Latest News

Weather and Tides

Archimedes Principle

It is the essence of diver buoyancy.

Introduction

Archimedes' Principle explains the nature of buoyancy.

An object immersed in a liquid,  either wholly or partially, receives an up thrust equal
to the weight of the liquid displaced by the object.

Using Archimedes' Principle, the buoyancy of a submerged object can be calculated by subtracting the weight of the submerged object from the weight of the displaced liquid.

• If the total displacement, i.e., the weight of the displaced liquid, is greater than the weight of the submerged object, the buoyancy will be positive and the object will float.
• If the weight of the object is equal to that of the displaced liquid, the buoyancy will be neutral and the object will remain suspended in the liquid.
• If the weight of the submerged object is greater than that of the displaced liquid, the buoyancy will be negative and the body will sink.

The buoyant force of water is dependent on its density, that is, its weight per unit volume. Sea water is more dense than fresh water, therefore a diver in seawater will be more buoyant than in fresh water, hence the need for a heavier weight belt when diving in the sea.

Lung capacity can have a significant effect on the buoyancy of a submerged person. A diver with full lungs  displaces a greater volume of water and therefore is more buoyant than the same diver with deflated lungs.

Density

Density is a measure of how densely packed matter is.  One standard brick weighs more than the same size piece of wood but less than the same size brick of lead.

          Density (Kg/m³) = Mass (Kg's)
                                     Volume (m³)

Specific Gravity

This is the tendency of solid and liquid substances to float or sink. Specific Gravity is the ratio of the density of a particular solid or liquid to the density of pure water, which is given an Specific Gravity.  Substances with a value less than 1000 will be less dense than pure water and by definition would float in pure water.  Substances with a value greater than 1000 will be more dense than pure water and would conversely sink in pure water.

Each tissue has its own Specific Gravity e.g., fat is 700 to 900, bone is 1,900 and muscle 1,080 hence the ratio of fat to muscle, the human body would have a Specific gravity 1000 making you a floater or a sinker depending on your build.

Lifting Underwater Objects

Divers may have to lift objects from the sea bed using lifting bags. Assume that divers are required too lift a concrete block from 20m to the surface.

1. The block measures 1m x 1m x .25m  (1 x 1 x 0.25 = 0.25 cubic meters of concrete)

2. The table above states that concrete weighs 2403 kg per cubic meter, therefore the concrete block will weigh 0.25 x 2403kgs = 600kgs (approx.) at the surface.

3. Archimedes principle states that the concrete block will have an up-thrust that is equal to the weight of the water displaced. The water displaced in this case is 250 kg's (0.25 cubic meters of water = 0.25 x 1000kgs or 250kgs) 

4. The underwater weight = 600kgs - 250kgs which equals 350kgs Therefore lifting bags with a total of 350kgs of lift will be required to lift this concrete block.