Scuba tanks are so critical to divers’ ability to stay underwater. They not only provide the air necessary to breathe underwater, but scuba tanks also affect one’s buoyancy. Since you literally cannot scuba dive without a tank, it’s important that you know more about how scuba cylinders affect you underwater.
The two most popular types of scuba tanks, steel and aluminum, float differently underwater. Steel scuba tanks are always negatively buoyant, meaning they will always sink whether they are full or empty. Aluminum tanks are negatively buoyant when full, but eventually becomes positively buoyant as it empties. In other words, aluminum tanks sometimes float but steel tanks never float.
Whether you float or sink depends on your buoyancy. In addition to your scuba tank, the rest of your scuba gear as well as your own body weight affects your buoyancy. In this article, we will go over why aluminum tanks sometimes float and discuss the differences between the two most common tank types on the market.
Scuba Tank Buoyancy
Most scuba tanks are made from either steel or aluminum. Now, there are even carbon fiber tanks that are exceptionally light, but whether it will catch on remains to be seen. Depending on the size of the tank, some steel and aluminum tanks can weigh over 40 pounds when full. It might sound alarming that divers are willing to strap something that heavy onto themselves when diving because that sounds like the fastest route to a watery grave.
Despite how heavy scuba cylinders are, they are not going to plunge you straight to the depths. To understand why this is the case, you need to learn about buoyancy. The basic principle is that the ocean will naturally exert an upwards force equal to the amount of water that you displace. Also factor in the positive buoyancy of the air stored in your lungs as well as in your neoprene equipment, and you’ll realize that there is a very high chance you’ll float.
You can either be positively buoyant, negatively buoyant, or neutrally buoyant. Positive buoyancy means you are constantly floating upwards. Negative buoyancy means you are constantly sinking. Neutral buoyancy refers to when you are floating in place, neither ascending nor descending. The best way to achieve neutral buoyancy is with a buoyancy control device (BCD).
In addition to a tank’s buoyancy, divers should consider where they will be diving as well as their size to determine the tank they should use. For leisurely, shallow dives, an aluminum tank may be the optimal choice. For deeper dives where more air capacity is needed, the negative buoyancy and larger size of steel tanks make them the optimal choice.
Do Steel Scuba Tanks Float?
You probably already know that steel is heavier than aluminum. Ironically, steel tanks are lighter than aluminum tanks. The reason is that because steel is the stronger material, less of it is needed to do the same work as an aluminum tank. Unfortunately, this extra durability, lower weight, and wider range of pressure options comes at a higher price. However they are worth the investment for most divers.
When you purchase a steel tank, or any tank for that matter, one of your considerations should NOT be whether it can float. In fact, steel tanks do NOT float. When full, they are negatively buoyant, meaning they will sink in water. When empty, they are still negatively buoyant, just less so. If you want a tank that provides some positive buoyancy at the end of the dive, then steel tanks are out of the picture.
Although steel tanks seem to be the superior option when it comes to durability and weight, it has some issues of its own. For example, using a steel tank in saltwater exposes it to rust and corrosion if it is not properly maintained. Furthermore, steel tanks must be completely dry when filling it. Any moisture inside can corrode it and cause it to be unsafe to use.
Do Aluminum Scuba Tanks Float?
Aluminum tanks are the most popular tank option because of how much more affordable they are compared to steel tanks. Aluminum is softer than steel and therefore more susceptible to damage. In order to match the same air capacity as a steel tank, aluminum tanks have to be much thicker and therefore heavier.
If you are looking for a tank that floats, aluminum tanks do float when they are almost empty. Therefore, as you dive you will notice the tank getting lighter until eventually it is helping you stay afloat. Some extra dive weights will be needed to counteract the buoyancy it provides when almost empty.
Check out this video to see how an empty aluminum tank floats compared to a full one:
One clear advantage that an aluminum tank has over a steel tank is its natural resistance to corrosion. When aluminum is exposed to air, it oxidizes which further helps it to prevent rust and corrosion. Less maintenance is required to keep an aluminum tank in top condition, so it is much less of a hassle to own.
Divers who plan on doing shallow dives, don’t want to spend that much money, and don’t want to deal with as much maintenance should invest in an aluminum tank.
Scuba Tank Considerations
Before purchasing a tank, you should figure out what kind of dives you do the most and choose a tank that is optimal for that. For instance, you mostly perform shallow dives, then you do not need a heavy-duty tank designed for technical or deep diving. That is why there are so many tank sizes and fill pressures to choose from. The most basic tanks are 80 cf aluminum and steel tanks, which can provide approximately 45-60 minutes of breathing time. It also doesn’t cost very much to fill up.
The size and weight of a tank determines what kind of diving it should be used for. Larger tanks provide more air capacity and are obviously heavier, but when it comes to how much a tank sinks or floats, the biggest factor is whether it is a steel or aluminum tank. If you want to learn more about how much scuba tanks weigh, then read our article on this topic here.
It’s also important to consider whether you will be diving in saltwater or freshwater. Saltwater is more positively buoyant than freshwater thanks to its high salt content. You can expect to have an extra 3 lbs of upwards force keeping you afloat, which means you don’t need as much dive weights.
When purchasing a tank for a child, female, or small male, a larger tank may be a detriment when diving due to how difficult it would be for a smaller individual to handle. Thus, a diver’s body weight should also be factored into the size and weight of the tank when purchasing one.
Regardless of the type, size, and weight of the scuba cylinder, you will always be able to float by using a buoyancy control device (BCD). A BCD is connected to your tank and will inflate or deflate based on your needs. BCDs help divers displace more water and manage their buoyancy. Dive weights are often used to assist with buoyancy control as well. During your pre-dive check, make sure that the BCD functions properly before heading into the water.
Do You Need a Tank that Floats?
Which tank you end up getting depends on your diving needs and preferences. For beginners, the extra buoyancy that an aluminum tank provides near the end of the dive is a godsend. For others, it is annoying that they need to bring more dive weights to counteract this. You will need to use a BCD to adjust your buoyancy to ensure that neutral buoyancy is maintained.
Most people who purchase an aluminum tank aren’t really thinking of any other factor except price. Unfortunately, scuba gear can be prohibitively expensive and an 80 cf aluminum tank is the cheapest option available besides renting. However, aluminum tanks are a perfectly suitable alternative to steel tanks.
With that said, steel tanks are arguably the best option if you have the funds for one. They provide longer bottom times thanks to their lower dry weight, as well as more air pressure and volume than a similarly sized aluminum tank. Steel tanks don’t float, but that doesn’t matter since you will be managing your buoyancy with a BCD.
You can check out our review of the best scuba tanks to learn more about which ones you should be getting.