One of the main physical concepts that needs to be fully understood during early scuba diving training is that of buoyancy. Buoyancy theory is encapsulated in Archimedes Principle which states that when a body is immersed in a liquid it experiences a force acting upwards on it that is equal to the weight of the fluid displaced by the body.

In effect, this is why a body floats or it sinks. A boat will float on the water displacing an amount of that water. The amount displaced equals the weight of water displaced which in turn equals the force acting upwards keeping the boat afloat. It equals the weight of the boat, therefore we have a situation of positive equilibrium.

When a stone sinks in water its volume displaces an amount of water that is less in weight than the weight of the stone. The up-thrust is not enough to keep the stone afloat and it sinks – a case of negative buoyancy. The stone is denser than the water and therefore cannot displace enough to provide the up thrust that would keep it afloat.

Consider the case of an object that is neutrally buoyant within the fluid that it sits. It neither sinks or pops to the surface – it stays where it is placed within the body of fluid. This is in fact the ideal situation that a diver should be in. A scuba diver does not want to struggle against the up-thrust to get beneath the surface of the water, nor does he or she want to bump to the bottom and then struggle to fin their way back to the surface.

By being neutrally buoyant a diver will expend the least energy not only moving up and down the water column but also swimming through the water as they will not be continually struggling to stay at a certain level. A diver will need to fully understand the concept of buoyancy so that the correct scuba diving equipment can be chosen and more importantly the diver understands how and why they are using that life supporting diving equipment.