Steel scuba tanks do not float, they always are negatively buoyant. Aluminum scuba tanks sink when full but can float when empty due to losing the weight of the compressed air. When referring to how and why scuba tanks float, it all relates to buoyancy.

Scuba tanks are one of the essential pieces of equipment for divers. Not only do they provide critical air needed, but scuba tanks can factor into a diver’s buoyancy as well. Since they play such a significant role in diving, it is essential to know how a scuba tank can assist you while you are in the water.

Steel and aluminum tanks perform differently. Factors including the water, diver, and additional accessories also play into the buoyancy a diver may experience. Let’s discuss why scuba tanks sometimes float and the differences in the most common tanks on the market.

Do Scuba Tanks Float?

Standard scuba tanks are made from either aluminum or steel. When a scuba tank is full, some models can weigh over forty pounds. Although scuba tanks are heavy equipment, that does not mean that they are going to sink you straight to the bottom of where you are diving. Most steel tanks have what is called a negative buoyancy. This means they are not as likely to float as an aluminum tank once it is empty. 

Archimedes Principle (source) helps explain why buoyancy plays such a key role in diving and scuba tanks. The principle says that the upward force an item experiences is equal to the liquid that item has displaced. When considering an aluminum tank, the weight of that tank towards the end of your dive is not going to be the same as it was at the beginning of the dive. The tank material is going to have the same buoyancy due to its size. So, when your aluminum tank becomes lighter, there is a stronger force pushing that tank to the surface.

In addition to each tank’s buoyancy, the diving area and the actual size of the diver should be considered when trying to determine which tank to use. For shallow dives, saltwater or for smaller divers, an aluminum tank may be the best choice. Due to the negative buoyancy, higher air capacity and durability of a steel tank, they are often used for longer and deeper dives.

What to Consider When Choosing a Scuba Tank

It would be best if you always chose a tank based on the dive you are doing. If you are doing a sport dive, you will not need a tank that is designed for technical diving. Most tanks are designed for different needs. If you plan on going for a deep dive or a cave dive, you will not need the same capacity as you would for a leisure dive in open water. It is also important to remember that you are much more buoyant in saltwater than freshwater due to the amount of salt dissolved in the water. 

The weight and size of the tank you are using should also be part of what helps you determine what tank you need. Many divers choose tanks based on their dry weight to help them determine the comfort of wearing one or more tanks and how it will work with the equipment they are wearing. You can read about the dry weights of tanks in my post Are Scuba Tanks Heavy?

If you are selecting a tank for a female or a child, larger tanks may be too big for them to use while diving. The diver’s body weight should also be considered when determining what size weights and tank they will need.

Divers use something called a Buoyancy Control Device. This is a piece of equipment that they can inflate or deflate depending on their needs. A BCD helps the diver displace more water, which can help them manage buoyancy while underwater. Divers also use weights to assist this as well. Before going on a device, it is recommended that a diver tests their BCD when their tanks are on.

Excellent video summary of the differences

Differences Between a Steel and Aluminum Tank

When it is time to choose a tank, you will have two options when it comes to the tank’s material. Aluminum and steel are the two most popular options for scuba tanks. Knowing the differences between the tanks is the best way to know which one is for you. While aluminum is one of the most popular options based on its tendency to float, steel tanks are ideal for more experienced divers.

Steel Tanks

Although you may think that steel tanks are going to be heavier than an aluminum tank, steel tanks take less material to do the same work. Steel tanks tend to come at a higher price, but the durability and ability to withstand multiple dives make them worthwhile for most divers. Since the material is more durable, they come in a broader range of pressure options. Something to consider before you purchase or use a steel tank is that they are not as likely to float. If you rely on that buoyancy at the end of your dive, this is not the tank for you.

Although a steel tank can be smaller and more durable than aluminum at the same air capacity, it does have problems of its own. If you are using a steel tank in saltwater, your tank may require more maintenance to keep the corrosion down to a minimum. You can read my prior post on painting a scuba tank, which can help with maintenance. Additionally, your steel tank has to be completely dry when you fill it. Any moisture in your tank will damage the tank and cause it to be unsafe to use. 

Aluminum Tanks

Aluminum tanks tend to be more popular with new divers because they are less expensive than steel tanks. Aluminum is a softer metal compared than steel, which means that you are more likely to damage your tank while you are using it. Due to the soft metal, they also cannot hold the same amount of pressure as a steel tank will be able to hold at the same size.

Aluminum tanks do lighten up during your dive. While that weight may help you at the beginning, you may find yourself fighting the buoyancy of your tank towards the end of your dive. For those looking to have shallow dives or are just in the beginning stages, aluminum tanks offer more benefits, which can help you improve your diving skill. For those who use aluminum tanks, you may need more dive weights.

So, Should You Pick a Tank that Floats?

The tank that you end up selecting is a personal preference. For some, the extra buoyancy an aluminum tank provides near the end of a dive is beneficial, while it is not for others. If you are using a tank that will become lighter towards the end of your dive, have a BCD that is adjusted to the levels you need to maintain a dive and weights to help make sure you do not float to the top of the water are crucial.

Most divers are not selecting an aluminum tank based on the floating aspect that they tend to have. Although they will float as they become empty, aluminum tanks are a more affordable way to get in and explore the water if you are a beginner. If you are looking for a more durable tank, a steel tank is the best option. Steel tanks are ideal for those looking for longer bottom times due to their lower dry weight, and higher air pressure and volume in the same size tank.