When you first start diving, you're likely to find the first cylinder you encounter is the AL80 aluminum cylinder, commonly known as an 11-liter aluminum cylinder. Nearly every dive shop and liveaboard in Southeast Asia uses this type of cylinder. However, when diving first became popular in the last century, most divers used steel cylinders. It wasn't until the cost of aluminum cylinders decreased that aluminum became the preferred choice. This article will use the AL80 aluminum cylinder, HP80 steel cylinder, and LP85 steel cylinder as examples to briefly explain the advantages and disadvantages of aluminum and steel cylinders, and offer some preliminary recommendations for choosing the right cylinder.
01 Cylinder Specifications
The numbers in the names AL80, HP80, and LP85 refer to the total volume of the uncompressed gas in the cylinder at operating pressure, expressed in cubic feet (cubic feet). HP stands for high pressure, with an operating pressure of 237 bar; LP stands for low pressure, with an operating pressure of 182 bar; and AL stands for aluminum. First, I've compiled a comparison of the parameters of these three cylinders. In the remainder of this article, I'll use this table to explain the advantages and disadvantages of each cylinder in different situations.
02 Cylinder Characteristics - Buoyancy
During tropical diving, due to the high water temperature, divers typically wear only 3mm wetsuits or even just jellyfish suits. Aluminum cylinders are therefore more suitable. Firstly, they are available at all dive shops; secondly, they are slightly positively buoyant when nearly empty, eliminating the need to carry excessive weight to maintain neutral buoyancy at a depth of 3-5 meters at the end of the dive. Choosing a cylinder with excessive negative buoyancy may overload the diver, requiring multiple inflates in the BCD to maintain neutral buoyancy. Using a cylinder with excessive negative buoyancy can make it more difficult for the diver to swim to the surface if the BCD fails when the cylinder is nearly full. Furthermore, excessive air in the BCD makes maintaining trim even more difficult. For more information on how BCD gas affects trim, please click here.
For cold water diving, using steel cylinders is more appropriate. Cold water diving requires divers to wear thicker wetsuits or drysuits, which inherently provide greater positive buoyancy. Using aluminum cylinders to increase this positive buoyancy would require more weight to achieve a normal descent and maintain neutral buoyancy at depths of 3-5 meters when the tank is nearly empty. Wearing a drysuit and a 400g undersuit, a diver would likely need 13-15kg of weight. This amount of weight is difficult to distribute evenly across the diver's body, and uneven weight distribution makes maintaining trim more difficult.
Large amounts of weight cannot be evenly distributed, and the result is that most of the weight is likely to be placed on the weight belt. If an unexpected underwater incident occurs and the weight belt is lost, the diver will be unable to recover and would rush to the surface, potentially exposing them to decompression sickness.
Therefore, using steel cylinders is more sensible for cold water diving. From the table, we can see that when empty, the AL80 has a buoyancy of +1.9 kg, while the HP80 has a buoyancy of -0.8 kg. If we switch from the AL80 to the HP80, while keeping all other equipment the same, we can save 0.8 kg + 1.9 kg = 2.7 kg of weight. Furthermore, this 2.7 kg of weight does not occupy any trim weight pockets, is not lost, and is evenly distributed.
LP85 steel cylinders are a very popular sidemount tank choice in North America because they have similar characteristics to aluminum cylinders: negative buoyancy when full and positive buoyancy when empty. However, the LP85 has less positive buoyancy, allowing for a smaller weight reduction.
03 Tank Characteristics - Length and Weight
The HP80 is 12.4 cm shorter than the AL80, making it more suitable for divers of shorter stature. Using an overly long cylinder can be uncomfortable, with the cylinder valve either pressing against the back of the head or the bottom of the cylinder extending past the hips. Overly long tanks also shift a diver's center of gravity downward, making it more difficult to maintain trim. Sitting in a boat with a BCD while waiting to enter the water can be uncomfortable because the tank on your back is pressing against you.
The HP80 is also more suitable for female divers because it weighs 1.3 kg less than the AL80. Adding the 2.7 kg of weight that can be removed compared to the AL80, the total weight carried by a diver is reduced by 4 kg, while the total amount of available air remains the same.
The LP85 and AL80 are very similar in size, with roughly the same length and weight, but they can hold 5 more cubic feet of uncompressed air. Because of the operating pressure of the LP85, many cave diving shops in Florida will inflate it above this operating pressure to carry more air (cave fill). If filled to 230 bar, it can hold approximately 107 cubic feet of air, far exceeding 80 cubic feet.
04 Summary
From the above analysis, we can conclude that aluminum tanks are better for tropical diving, while steel tanks are more suitable for cold water diving. For the same total gas storage, carrying a steel cylinder weighs less than carrying an aluminum cylinder. Given the same total weight, a steel cylinder can carry more gas. Steel cylinders can also replace some of the ballast weights, distributing the gas more evenly than traditional ballast weights. Steel cylinders come in a variety of sizes, making them more suitable for people of all sizes. This article only uses the HP80 and LP85 as examples. In reality, there are many other sizes, and I will later write a dedicated gas cylinder buying guide to explain in detail each suitable situation. Of course, steel cylinders do have some disadvantages. First, most steel cylinders have rounded bottoms, making them difficult to store upright (some have concave bottoms, which allow them to stand upright). To store them upright, they require a tank boot, which is not good for the cylinder. Second, steel cylinders are generally more expensive, with the HP80 and LP85 costing roughly twice as much as the AL80. Of course, you get what you pay for. With proper care, a steel cylinder can last up to 50 or 60 years, while aluminum cylinders typically only last about 20.
