests a crack in the tank where the valve threads to it. You will probably need to submerge the tank and look for bubbles to find this leak. If you do, report it and ask for another tank for the next dive.
- Check the VIP Sticker and hydro stamp. They are required in the U.S., Canada, Australia and New Zealand—but not in many other popular dive destinations. Foreign operators may still inspect tanks and record the info in a log book instead of on a sticker, so don't panic if you don't see one. Almost all the danger of tank failure is during the fill when, presumably, you'll be somewhere else.
- Check the valve action. If a valve is hard to turn, slips or sticks, it needs service, but as long as it doesn't leak it's nothing to worry about.
- Check the appearance. Does the tank look dinged, scratched, battered and bruised? You would be too if you were handled that way. It's not likely to be a problem unless there's a deep gouge, say one-eighth inch or more. Even then, it's the guy who fills it who's most at risk.
For Sale: Tank Tips for Buyers
If you're in the market for a tank of your own, you must address one basic question. Steel or aluminum: which is better? The answer depends on several factors. Compare, for example, a Faber steel 78 to a Luxfer aluminum "80," which actually holds 77.7 cubic feet of gas.
- Weight. The steel tank weighs less. Although aluminum is lighter than steel, it's also weaker, so you need more of it—the wall is about twice as thick. The steel tank weighs 30 pounds empty, the aluminum tank weighs 31.7 pounds.
- Size. Because it needs less metal, the steel tank is also smaller, almost 3 inches shorter and the same diameter.
- Buoyancy. The steel tank is also less buoyant, less than 1 pound positive when empty compared to 4 pounds for the aluminum tank. That's 3 pounds you can take off your weight belt and still be neutral at the end of your dive.
- Air capacity. We picked tanks of nearly equal capacity, but it's not so simple. For example, the steel tank holds 78 cubic feet only when overfilled by 10 percent, from its rated psi of 2,400 to 2,640. Many steel tanks are stamped with a "+," which is supposed to permit a 10 percent overfill for only the first five years after manufacture. A hydro with a special procedure that renews the "+" is possible, but hard to find. So, technically, your steel 78 will probably shrink to 71 cubic feet in five years. In practice, most fill stations ignore the five-year limit and continue to give the 10 percent overfill anyway. But some may not.
On the other hand, sometimes less is more. When tanks are "hot filled" by whips on a dive boat, all tanks, aluminum or steel, will be filled to the same 3,000 psi. (That's right, dive boats usually overfill steel tanks.) When the tank cools, this will result in about 2,700 psi. So the steel tank will get its full 78 cubic feet, while the aluminum "80" will get less—about 70.
- Longevity. With proper care, both are good for hundreds of thousands of fill/empty cycles. Who's going to live that long? No difference.
- Durability and maintenance. Aluminum tanks have had cracking issues and are susceptible to oxidation, usually minor. But steel tanks can rust, a more serious problem. Aluminum is softer than steel, so more susceptible to external dings and dents. Both require the same inspection protocol: visual inspections annually (at least) and hydros every five years (at least).
- Price. Ah, there's the rub. The steel tank will cost at least twice as much as the aluminum one.
Tank Maintenance Tips
Tank owners, there are only two cardinal rules of tank maintenance:
1) Keep Water Out
It's critical to keep the inside of any tank, especially a steel tank, dry. Both steel and aluminum are subject to internal oxidation, a process that contaminates the air in the tank and weakens the metal.
Aluminum oxidation is only a minor problem, because once a thin layer of oxide forms, it protects the underlying metal from further oxidation. Although the white powdery stuff will not be good for the insides of your regulator, the tank itself is rarely harmed and can usually be cleaned.
Steel oxidation—rust—is more serious because the process can continue deep into the metal. Salt water inside the tank, and storage of the tank under pressure in one position, which concentrates the water and rust in one spot, can even turn a tank into a missile. Lying on its side will be worse than standing on end, because the walls of the tank are thinner than the bottom. A tank charged with nitrox will also be at greater risk because of the greater concentration of oxygen in the mixture.
Water usually gets inside a tank during the fill process. To keep water out:
- Dry the stem of the valve with a towel, and open the valve for a moment before attaching the fill whip. The short blast of air will drive out any water.
- Dry the whip. A good fill operator will let out a short blast of air from both the tank and the whip before putting the two together.
- Avoid the fill tank of doom. The ubiquitous tub of water at the fill station may may not be as good an idea as it sounds. That's because the water tub is a prime source of moisture getting inside scuba tanks. The alleged cooling effect of the water actually has little impact on the fill, and the tub provides little buffer in the unlikely event of a tank explosion. Better to reduce the possibility of explosions by dry-filling.
- Never completely empty a tank, and if you do, close the valve immediately. If a tank is left empty and open, it will pump moist ambient air in and out as it changes temperature.
2) Have Your Tank Routinely Inspected
Visual inspection, required annually, is your best insurance against corrosion, cracking and premature demise of your tank. A good visual will detect minor corrosion and correct it before it gets worse.
A good inspector will check the outside for corrosion, dents, cuts and bulges. He'll check the inside with a bright light for corrosion or contamination. He'll check the threads with a dentist's mirror for cracking and galling. He'll check the valve, the burst disc and the O-rings.
Unfortunately, many inspectors are not trained and don't really know what to look for. How can you tell?
- Look for a PSI certificate. PSI is the only full-service organization training scuba tank inspectors. About half of all tank inspectors are PSI certified.
- Ask lots of questions: "What are you looking for? How can you tell? How did you learn this?" Answers should be clear and confident.
- Leave a sticker on your tank. If the inspector doesn't peel off your "I Love Caymans" sticker to look underneath, he hasn't done a thorough job.
Hydro tests, required every five years, include a visual inspection. Then the tank is pressurized with water, usually to five-thirds the service pressure. The amount the tank expands under pressure is measured, then the amount it contracts again when pressure is relieved is measured. These figures indicate whether metal fatigue has occurred.
What About High-Pressure Cylinders
High-pressure cylinders have become popular recently because they offer more air from the same package, or alternately, the same air from a smaller tank. Aluminum cylinders are available rated at 3,300 psi, as are steel cylinders rated at 3,180 psi—with a 10 percent overfill, that's 3,500. Other steel cylinders are permanently rated at 3,500 psi.
More is better, right? Sounds like a no-brainer, but there's still no free lunch. Because higher pressure requires more metal to contain it, the high-pressure tank is usually as big as the low-pressure one, and heavier.
Compare a Luxfer 3,300 psi "80" to the same company's 3,000 psi "80" (actually 77.4 and 77.7 cubic feet, respectively). The high-pressure tank is the same diameter, and only three-quarters of an inch shorter. The high-pressure tank is heavier, but more negative, so the "real" weight (dry weight plus buoyancy) is about half a pound less. Not exactly breathtaking differences.
In steel there's more advantage to high pressure than in aluminum, because the pressure increase is so much more—about 32 percent. Faber makes a low-pressure 95-cubic-foot tank and a high-pressure 100. The size of the two is about the same: both are the same diameter, and the high-pressure tank is about a half-inch longer. The biggest difference is in buoyancy, which makes the "real" weight of the high-pressure tank about eight-and-a-half pounds less.
Other considerations with high-pressure tanks:
- DIN Valves. High-pressure tanks come with DIN valves, which require an adapter to fit standard first stage regulators. DIN valves also tend to trap water, which is then blown into the tank on the next fill.
- Price. High-pressure tanks cost more.
Fill capacity. Will dive shops be able to fill it to capacity? Most dive boats, for example, can rarely fill past 3,000 psi—a hot fill at that.
Tank Travel Savvy
You called ahead and found out the dive shop in paradise doesn't have your preferred tank size. Should you bring your own?
Probably not. Flying with a tank, while not impossible, isn't worth the hassle. Not only will your tanks have to be emptied, the airline or customs agents may demand that the valves be removed, which will let moisture and contamination enter. Plus you'll probably (rightfully, in this case) be charged extra baggage fees.
Most of us learned to dive on the trusty aluminum 80 and can live with one for a week.
"Visual Plus" Inspection: A Ripoff?
Increasing numbers of dive shops demand evidence of a "visual plus" inspection, which means testing with an electronic device called an "eddy current tester" in addition to the usual visual examination, before they will fill a tank. The device is supposed to be able to detect cracks in aluminum tanks. Naturally, they sell the inspection, at a premium price. Do you really need it?
In a perfect world, probably not. A competent visual inspection alone will detect a crack long before it becomes dangerous, says Bill High, founder of Professional Scuba Inspectors, and the test device is not foolproof anyway. Many tanks have been falsely condemned because, for example, the threads weren't properly cleaned before testing. No doubt, some shop owners are panicky and a few are greedy: some shops are demanding (and charging for) the test on 6061 tanks and steel tanks, neither of which has a history of explosive cracking.
But the world isn't perfect, many visual inspections are incompetent and we're not the ones standing next to those tanks as they are being filled.
The Exploding Tank: Fact or Fiction?
Stories of exploding aluminum tanks have sent chills through the on-line message boards recently. The way it's usually told, an aluminum tank had developed unseen cracks in its neck, and while being filled it exploded, killing, maiming or (fill in the blank). What's the truth?
Fact: Tanks do explode—but not often.
- The usual suspects are aluminum tanks made by Luxfer, Walter Kidde, Norris Industries and Reynolds Aluminum before 1988 from an alloy called "6351." Of these manufacturers, only Luxfer continues to make scuba tanks, and has used a different alloy since then.
According to Department of Transportation records, 12 of these tanks in the United States, 17 worldwide, have exploded since 1986, almost all of them while being filled. For some proportion, consider that approximately 25.4 million tanks were made from the 6351 alloy.
Most of the explosions were due to what is called "sustained load cracking," apparently caused when lead in the 6351 alloy migrates, weakening the metal structure. Cracks begin to appear near the crown of the tank and spread slowly, usually over many years, up through the threads. As "sustained load" indicates, storing tanks under full pressure aggravates the problem.
Actually, small cracks are not that uncommon. Most cracks do not cause explosions and those that do should have been obvious to visual inspection for several years. The explosions that have occurred signal not only a problem with the aluminum alloy, but a more serious problem with the quality of many visual inspections.
In 1988, Luxfer switched to a "6061" alloy, which does not contain lead. (Catalina began making tanks in 1986 and has used 6061 from the beginning.) According to Bill High, founder of Professional Scuba Inspectors, Inc., the industry's only recognized training organization for tank inspectors, there have been almost no cracking problems in post-1988 tanks, nor in steel tanks.
High has had a few reports of "stress corrosion cracking" in Catalina 6061 tanks, "but this is very unusual," he says. None have exploded.
Pre-1988 aluminum tanks can be identified by their neck markings. Luxfer will replace any cracked Luxfer tank that's newer than 10 years old for free. For older tanks, it charges $50. For details, contact Luxfer at (800) 764-0366; fax (909) 781-6598.
Fiction: The Killer Car Trunk.
- While Dave Diver works in his air-conditioned office, his full scuba tank is baking in the trunk of his car parked in the hot sun. His black car. The temperature rises. 1 p.m. 2 p.m. Suddenly, there's a loud "boom" and Toyota debris rains down over the parking lot.
True story? Not a chance. In 90F heat a car trunk might reach 150F. You'd have to heat a full aluminum "80" to about 400F to reach even the hydro test pressure, which is 5,000 psi. And the burst disc (which can sound like an exploding Toyota) should blow out at about 4,500 psi, relieving the pressure.
Fact: We Have Liftoff.
- On the other hand, Bill High swears this is a true story: A tank stored in a house, standing upright, fully charged and rusting from the inside out, burst a two-inch hole through its bottom. The tank shot up through the roof of the house and came down through the roof of a factory a mile away.