How to Understand Floating and Sinking

Understanding density

The Greek mathematician Archimedes discovered in the second century BC that when a body is immersed in a fluid, a force equal to the weight of the fluid displaced buoys the body up. This principle applies to both floating and submerged bodies and to all liquids and gases.

A fully equipped diver is made up of many constituent parts, some of which will naturally sink, such as the tank and weights, and others that tend to float, such as the suit buoyed up by the air contained within it.

A diver’s body can either sink or float, depending on the amount of air in the lungs. The BC also has variable buoyancy. The equipped diver must be viewed as a whole, with the overall weight balanced against the amount of water displaced. This equals the overall density. Water has a different density depending on whether it is fresh or salt water.

If the diver’s overall density – complete with suit and equipment – is less than the density of the water, the diver will rise in the water column (float). If the diver’s body and kit is denser than the water, the diver will sink in the water.

Adjusting buoyancy

The relative density of an object to the water also determines the proportion of a floating body that will be partly submerged in the water, as in the case of a diver with a BC inflated at the surface. For a submerged body, the apparent weight of the body is equal to its weight in air less the weight of an equal volume of water.

A diver has two ways to continually adjust his or her density. First, lung volume can be varied by altering the amount of air inhaled and exhaled. Second, the volume of air in the BC or drysuit can be adjusted.

A diver can become lighter than the water displaced by inflating the BC and floating at the surface. To sink, the diver can become heavier than the water displaced, and can do this quickly or slowly by altering the amount of air put into the BC or drysuit.

When a diver gets to the depth required, sufficient air should be added so that the diver neither continues to sink nor floats up. This is called ‘neutral buoyancy’. At this point, little effort is needed by the diver to propel forward using a few fin strokes.

The art of diving depends on buoyancy control, and to be neutrally buoyant means that a diver becomes free and weightless in a liquid world.

Breath control

Of course, a diver is breathing in and out all the time. This means that their buoyancy changes slightly from moment to moment. But a diver doesn’t have to keep fiddling with the direct-feed of air to the BC or drysuit to maintain buoyancy. It is easier to make fine adjustments to the buoyancy by simply altering lung volume. For example, taking a large breath will help a diver rise over an obstruction and a large breath out will help them to sink a little after passing it.

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  4. How to Choose a Buoyancy Compensator for Scuba Diving
  5. How to Understand the Breathing and Buoyancy of Fish

Filed Under: Sports & Fitness


About the Author: Cody Riffel is a regular contributor to MegaHowTo. She likes to write on variety of topics, whatever interests her. She also likes to share what she learns over the Internet and her day-to-day life.

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