Online Scuba Training

Intro to Buoyancy – for scuba divers

BUOYANCY – intro

It is important for divers to understand the factors affecting buoyancy. These are:


Density is defined as mass per unit volume (density = mass ÷ volume).

For our purposes, mass can be considered to be the same as weight, so density is equivalent to weight per unit volume. A substance is more dense than another if the same volume has more weight.  Try lifting a bucket of water and then a bucket of lead, to illustrate this.

Specific Gravity

Specific gravity (S.G.) is the density of a substance compared to the density of fresh water which is given a value of one.  Lead has a specific gravity of 13.5 so it is 13.5 times as dense as water.

e.g.1 liter of water will weigh 1 kg., while the same volume of lead will weigh 13.5 kg.

The concept of specific gravity is important since the specific gravity of a substance determines whether it will float or sink in water.

A substance with a specific gravity greater than 1 (i.e. denser than water) will sink. Lead, with a specific gravity of 13.5, does not float well, whereas oil, with a specific gravity of 0.8, floats easily — producing an oil slick.  The human body has a specific gravity of slightly greater than 1, depending on its content (fat has a specific gravity less than 1, and bones are greater than 1) but the air content of the lungs provides enough buoyancy to allow most people to float.

Archimedes Principle

free online scuba trainingThe ancient Greek, Archimedes (apparently while reclining in his bath), discovered that when an object is immersed in a fluid, it appears to be lighter, and that the apparent loss of weight (or buoyancy) is equal to the weight of water displaced by the object.  That is – the buoyant effect will be equivalent to the weight of fluid of equal volume to the immersed object.  Depending on whether the weight of fluid displaced is greater than, equal to or less than the weight of the object, an object immersed in the fluid will either float, remain suspended or sink. Even an object which sinks will still appear to be lighter than it would out of the
fluid.  Sea water is denser than fresh water because of the salt content, so a greater weight of sea water will be displaced by an object.  Hence objects in sea water are more buoyant than in fresh water.

Air (in the abdomen, buoyancy compensator and wet suit) contributes to buoyancy.  Unfortunately air in these compartments varies in volume in response to the pressure changes with varying depth, making constant buoyancy adjustments necessary. This is usually accomplished by adding air to, or releasing it from, the diver’s buoyancy compensator.  Divers go to considerable lengths to vary their buoyancy to help them submerge, to stay at a given depth, or to ascend or stay afloat in an emergency.

Proper  Buoyancy

One of the most critical skills you must learn during your open water training is a mastery of proper buoyancy, trim, and control in the water column!

One of the biggest mistakes many instructors make during classes is to not demand a mastery of buoyancy control. Many new students learn to dive and develop skills in a pool setting, over-weighted, and sitting on their knees for the majority of skill development. Take these examples below…. do they represent a natural or desirable diving position?   NO..!!!

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If the first thing a new diver learns is to be negatively buoyant during their Open Water Training, they will have a very hard time with comfort and control. A lack of this skill produces a diver with very little confidence as well.  It is believed that this is a big reason new divers quit diving within the first year of certification. Here are a few examples of divers that were the result of improper training and skill development.

Keep in mind, the first video is of a Scuba Instructor…!!!

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This Second video is of a young student, pushed through training way to quickly…in what we call weekend crash courses. This student is obviously not comfortable and has not had the chance to develop true skill

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As you can see in the above pictures, a lot of students learn and perform skills while firmly planted to the bottom. This is completely backwards to what should be taught.  As a new diver it is very important to develop true neutral buoyancy while performing and practicing skills. To master your buoyancy, this is the way it should be. Here are some examples of proper buoyancy in the water column.

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