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Tank Definition Tank Choices The Big Lie About Capacity Sport Diving with Air Sport Diving with Nitrox Technical Diving Aluminum versus Steel	Low-Pressure versus High-Pressure Yoke and DIN Valves Dimensions Paints, Coatings, and Finishes Manufacturers Recommendations
If you are interested in purchasing ready-to-dive SCUBA tanks over the internet and having them shipped directly to you please to see our inventory and pricing.

A Rose by Any Other Name

The actual proper name for the big heavy thing we use to go diving is a cylindrical pressure vessel. A tank is the proper term for an unpressurized container like a fuel tank. The people who make pressure vessels for diving, call them compressed gas SCUBA cylinders. However, SCUBA tank or dive tank is the term used by nearly all divers. For the purposes of this article, we'll adopt the term "tank", but you may find elsewhere on our site that we refer to them as cylinders.

Tank Choices

There are several SCUBA tank manufacturers, and the specifications for their tank pressures, dimensions, and capacities are constantly changing. Divers can choose between SCUBA tanks manufactured from aluminum or steel. Tank pressures span a wide range, but the most common pressures are "low" (2400 to 2640 psi), "standard" (3000 psi), and "high" (3300 to 3500 psi). They are available in a huge selection of capacities as the primary dive supply ranging from 50 cubic feet to 150 cubic feet. Tank coatings fall into three general classifications of painted, galvanized, and uncoated. A recent new design, high-pressure 3442 psi steel tanks manufactured under DOT Special Permit (previouly termed an exemption from the 3AA class design), is becoming widely available and causing many experienced divers to re-think their tank selection. This article will help guide divers in the selection of tanks. However, keep in mind that there is no "perfect" tank for all diving conditions and all divers.

The Big Lie About Capacity

Volume is defined by the internal dimensions of the tank, and normally measured in cubic inches of water. Capacity is defined by the amount of breathing gas the scuba tank will hold at its maximum rated service pressure. Determining the actual capacity from the water volume is troublesome, because not everybody does their math quite the same way when converting volume to capacity. In fact, several tanks actually hold a significantly different capacity of gas than the size often used to describe them. What is actually the same tank may appear as two different capacities when listed in two different references. For example, the same Italian import low-pressure steel tank is called a 95 or 98, depending upon the importer. Another example is that a Pressed Steel Tank Co. low-pressure steel 104 has the same water volume that Faber describes as a low-pressure 108. By the way, the manufacturers are allowed some leeway in the specifications for tanks to account for variations in the manufacturing process. Thus any particular tank may be a little smaller or larger than actually stated in their technical specifications.

Selecting a Tank Capacity for Sport Diving with Air

Start with the benchmark of the "Standard Aluminum 80" (AL80) that actually holds about 77 cubic feet of gas at 3000 psi. This is plenty of gas for the experienced air diver, and usually the air no-stop-required time is what limits the dive. The most consistent complaint about the AL80 is its poor buoyancy characteristics. It is very positively buoyant near the end of the dive, which means that you have to start your dive with too much weight, yet sometimes you are still struggling to stay neutral at your safety stop.

If you usually return to the boat with 800 psi or more of gas in an AL80, then you might consider a smaller tank such as the Aluminum 63 (AL63) or high-pressure steel 65 (HP65). Obviously, one advantage of the smaller tank is less weight to carry and haul up the boat ladder, but more important are significantly improved buoyancy characteristics over the AL80. Many women prefer these compact and lightweight tanks for sport diving.

On the other hand, if your gas consumption is sometimes the limiting factor on your bottom time, you might choose a larger tank like the high-pressure steel 100 (HP100). The aluminum 100 is the largest aluminum tank available, but at a weight of 40 pounds, is considered quite heavy. The HP100 is the same weight as aluminum AL80, but has a service pressure of 3442 psi. The HP100 is the most popular steel tank choosen by sport divers.

An excellent compromise solution, if 80 cubic feet of gas is enough, but you don't want to deal with the additional weight and buoyancy hassles of the larger tanks, is the low-pressure steel 80 (LP80) and the high-pressure steel 80 (HP80). These tanks have a little more volume (remember the AL80 volume is only 77 cubic feet) and the much better buoyancy characteristics of steel, yet are the same weight or even slightly less weight than the AL80. The LP80 service pressure of 2640 is easy on equipment, and the HP80 is exceptionally compact. The low-pressure 85 (LP85) represents a option for those divers who need a bit more gas but balk at strapping on a really big tank.

Selecting a Tank Capacity for Sport Diving with Nitrox

Diving on Nitrox does change our recommendations. Nitrox divers are more often limited by the volume of their tank, because they tend to stay down longer Even if they are using the Nitrox advantage for safety with an air table, Nitrox divers usually push the air limits right to the edge because they know they have the advantage of the additional time margin granted by the Nitrox. Again, the HP100 is very popular among sport Nitrox divers. . Nitrox divers using the Nitrox advantage for time stay down a LOT longer, and their tank of choice is usually one of the very large tanks. They might choose the high-pressure steel 119 (HP119), high-pressure steel 120 (HP120) or sometimes even the high-pressure steel 130 (HP130).

Selecting a Tank Capacity for Technical Diving

More is not always better, and you should match your tank volume to your gas requirements. If you are not going too deep or staying too long, use an H-valve with a single HP130. Staying with a single tank streamlines your profile and reduces buoyancy compensation resulting in improved gas consumption. If you need more gas, then choose double HP100s, or double HP119s. Cave divers like LP104s and HP130s, but for many divers, they are unnecessarily heavy and cumbersome for technical diving done from boats. We've seen even larger doubles configurations, but divers using them typically need assistance to get in and out of the water. Some divers choose double AL80s, but these become so buoyant near the end of the dive that they work best with a 7-pound trim weight.

For deco tanks the aluminum 19s (AL19), aluminum 30s (AL30) and aluminum 40s (AL40) are most popular. Contrary to popular opinion, when the weight of a valve and regulator is considered, these cylinders are NOT positively buoyant when near empty. They do lay in the water a bit strange, with the valve down and the base floating up, but because of the way they are usually rigged; most divers actually take advantage of that behavior. The AL19 has become popular as an oxygen deco tank, because it's compact and 19 cu ft of oxygen is plenty for all but the most extreme dives. The AL19 also makes an excellent choice as a bailout for rebreather divers or a pony for sport divers. For stops in the 70 to 30 foot range on rich Nitrox mixtures, the AL40 has become the cylinder of choice. Some tech divers prefer the low-pressure steel 45s (LP45), but a pair of these is relatively heavy and only makes sense if you are doing longer decompression stops. For deep stops in the 200 to 100 foot range, or if you are doing decos so long that you want to take a nap on your stops, the deco tank of choice seems to be the AL72.

Aluminum Tanks versus Steel Tanks

Both steel and aluminum corrode in the presence of sea water. Steel tanks produce iron oxide, and aluminum tanks produce aluminum oxide. Regardless of the metal, the corrosion can occur rapidly in the right conditions and can significantly shorten tank life. With properly cared for tanks, corrosion is not an issue no matter what type of metal. However, for tanks that have not received proper care, corrosion is a major concern. Internal corrosion and line corrosion around the boot are the most common reasons both steel and aluminum tanks fail inspection.

Because steel is stronger and more ductile than aluminum, construction of a tank requires less steel than aluminum. This usually means that for an equal gas capacity, a steel tank will have a total weight less than an aluminum tank yet have better buoyancy characteristics. For example, the standard aluminum 80-cubic-foot tank weighs about 35 pounds, while similar capacity steel tanks weigh in at about 30 pounds.

There have been lengthy discussions about the service life of steel tanks vs. aluminum tanks. The service life of a properly cared for steel tank is widely considered to be at least 40 years. The service life of a properly cared for aluminum tank is more controversial, but some dive shops no longer fill an aluminum tank over 15 years old.

Aluminum tanks are particularly susceptible to damage from heat. During the manufacturing process, the aluminum tank is strengthened using a special heating and cooling process. If an aluminum tank is later heated beyond 350°F, the tank walls lose their strength and the tank fails at normal service pressures. There are some well-publicized failures of aluminum tanks that had been previously repainted with heat-cured paint.

In general, most experienced divers strongly prefer the buoyancy characteristics of steel tanks to those of aluminum. Aluminum tanks tend to shift from negative to positive buoyancy as the diver consumes the contents. This requires the diver to carry additional weight at the start of the dive in order to be properly weighted at the end of the dive. Steel tanks shift from negative to neutral buoyancy, and this allows the diver to begin the dive with less weight and still be properly weighted at the end of the dive. There is a line of aluminum "compact" high pressure tanks that offers somewhat improved buoyancy characteristics when compared to standard aluminum tanks. However, some divers feel the uneven distribution of weight in the compact aluminum tanks causes them to be bottom-heavy in the water.

The final consideration of aluminum vs. steel is the price. The initial purchase price of aluminum tanks is significantly less than comparably sized steel tanks. However, many people feel that when the costs are amortized over the life of the tank, the true costs are roughly equal.

Low-Pressure Tanks versus High-Pressure Tanks

The most common service pressures seen in modern SCUBA tanks are 2640 psi, 3000 psi, 3300 psi, 3442 psi, and 3500 psi. Most aluminum SCUBA tanks are rated for 3000 psi. The compact aluminum tanks and the standard AL100 tank have a service pressure of 3300 psi. The 2640 psi low-pressure steel tanks are preferred by Nitrox and technical divers who expect to be blending their gases using the partial-pressure method. The 3442 and 3500 psi high-pressure steel tanks offer the advantage of more compact size when compared to similar volume low-pressure steel tanks and even some aluminum tanks.

The lowest service pressure is best from the point of view of equipment. The higher the pressure, the more stress on regulator and valve components, such as O-rings and seats, increasing the likelihood of a failure. At 3442 and 3500 psi, the high-pressure seat of the regulator first stage is often the first component to fail. Some evidence suggests that a tank interior contaminated with sea water will corrode significantly faster at higher pressures. Our hydrostatic test facility operator has told us that the high-pressure steel SCUBA tanks seem to be the most common type of tank to fail hydro, while the low-pressure steel SCUBA tanks seem to have very low failure rates.

Some types of steel tanks can be qualified for a 10% overfill with each hydrostatic test. This is indicated by a "+" mark following the hydrostatic test stamp. For example, a 2400 psi tank with a "+" has a service pressure of 2640 psi. An extra step in the hydro test process must be performed in order to qualify the tank for an overfill "+" mark. It's always performed on the first factory hydrostatic test, but often overlooked on subsequent tests in the field. Technically, its absence means that it no longer qualifies for overfilling, but in reality its absence almost always means the additional step was never performed. Regardless, the absence of the "+" mark on subsequent hydro stamps is often ignored by dive shop fill station operators who fill to the higher service pressure anyway.

The practice of overfilling occurs primarily in the technical diving community but is occasionally seen in the Nitrox sport-diving community as well. Although the dangers and risks of overfilling is a separate discussion, modern chrome-moly alloy low-pressure 2640-psi steel tanks are sometimes overfilled to 3000 psi and as much as 3500 psi in order to gain very significant increases in volume. Overfilling the high-pressure tanks is uncommon because few fill stations can deliver pressures much beyond 3500 psi, and the relative volume benefit is significantly diminished. Regardless of the practice of overfilling steel tanks, almost no one advocates overfilling aluminum tanks. Irrespective of the risks, overfilling is also a logistics hassle because many fill stations will not overfill, and it requires changing the burst disks in the valve.

In our experience, many dive shops consistently under fill certain high-pressure tanks. Fill station operators will often assume all aluminum tanks are filled to 3000 psi, causing the 3300 psi service pressure aluminums to receive short fills. We've also noticed that the very high capacity tanks like the 3442 psi HP130 are also sometimes under filled when they are near empty because they heat up so much due to the large amount of gas being delivered.

One idea we like is selecting a large-capacity high-pressure tank and then routinely deliberately under fill it to 2700 - 3000 psi. For example the 3442 psi HP100 yields 89 cubic feet at 3000 psi, and the 3442 psi HP119 yields 107 cubic feet at 3000 psi. That is plenty of gas for most dives, puts an end to short fills, and the lower pressure is easy on equipment, yet allows the option of a high-pressure fill when you need really need more gas. This is how divers have for years been using overfilling with low-pressure tanks, but with the advent of the large-capacity high-pressure tanks, it eliminates the concerns regarding overfills.

Yoke and DIN Valves

Although a discussion of the merits of yoke versus DIN valve and regulator fittings is outside the scope of this article, some issues need to be addressed. In the past, the most common type of valve found on aluminum and low-pressure steel scuba tanks was the classic yoke K-valve. More recently, many divers have been using a 200-Bar DIN valve that fits aluminum and low-pressure steel tanks. This valve also accepts a small screw-in adapter that allows the DIN valve to be converted for use with yoke-type regulators. All high-pressure 3500 psi steel tanks had a smaller neck opening that accepted only a 300-Bar DIN valve, for which no yoke adapter existed and thus required a 300-Bar DIN fitting regulator.

In 2003,a new a high-pressure (3442 psi) tank design having weight and buoyancy characteristics of low-pressure tank designs has been introduced including the same neck threads as aluminum and low-pressure tanks. The tank is normally offered with a 200-Bar DIN valve and yoke adapter installed. However, we have seen some yoke regulators with the yoke assembly marked for 3000 psi max service (or unmarked), and we do not recommend using those yoke regulators on the high-pressure tanks.

Tank Dimensions

For most people, the standard AL80 at 26 inches in length is a bit too long to be comfortable. The steel HP100 at 24 inches in length seems about right for the average diver. Even the larger-volume HP119's are very manageable at 24 inches. The AL63 and the HP80 at 21 inches are a nice length for divers of smaller stature. For children and smaller women the HP65 at 17-inches is a good choice. The really large-volume tanks like the LP120, HP120 and the HP130 that range from 28 to 30 inches in length are a bit unwieldy for most divers, but a viable choice for tall divers over six feet in height. With longer tanks, you must take care when you set your BC harness to make sure it's not too low on the body of the tank. Otherwise, you may hit the back of your head on the regulator first stage when you enter the water.

Height	Tanks
17 Inches	HP65
20-21 Inches	AL63, LP66, HP80
24 Inches	LP80, LP95, HP100, HP119
26 Inches	AL80, AL100, LP85, LP104
28-30 Inches	LP120, HP120, HP130

Tank diameters are a more significant consideration. Most tanks, aluminum or steel, are the common diameter of 7-¼ inches. The AL100 and the large-capacity low-pressure tanks, such as the LP95, are 8 inches in diameter. Some of the larger-capacity high-pressure tanks, such as the HP119 and HP130, are also 8 inches in diameter, although the HP120 is a 7-¼ inch tank. Your BC will fit the 8-inch diameter tanks just fine, but if you have to switch it back and forth between two different tank diameters, constantly readjusting the BC tank band(s) can be annoying. Also, some dive boats have tank storage racks that don't accommodate the 8-inch diameter tanks very well. However, the 8-inch tanks are rapidly increasing in popularity, and boats are beginning to make accommodations for them.

Diameter	Tanks
7-¼ Inches	AL63, AL80, LP80, LP85, HP80, HP100, HP120
8 Inches	LP95, AL100, LP104, HP119, LP120, HP130

Tank Paints, Coatings, and Finishes

Standard aluminum tanks are available in a very wide range of colors. Steel tanks and the smaller aluminum tanks (i.e., less than 50 cu. ft. in volume) are often available only in a very limited selection of colors. At any given time, the availability of specific colors on specific sizes is very erratic due to relatively small production runs.

With aluminum tanks, it seems that most coatings do more to encourage corrosion than to prevent it. Paint on aluminum tanks just does not seem durable and does not adhere well, often peeling and flaking. Once water makes its way under the paint, corrosion is actually encouraged. The opaque paint colors hide any corrosion that might be occurring underneath the paint. Aluminum tank manufacturers have attempted to address this by offering clear-coat and translucent colors that allow the corrosion to be visible, but our experience has been these finishes seem to really promote even faster corrosion. Ironicly, the most popular paint color for tanks is "Yellow", yet this seems to be one of the least durable paints, beginning to peel and flake with even limited exposure to saltwater. The aluminum finishes of choice have become brushed aluminum or what is termed "shot blast," which is an unpainted bare metal surface that has been textured and is a uniform dark gray. Recently a "natural" finish has become available. The natural finish is just the appearance of the cylinder immediately after heat treatment when it forms an aluminum oxide coating. The natural streaked gray oxide coating is ugly, but a few dollars lower in cost. All, the unpainted finishes hold up very nicely and do not encourage or hide corrosion.

Uncoated steel tanks quickly develop a coating of rust in the presence of water, so all steel tanks are coated. The most durable coating for steel tanks is hot-dip galvanizing. When a steel tank is galvanized, a zinc metal coating is bonded to the surface of the steel. The hot-dipped galvanized coating comes from the factory as a shiny silver color that rapidly ages to a mottled gray once exposed to sea water. Some experts believe that environmental concerns about the zinc galvanizing operation itself outweigh the benefits of hot-dipped galvanization. Steel tanks are also available in a durable and environmentally friendly cold-galvanized coating that is further protected with an epoxy paint that seems to adhere very well.

We have observed debates over the corrosion resistance and durability of hot-dipped galvanized steel versus cold-galvanized painted steel tanks. In fact, we have seen steel tanks of every type and manufacture in excellent condition after many years of heavy service and we have also seen them fail the first annual inspection. The truth is both types of steel tanks will give many years of corrosion free service when they receive proper care. All tanks must be thoroughly rinsed in fresh water after use in salt water and during storage the boot should be removed. Both unpainted and painted steel tanks require some additional care in comparison to unpainted aluminum. Any deep gouges or scratches that cut through the galvanizing and expose bare metal need to be touched up with either a rust-inhibiting paint or a cold-galvanizing compound, both available in aerosol sprays and brush applicators at hardware stores.

to see the different aluminum tank finishes.

Tank Manufacturers

Just a handful of companies currently produce SCUBA tanks for the U.S. market. Several dive equipment distributors also market these same tanks under various brand names. All of the tank manufacturers produce quality products, but even similar-capacity tanks from various manufacturers often differ in their weight, dimensions, buoyancy, and finishes. Some sizes and service pressure combinations are available only from a specific manufacturer.

Luxfer Gas Cylinders and Catalina manufacture aluminum tanks in the U.S. Luxfer is by far the largest manufacturer of aluminum tanks in the world. Catalina offers a traditional line of aluminum tanks and a line of compact aluminum tanks that have better buoyancy characteristics when compared to traditional aluminum tanks, but require a higher 3300-psi service pressure. Both Luxfer and Catalina offer their tanks in a wide array of paint colors as well as clear, brushed aluminum, and shot blast finishes. Luxfer has recently introduced the natural finish.

Pressed Steel Tank Co manufactured hot-dip galvanized steel tanks in the U.S. for many years. PST was the first company to introduce 3442 psi high-pressure tanks that have buoyancy/trim characteristics similar to low-pressure steel tanks. In 2006, PST ceased operations, was sold, and the new owners have announced plans to resume operations in the future. Although PST cylinders are in wide use, on-going availability of PST tanks is currently in question.

Worthington Cylinders, headquartered in the U.S., has recently introduced both low-pressure and high-pressure steel SCUBA tanks manufactured at their plant in Canada. When first introduced the Worthington cylinders were cold-galvanized and then coated in white epoxy paint. In the Fall of 2005, Worthington began shipping hot-dip galvanized cylinders including 3442 psi high-pressure versions. Worthington also manufactures a line of small 'pony' size high-pressure 3AA class steel tanks (3130 psi with + 10% overfill = 3443 psi) that are popular for use as deco and rebreather bottles.

Norris Cylinders is also producing hot-dip galvanized steel SCUBA tanks manufactured in the U.S. Norris does not provide much information about their tanks and we have seen only seen 100 cubic foot high pressure steel tanks that seem relatively heavy.

Faber of Italy manufactures and exports steel SCUBA tanks for the U.S. market. The Faber tanks are cold-galvanized and then coated in white epoxy paint. Faber manufactures a line of high-pressure 3AA class primary steel tanks (3180 psi with + 10% overfill = 3500 psi) that are not well received because of their relatively heavy weight. Faber has also introduced 3442 psi cylinders in a range of sizes including very large capacities up to 149 cubic feet. Recently, Faber has established a new U.S. distributor and the tanks are sold in the U.S. under the name "Blue Steel".

ASHI of Japan also manufactures and exports steel SCUBA tanks for the U.S. market. The ASHI tanks are cold-galvanized and then coated in silver epoxy paint that at first glance has a similar appearance to hot-dip galvanizing. Although many assume ASHI tanks are hot-dip galvanized, they are in-fact cold-galvanized and painted. The ASHI tanks are also relatively heavy weight.

Recommendations

When purchasing tanks, the long-term advantages of steel's excellent buoyancy characteristics and long life make it the best choice for most divers. Choose a high-pressure steel tank size that meets your needs when it is under filled, putting an end to short fill concerns. If your budget is tight, then aluminum initially costs significantly less. If going with aluminum, avoid paints, and choose the shot blast, natural or brushed finish. The standard aluminum 80 is not a "one-size-fits-all" tank. Making the right tank choice can significantly improve your diving enjoyment.

If you are interested in purchasing ready-to-dive SCUBA tanks over the internet
and having them shipped directly to you
please to see our inventory and pricing.

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