What's the Difference Between Steel and Aluminum Scuba Tanks?

Couple Preparing for a Deep Sea SCUBA Dive
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Even if a diver is not interested in purchasing his own scuba tank, it is helpful to understand the difference between steel and aluminum tanks becuase an increasing number of dive shops offer clients a choice of rental tanks. 

Physical Differences Between Aluminum and Steel

Aluminum is softer than steel. Aluminum tanks must have thicker walls than steel tanks to hold air at a comparable pressure. Because aluminum is softer than steel, it scratches and dents more easily.

Steel tanks may rust in the presence of moisture. They are more likely to be damaged by improper fills containing moisture than aluminum tanks and may require periodic tumbling, a process that removes oxidation from the inside of the tank.

What Is the Difference Between Low Pressure and High Pressure Tanks?

Scuba tanks are rated to hold a maximum pressure (given in pounds per a square inch). The higher the pressure, the more compressed the air inside the tank is, and the stronger or thicker the tank walls must be to safely contain the air. A tank filled to 3300 psi contains a higher volume of air (basically more air) than the equally-sized tank filled to 2400 psi.

• Standard Pressure is 3000 psi
• Low Pressure (LP) is 2400-2650 psi
• High Pressure (HP) is 3300 to 3500 psi

LP steel tanks hold a high volume of air at a low pressure. They are generally larger and heavier than HP steel tanks. LP steel tanks are usually given a 10 percent overfill rating.

This rating allows the tank to be pumped to 10 percent more pressure than its official pressure rating. For example, a LP steel tank rated to 2400 psi may be filled to 2640 psi with a 10 percent overfill rating. This rating must be confirmed every time the tank undergoes hydrostatic testing.

Dry Weight of Steel and Aluminum Tanks

Dry weight refers to how much a scuba tank weighs on land, and is an important consideration for divers who plan to hike their tanks a significant distance.

Steel tanks are lighter than aluminum tanks that hold the same volume of air because the tank walls are thinner. Tanks tend to weigh between 25 and 36 pounds, with specialty tanks weighing 40 pounds or more.

Size of Steel vs Aluminum Tanks

Steel tanks have thinner walls than aluminum tanks with an equal pressure rating. An 80-cubic-foot steel tank rated to 3000 psi will be slightly smaller than an 80-cubic-foot aluminum tank rated to 3000 psi because the tank walls are thinner.

High-pressure steel tanks hold air compressed to a higher pressure. Because the more compressed air is, the less space a given volume of air occupies, HP tanks are usually smaller than standard-pressure tanks that hold a comparable volume of air.

Tank size is an important consideration for young or small divers who may find that standard or large tanks bang into their heads or legs underwater. Most standard tanks are 7.25 inches in diameter, but can range between 20 and 30 inches long or more.

Capacity of Steel and Aluminum Tanks

Tank capacity refers to the volume of gas (in cubic feet) a tank can hold at its rated pressure. The higher the tank capacity, the greater the volume of air available to the diver, and the longer the air will last underwater.

Tank capacity is an important consideration for divers who plan on doing deep or long dives, or divers who have a high air consumption and may benefit from the additional air of a high-capacity tank. Conversely, smaller divers with low air consumption or divers who only engage in shallow or short dives may find the capacity of an Al 80 excessive and prefer smaller, lighter tanks with lower capacities.

Buoyancy Characteristics of Steel and Aluminum Tanks

Steel tanks are generally more negatively buoyant than aluminum tanks.

As a diver empties his tank by breathing from it, the tank becomes lighter. One difference between steel and aluminum tanks is that aluminum tanks become positively buoyant (float) as they are emptied while steel tanks only become less negatively buoyant (don't sink so much) as they are emptied.

Whether he dives with a steel or an aluminum tank, a diver must compensate for the increased buoyancy of his tanks near the end of a dive. However, a diver using a steel tank will need significantly less weight than a diver using an aluminum tank, because steel tanks are more negatively buoyant overall.

Durability of Steel vs Aluminum Tanks

When properly cared for, steel tanks generally last longer than aluminum tanks. Steel is a harder metal than aluminum and is less likely to pit or dent, compromising a tank's integrity and rendering it unusable. Unlike aluminum, steel may rust, but with proper care (filling the tank only at reputable fill stations with completely dry air and never completely emptying the tank) most rust can be avoided. Any rust discovered during a visual inspection can be removed by tumbling the tank.

It is not uncommon for aluminum tanks to develop cracks or fractures in the tank neck threads where the valve screws into the tank. These cracks can cause a catastrophic gas loss, and a tank with a cracked thread is unusable. The tank neck threads of aluminum tanks are inspected during the standard visual inspection so this problem is usually caught before it becomes dangerous.

Tank Valves

Aluminum tanks commonly have yoke valves, while steel tanks (especially high-pressure steel tanks) are likely to have DIN valves. Divers should consider which style of tank they are likely to use when investing in a scuba regulator.

Price of Steel vs. Aluminum Tanks

Steel tanks are usually more expensive than aluminum tanks. If price is a major factor, you'll probably want to go for aluminum.

The Take-Home Message

Steel tanks weigh less, are smaller and more durable, and require that a diver use less weight than standard aluminum tanks. However, aluminum tanks are so much cheaper than steel tanks that they have rapidly become the industry standard.