How Do Scuba Divers Sink and Float? Buoyancy Control

Last Updated: July 18, 2023

The deep ocean is a beautiful place with coral reefs, aqua-plants, aquamarine life. Plus the thrill of scuba diving is unmatchable.

If you are curious about scuba diving, you might have wondered how are scuba divers able to float and sink.

By reading this guide you get know how scuba divers go up and down and what buoyancy is and how to use it while scuba diving.

So, dive into the underwater depths of sinking and floating.

How Do Scuba Divers Descend and Ascend?

how scuba divers go up and down

While scuba diving, it is necessary to descend and ascend properly. You need to use the combination of your fins and your lungs to help you.

When you begin descending, don’t empty air out of your Buoyancy Control Device. Ensure you breathe deeply and establish a negative buoyancy while descending. When you are ready to come up, ensure your BCD is partially inflated so you can float vertically.

For a successful ascent and descent, you must bear the following tips in mind:

  • Ensure you aren’t going up or down too fast. You’ll be at risk of decompression sickness.
  • Exhale completely while going down and inhale deeply while coming up.
  • Hold on tight to a dive rope that will guide you downwards or upwards in water. It will control your speed and allow you to adjust your buoyancy comfortably when required.
  • Your feet need to point downwards so that you can stay upright while ascending.
  • Equalize to ensure there’s no air trapped in your ears.

What is Buoyancy?

Buoyancy is your ability to float. Essentially, when a diver enters the water, they push aside the water to create space for themselves.

Buoyancy works on the principle of water getting displaced, and when it does, it starts trying to fill in the space the diver occupies, so this pressure starts to move the diver upwards. This is called buoyant force. In contrast, gravity will make the diver go downwards.

There are some common terminologies related to buoyancy that you should be aware of. It affects how scuba divers are going up and down:

  • Positive Buoyancy – used to ascend or when you want to float upwards.
  • Negative Buoyancy – used to descend or when you want to sink downwards.
  • Neutral Buoyancy – used when you neither want to sink nor float. You maintain a single depth and remain suspended in water. It helps you control your speed.

How Do Scuba Divers Adjust Buoyancy?

Now let’s see how scuba divers adjust buoyancy so they can float and sink.

Divers will use a device called Buoyancy Control Device. The device is a bladder or a wrap-around jacket that connects to a Low Pressure Inflator (LPI). The LPI connects with your tank.

Your inflator hose will have two buttons, namely the deflate and inflate buttons. You use these buttons to adjust the buoyancy.

It’s important to note that the amount of air in your BCD is what controls whether you go up or down.

Let’s see how you adjust your buoyancy according to the various stages of your dive:

  • Before you start your dive, you need to float. So, you need to be positively buoyant. In this case, you will inflate your BCD. When you inflate, you fill the BCD with air. Ensure you don’t overfill it.
  • When you are ready to dive, ensure you are negatively buoyant so that you can sink. So, you press the deflate button on your BCD. As you exhale and the air pumps out of the BCD, you will slowly start to sink. Ensure you are in a vertical position while going down
  • Well-before you hit the rock-bottom of the ocean, ensure you reach neutral buoyancy. You achieve this by adding a little amount of air into your BCD. You need to ensure you don’t fill too much, or you’ll start floating. So, add a wee bit of air, then wait for few minutes.
  • You can cruise underwater with neutral buoyancy comfortably. You’ll save energy, oxygen, and you’ll have a longer and more fun dive session.

Learn to control the BCD correctly for a long and fun dive. Ensure you follow the advice of your dive instructor.

Factors That Affect Your Buoyancy

Some factors affect your buoyancy. This will help you understand how does a scuba diver float and sink.


You use the BCD to adjust your buoyancy, as mentioned above. Your BCD can change the amount of water getting displaced through the inflation and deflation button. Inflation causes you to displace more water and float, while deflation will displace less water and help you sink.


Generally, your scuba diving gear with the BCD, tank, and more will make you positively buoyant even if there is no air in your BCD. So, you use some lead weights to overcome this issue. Plus, the weights help you descend and stay underwater easily.

Your Suit

Your suit is your exposure protection. You wear either a wetsuit or a drysuit, both of which are positively buoyant. Wetsuits consist of air bubbles trapped within the neoprene. Your drysuit gives you an insulating layer of air.

Moreover, if your drysuit or wetsuit is thicker, it will have more air trapped in it, making you even more positively buoyant.

Additional Dive Gear

Any additional gear you take with you will add to your overall buoyancy. For example, if you use heavy fins or regulators, you will be negatively buoyant. So, you’ll need fewer lead weights to balance it out.

It’s crucial to test your buoyancy when you change any piece of equipment. It determines your ease in sinking and floating while diving.

Tank Pressure

The tank you carry also has weight. While the volume and weight of the tank remain constant, the air inside the tank doesn’t. The tank becomes progressively lighter as you breathe the oxygen out of it.

In the beginning, the standard weight of an 80-cubic foot aluminum tank is 1.5 pounds negatively buoyant, while at the end, it becomes 4 pounds positively buoyant.

So, you need to weigh yourself to maintain a negative or neutral buoyancy even when your tank is lighter.

Air in the Lungs

Even the air in your lungs will affect your buoyancy when scuba diving. When you breathe out, your lungs become smaller. That means you’ll displace less water, which will make you negatively buoyant.

In contrast, your lungs and chest grow larger when you inhale, so you displace more water, making you positively buoyant.

So, you need to exhale on the surface when you want to start your descent.

Salt Water or Fresh Water

The salinity of the water you choose to dive in also influences your buoyancy. Oceans with saltwater generally weigh more due to the salt content. So, a diver in seawater will have more buoyant force as they will displace more water.

In contrast, freshwater doesn’t have much salt content. Although the volume of the water might be the same, the diver will displace less water, and buoyant force decreases.

That is just one of the differences you need to be aware of in saltwater vs freshwater diving.


The surface buoyance of your wetsuit changes as you go deeper. As you go down, the pressure increases and will flatten the air bubbles in your suit, making it thinner and flatter. That makes your suit heavier, so you displace less water.

You’ll notice that your buoyancy changes the fastest in the first few feet. At the first 33 feet, you’ll lose half your surface buoyancy and so forth as you go down. By the time you hit 66 feet below, you’ll have only 1/6th of buoyancy remaining.

The opposite happens when you ascend. Your wetsuit buoyancy will increase instantly as you come up. You’ll need to watch out and adjust your buoyancy at different depths.

How Fast Can You Ascend while Scuba Diving?

When ascending, it’s necessary to go as slow as possible. The Professional Association of Diving Instructors recommends not to ascend faster than 60 feet (18 meters) per minute. Some other agencies recommend not going faster than 30 feet (10 meters) per minute.

Suppose you feel you are going too fast; deflate your BCD to remove some air.

It’s inadvisable to go fast because you can get the bends if you go too quickly at such high pressure.

Final Thoughts

Scuba diving is a thrilling and fun activity. But like any sport, you need to know how to scuba dive safely.

To sum it up, divers go up and down using a combination of their lungs, fins and Buoyancy Control Device. Adjusting the buoyancy and understanding how it works explains how divers sink and float.

So, now that you know all about it book that vacation and go diving for an exhilarating experience.

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