The Basics of Buoyancy

Young woman scuba diving signals okay

Scuba diving has a lot of important physics and physiology behind it, but the most important thing for a diver to understand is buoyancy. Knowing how to control it separates the good divers from the bad ones. Although at first it may seem a little confusing, it soon becomes clear, especially as you practice controlling buoyancy in the water.

Buoyancy refers to how an object acts in the water, whether it floats, sinks, or hovers mid-water. Divers use 3 terms to describe the buoyancy of an object:

1) Positive Buoyancy, or Positively Buoyant = An object that is positively buoyant will float towards to surface, and stays there.

2) Negative Buoyancy, or Negatively Buoyant = An object that is negatively buoyant will sink towards the bottom, and will remain on the bottom.

3) Neutral Buoyancy, or Neutrally Buoyant = An object that is neutrally buoyant will remain suspended at a single depth underwater.

Imagine you are running a bath, and you fill it up right to the top. When you get into the tub, the water level will rise, and some will spill over the top and pour onto the floor. The amount that spilled over the side is the same volume as your body. The water has been displaced to allow for your volume. Whenever an object enters the water, the water surrounding it pushes (exerts pressure) against it to try to fill the space that the object is occupying. This pressure is trying to push the object upwards, this is called the buoyant force.

In order to work out whether an object will float or sink, we use ‘Archimedes’ Principle’. For this, you need to understand the two forces that are at work.

1) Gravity & objects weight: This is the force that is pushing the object down

2) Buoyant force: This is the force that is pushing the object upwards.

So basically, if the force from the weight of an object is greater than the force of the buoyancy, it will sink. If the buoyant force is more, then the object will float.

buoyancy

A way to work out where or not something will float or sink, you need to know how much water (the weight of the water) the item displaces. If the object displaces more water that its own weight, it will float, and if it displaces less than its own weight, it will sink. If the weight of the water displaced is exactly the same as the weight of the object, it will remain suspended at one level. This is why huge ships will float, but a nail will sink. Despite the huge weight of a ship, its design means it displaces far more water than it weighs. The nail however, displaces very little water due to its shape, therefore it sinks.

What effects a Scuba Divers buoyancy?

There are many factors involving the buoyancy of a scuba diver:

1) BCD (Buoyancy control device)
We control our buoyancy by using our BCD. As we inflate it, we add air to the BCD, which adds volume, but very little weight. Now we are displacing more water, which means the buoyant force pushes us further upwards. Likewise when we deflate it, we displace less water, therefore we decrease our buoyancy.

2) Lungs
When people learn how to swim, they are usually told to hold their breathe to help them float at the surface. This is because when we inhale, our lungs expand, pushing our chest outwards. Again, now we have a bigger volume, so we are displacing more water. When we breathe out, our volume decreases, and we lose buoyancy. Because of this, we can never be truly neutrally buoyant.

We use our BCD to become almost neutrally buoyant, then as we inhale we should become slightly positively buoyant, and as we exhale, we should become slightly negatively buoyant. This takes some time to learn, which is why in the Open Water Diver course there are a few skills that need to be practiced, such as the hover, or the fin pivot.

3) Cylinder Pressure
Although air doesn’t weigh much, it does have weight. In our scuba cylinders we have so much compressed gas that the pressure does make a difference to our buoyancy. When we breathe from the tank, we take some of the air out, therefore each breathe is making it a little lighter. This doesn’t really effect steel tanks, because they are so heavy, however a full aluminium tank start a dive being negatively buoyant, and when its gas pressure has reduced to 50 bar, it is very positively buoyant. Divers need to take this into account when weighting themselves otherwise they will find themselves floating upwards at the end of the dive.

4) Weights
Humans are naturally positively buoyant, as is most of the scuba equipment we use. Because of this, we need to use weights to help us descend, and stay comfortably underwater. Some people find they can start a dive with no weights, but will need them later on as their cylinder gets lighter.

5) Exposure suits, and the rest of the equipment
All exposure suits are positively buoyant. Wetsuits are made of neoprene, which has thousands of tiny air bubbles sealed within the material, and drysuits work by trapping a layer of air between the diver and the suit. The more material the suit has (thickness/length) the more buoyant it will be.

Every piece of equipment we carry effects our buoyancy. BCDs are usually positively buoyant, even with no air inside them. But regulators, fins, torches etc. are negatively buoyant. This is why a diver should conduct a buoyancy check every time they change their equipment.

6) Body type
As mean as it might sound, fat floats. If a diver has higher body fat levels, they will need more weight to sink than those with lower fat levels. Women naturally have higher body fat levels than men, so tend to need more weight.

7) Water type
In fresh water, you will sink much easier than you would in the ocean. This is because the dissolved salts in sea water causes it to weigh more than fresh water. If a diver was submerged in either salt water or fresh water, the water volume displaced would be equal, however the weight of the salt water would be more than it would be for the same volume of freshwater. Because the buoyant force is equal to the weight of the water displaced, a diver would be far more buoyant in salt water than they would be in fresh water.

The Key to Buoyancy Control

The most important thing to understand as a diver is this, buoyancy changes with depth. Under pressure, air compresses and becomes denser. If the pressure increases (e.g. if the diver goes deeper) the air volume inside the divers lungs and BCD will decrease, and the once neutrally buoyant diver will now be negatively buoyant. On the other hand, if the pressure decreases (e.g. the diver ascends) the air volume will increase, making the diver more positively buoyant. This means the diver must always make micro adjustments in order to keep in control in the water. It is common to see new divers always adjusting their BCD to try to stay neutral, but this is difficult. An experienced diver will always use minimal air in their BCD, and use proper breathing patterns to stay in control. This seems complicated at first, but with practice, it becomes almost second nature. All you need to do is head into the water and practice.

‘The Basics of Buoyancy’ was written by Mike

Check out our Blog: Buoyancy Control when Scuba Diving.