The reason that an iron ingot sinks and a ship built from iron floats are described by Archimedes’ Principle. Archimedes, a scholar from Syracuse, Italy explained, “Any object, totally or partially, immersed in a fluid is buoyed up by a force equal to the weight of the fluid displaced by the object”.
It is said that Archimedes had developed this theory while in the bathtub, and that after he had worked out the basics of the principle in his mind, he ran naked through the city streets shouting “Eureka! Eureka! I have discovered it! I have found it!”.
We can infer from Archimedes’ Principle that an immersed object that displaces an amount of water that weighs more than the object itself has Positive Buoyancy. In other words, the object floats.
When the weight of the displaced water weighs the same as the object, the object has Neutral Buoyancy, and neither sinks nor floats.
If the weight of the displaced water is less than the weight of the object, the object has Negative Buoyancy and will sink.
The Buoyant Force of a liquid is dependent on the liquid’s density; its mass per unit of volume. For example, freshwater has a density equal to 1.0 kg per cubic decimeter (about 1 liter) or 62.4 lbs per cubic foot. All things being equal, seawater is heavier; it has a density of 1.026kg per cubic decimeter or 64 lbs per cubic foot, about three-percent heavier.
Let’s take an example to understand how much this difference affects the amount of weights we use. When diving, an adult diver of average build will move about 80 liters / 21 gallons of water and receive a buoyancy of about 80 kg / 176 lbs in fresh water and about 82 kg / 181 lbs in salt water. When the diver passes from the pool to the ocean, he must then add at least 2 kg / 5 lbs of weights to be able to descend.
The difference in a diver’s buoyancy is evident when the diver moves from freshwater to saltwater, as the diver will be more buoyant in saltwater. However, the density of a saltwater body is directly related to the actual salinity of the water. Therefore it’s not uncommon for the buoyancy of a diver to change between bodies of water.
A good example of this variation is the Dead Sea, where the water evaporation is not compensated for by rain nor replenished by tributaries. So, the relative salinity is very high. In fact, the weight per volume of water from the Dead Sea is higher than that of any other body of water in the world. People that visit the Dead Sea are often surprised to find that they can float in the water as if they were sitting in an armchair, with their feet above the surface and their arms by their side.
This is because of Archimedes’ Principle that divers, under most circumstances, have to wear a weight belt in order to be able to dive.
The average human body, when immersed in water, is more or less neutrally buoyant. When wearing scuba equipment (with the exception of additional weight) the diver becomes positively buoyant due to the increased displacement created by the dive suit and Buoyancy Control Device (BCD), and therefore is unable to dive below the surface.
Even the added weight of a full cylinder is not enough to offset the increased buoyancy.
The addition of weights, either to a belt or weight integrated BCD, helps the diver offset the added buoyancy, and return to neutral.
When a diver begins to descend, additional laws of physics come into play that further affect the diver’s buoyancy. The BCD allows the diver to adjust their buoyancy while underwater to manage his position.
A properly weighted diver will have added enough weight to be neutrally buoyant while on the surface. Due to the effects of Boyle’s Law—which states that pressure is inversely proportional to volume, as the diver descends and the amount of pressure he is under increases, he will become more negatively buoyant.
In order to counteract the increase in pressure and decrease in volume, the diver must increase his overall volume by adding gas to his BCD.
In addition to the physical laws, other factors, that might not be obvious, also affect a diver’s buoyancy. For instance, slight movements, whether voluntary or involuntary, may change the diver’s trim and buoyancy. Also, inhaling and exhaling change the characteristics of the diver’s lungs, causing a diver to slightly rise or sink, much like inflating and deflating an air bladder.
In fact, before the BCD became mainstream, divers would rely solely on their lungs for slight variations in buoyancy.
As divers, we are basically trying to imitate fish. There is much we can learn and apply to our diving techniques by observing the actions of fish in their natural habitat. For example, fish are very precise in their movement. They make buoyancy adjustments in coordination with their forward propulsion with the least amount of effort required.
On the other hand, when observing a diver in the water, we will immediately notice how many more movements he performs for each simple movement that he wishes to carry out. This means that the diver generally moves too much.
However, both divers and fish are equipped with similar tools: a swim bladder and thruster fins in the case of fish, lungs and thruster fins in the case of divers. The difference lies fundamentally in how the two interact with the surrounding environment: the water.
Water is 700-800 times denser than air. As such, movement in water is a much more laborious task than it is on the beach.
The human body was not designed for efficiency in water, and when we put on scuba equipment, we create a significant amount of additional drag. Therefore, it becomes necessary to optimize our position in the water so we can take advantage of every forceful action we make.
Again, by observing fish, we see that they optimize their energy via perfect buoyancy. This allows them to use their movement energy to move forward and side to side, rather than to remain at the same depth. To emulate fish, divers optimize their energy using the BCD and their lungs to establish and maintain ideal buoyancy throughout the dive.
Consequently, it can be concluded that the factors that affect a diver’s buoyancy are a combination of the physical laws that we are subjected to as well as the movements that we make during a dive.




Advanced Open Water Diver