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What Is the Difference Between Steel and Aluminum Scuba Tanks?

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colorful tanks

These colorful aluminum tanks have different characteristics from steel tanks.

image copyright istockphoto.com, moens

What is the difference between steel and aluminum scuba tanks? 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 as an increasing number of dive shops offer clients a choice of rental tanks. This article outlines the essential differences between steel and aluminum scuba tanks using four different kinds of tanks as examples:

• Catalina aluminum 80 cubic-foot tanks (Al 80's)
• Sherwood Compact Neutral Al 80's
• Worthington steel low-pressure (LP) 85 cubic-foot tanks (85's)
• Worthington steel high pressure (HP) 80's

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 which 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 or "psi"). 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% overfill rating". This rating allows the tank to be pumped to 10% 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% 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.

• Catalina Al 80 - 31.3 pounds
• Sherwood Compact Neutral Al 80 - 34.4 lbs. These tanks have thicker walls and are therefore heavier than standard Al 80's
• Worthington Steel HP 80 - 28 lbs. These tanks are smaller and may have thinner walls than standard Al 80's
• Worthington Steel LP 85 - 34.3 lbs. These tanks are slightly larger than standard Al 80's, but have thinner walls

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 a 80 cubic-foot aluminum tank rated to 3000 psi because the tank walls are thinner.

High pressure (HP) 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 takes 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.

• Catalina Al 80 - 7.25 inches in diameter by 28.8 inches long
• Sherwood Compact Neutral HP Al 80 - 7.25 inches in diameter by 25.1 inches long
- This tank holds air at 3300 psi and is therefore smaller than as standard Al 80
• Worthington Steel HP 80 - 7.25 inches in diameter by 19.7 inches long
• Worthington Steel LP 85 - 7.25 inches in diameter by 24.7 inches long

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.

A tank's capacity is not always what one might assume. For example, many Al 80's don't hold a full 80 cubic feet of air:

• Luxfer Al 80 - 77.4 cu ft capacity at 3000 psi
• Sherwood Compact Neutral Al 80 - 80 cu ft capacity at 3300 psi
• Worthington HP steel 80 - 80 cu ft capacity at 3442 psi
• Worthington LP steel 85 - 82.9 cu ft capacity if filled to the 10+ pressure of 2640 psi

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.

The exception to this rule is the "Compact Neutral" Al 80, which is designed with thick tank walls and a higher working pressure. The Sherwood Compact Neutral Al 80 is smaller than the average aluminum tank, and remains slightly negatively buoyant when empty.

The following list assumes all tanks are in salt water.
• Catalina Al 80 - 1.6 lbs negative full and 4.1 lbs positive empty
• Sherwood Compact Neutral HP Al 80 - 5.9 lbs negative full and 1.4 lbs negative empty
• Worthington Steel LP 85 - 7.1 lbs negative full and 0.7 lbs negative empty
• Worthington Steel HP 80 - 9 lbs negative full and 3 lbs negative empty

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, and 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:

Steel tanks are usually more expensive than aluminum tanks. Here are some sample prices (including tank valves) I found online.

• Catalina Al 80 - $149
• Sherwood Compact Neutral Al 80 - $244
• Worthington LP Steel 85 - $270
• Worthington HP Steel 80 - $295

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.

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