Archimedes' Principle in Diving

The principle

Archimedes' principle says that any object in a fluid is pushed upward by a force equal to the weight of the fluid it displaces. If you weigh less than the water you push aside, you float. If you weigh more, you sink. If the two are equal, you hover, which divers call neutral buoyancy.

The Greek mathematician Archimedes described this more than 2,000 years ago. For divers it is the basis of weighting, buoyancy control and every adjustment you make to your buoyancy control device.

Positive, negative and neutral

• Positively buoyant: you float. You want this at the surface, with some air in your BCD.
• Negatively buoyant: you sink. You want this briefly at the start of a descent.
• Neutrally buoyant: you neither rise nor sink. You want this for almost all of the dive.

Divers change their buoyancy in two ways: by changing their volume (adding air to the BCD or a drysuit, or taking a deeper breath) or by changing their weight (adding or removing lead).

Salt water and fresh water

Seawater contains dissolved salt, so a litre of it weighs about 1.025 kg, while a litre of fresh water weighs about 1.000 kg. Because salt water is heavier, it pushes up harder, and you need more weight to sink in the sea than in a lake or quarry. The difference is roughly 2.5 percent of the total weight of you and your gear. A diver who switches from the ocean to fresh water usually needs to remove some lead, and the reverse.

Why buoyancy changes during a dive

Your buoyancy is never fixed. Several things change it as you dive:

• Suit compression. Neoprene is full of gas bubbles that shrink as you go deeper, as Boyle's law predicts. A thick wetsuit can lose several kilograms of buoyancy at depth and regain it as you ascend.
• BCD air. Air in the BCD shrinks on descent and expands on ascent, so it needs adjusting all the way.
• Your tank. Compressed gas has weight. A full aluminium 80 cubic foot tank holds roughly 2.5 to 3 kg (about 6 lb) of air, so it becomes lighter and more buoyant as you breathe it down (see scuba tanks).
• Your lungs. A full breath adds several litres of volume. Small changes in breathing are the finest buoyancy control you have.

Weighting correctly

The right amount of weight lets you descend with an empty BCD and stay at your safety stop with a nearly empty tank. A common check is to float upright at the surface with an empty BCD and a normal breath held: correctly weighted, you float at about eye level and sink slowly as you exhale. Being overweighted is a common mistake. It forces you to carry extra air in your BCD, makes buoyancy harder to control and increases your air use. The skills side of this is covered in neutral buoyancy.

Frequently asked questions