Steel versus aluminum:
Which is better?

In this article, we’ll compare steel and aluminum cylinders in several critical areas. As you will see, steel is almost always the better choice.

Weight

Let’s compare two cylinders of equal size. Both will be brand-new 80-cubic-foot cylinders with a working pressure of 3,000 psi.

By volume, steel is heavier than aluminum. Offsetting this, however, if the fact steel is stronger. Thus, it takes a greater volume of aluminum to contain 3,000 psi. The result is that an Aluminum 80 ends up weighing more than a Steel 80.

Corrosion

Moisture inside an aluminum cylinder creates aluminum oxide. This is dangerous to breathe. When we find aluminum oxide inside a cylinder, there are only two options:

• Condemn the cylinder.
• Attempt to remove the oxide via tumbling.

Tumbling aluminum cylinders requires a special media that will not damage the softer metal. It is not always successful.

Moisture inside a steel cylinder can form ferrous oxide (rust). Like aluminum oxide, this is not something you want to breathe. It is not as potentially dangerous, though.

• No matter how small, rust particles are far less likely to penetrate your lungs.

• Aluminum oxide particles, on the other hand, can easily penetrate your lungs.

When visually inspecting cylinders, the first thing we check for are impurities. If we find rust in a steel cylinder, we have two options.

• We can usually remove a light coating of rust with a wire whip. We then use compressed air to remove all the loose particles.

• If this is not an option, we tumble the cylinder using porcelain chips to remove the rust. This can take anywhere from 3 to 48 hours, depending on the severity of the rust.

Why aren’t these an option with aluminum cylinders? Both wire whipping and tumbling with porcelain chips can damage the softer metal. This is why steel cylinders can last up to 30 or more years. In contrast, aluminum cylinders seldom last this long.

Working Pressure

In our previous example, both the steel and aluminum cylinders had working pressures of 3,000 psi. Is it allowable to fill these cylinders past this?

• In the case of aluminum cylinders, the answer is no. Aluminum is simply too weak to withstand repeated overfilling.

• Steel, on the other hand, is a more robust metal. This is why, when new, you will often see a plus sign (+) next to the hydro date. This means you can fill the cylinder to 110 percent of its rated pressure.

Using the cylinder in our previous example, you can fill the tank to 3.300 psi if a plus sign appears next to the most recent hydro date.

Bear in mind hydrostatic testing involves overfilling cylinders by an additional 66.6 percent (five-thirds of their rated pressure). No cylinder can withstand this indefinitely. Eventually, it will fail to pass hydro. However, as the stronger metal, steel will stand up to repeated hydrostatic tests longer.

Cost

Steel cylinders generally cost up to twice as much as comparable aluminum cylinders.

• Before COVID, prices for a Steel 80 ranged from $300 to $330. Now that COVID has impacted production, prices can range up to $595.

• In contrast, pre-COVID prices for an Aluminum 80 ranged from $120 to $150. Post-COVID prices can range up to $295.

It appears that aluminum cylinders have a clear advantage in terms of price. But this does not account for longevity.

• With proper care, an aluminum cylinder can last years.

• Steel cylinders, on the other hand, can last decades.

When you factor in service life, steel can still be the better value.

Buoyancy

Out of the water, an Aluminum 80 weighs around 34 pounds. A Steel 80 weighs 32.4 pounds. This makes it slightly easier to carry. Underwater, however, the situation is reversed.

• A full Aluminum 80 weighs around 3.6 pounds underwater.

• A full Steel 80 weights 13.2 pounds underwater. This makes it 9.6 pounds heavier.

Bear in mind, as you breathe, both tanks will become more buoyant. An Aluminum 80 can actually be positively buoyant by dive’s end. This is a good thing…right? Not necessarily.

Let’s say a diver needs 20 pounds of ballast to offset his body’s natural buoyancy and that of his equipment.

• If diving an aluminum cylinder, that ballast must be almost entirely lead.

• If diving a steel cylinder, nearly half that ballast will be provided by the tank itself. As a result, the diver will only need to wear half as much lead.

This helps explain the increasing popularity of steel cylinders. Many divers find it more comfortable to have much of their ballast centered on their backs. Steel cylinders also make it easier to maintain horizontal trim.

Remember that air has weight

A cubic foot of air weighs 0.073 pounds. If you multiply this by 80 cubic feet, you get 5.9 pounds. Bear in mind we always want to surface with at least 500 psi remaining. This means that if diving either of the cylinders in our examples and you surface with 500 psi, you will have consumed a little less than five pounds of gas and be five pounds more buoyant.

Assuming cylinders with identical capacities and working pressures, neither steel nor aluminum offers any clear advantage. However, divers who use aluminum cylinders seldom dive ones with capacities greater than 80 cubic feet. Divers who use steel cylinders, on the other hand, often opt for larger sizes. This means they may experience an even greater shift in buoyancy during dives.

The following chart will give you a better idea of the differences in weight and buoyancy between steel and aluminum cylinders of different capacities.