Basic Buoyancy and Pressure

Before entering the water it’s a good idea to learn a little very elementary physics. Divers don’t have to be scientists – though many are. But if you want to enjoy the experience a second time, a little knowledge goes a long way toward keeping you safe.

Buoyancy and Archimedes Principle

Common observation shows that some things placed in water float, others sink. The same principle applies to a human body when it enters the sea. It all depends on the buoyancy of your body. For example, in swimming, it is a common knowledge that men’s legs sink as oppose to women’s legs are more than likely afloat.

Archimedes’ Principle states that an object immersed in a fluid experiences an upward force equal to the weight of the fluid displaced by the object. It isn’t just the weight of the object that counts – loaded aircraft carriers weighing 100,000 tons can float. They can because they displace enough water so that the force pushing up from underneath balances the force of gravity acting down. The same principle applies to people in fresh or salt water.

Here’s a physical idea that trips up a lot of beginners. If a diver weighs 200 lbs out of the water, how much does he weigh in the water? Answer: 200 lbs.

Your weight is the product of how much mass, m, you have multiplied by the acceleration due to gravity, g. W = mg. You don’t have less mass in water and g (near the Earth) is about 32 ft/s^2 or 9.8 m/s^2 in or out of the water. What’s changed in water, is that there is an upward directed buoyant force produced by the water underneath you. The water on top pushes downward. The balance of these forces, which is greater, determines whether you’ll sink or float. When you float you are said to have positive buoyancy. Sink and you are negatively buoyant. If you’re stationary you have neutral buoyancy. Since air is less dense than water, it floats. So, the air in your lungs tends to make you positively buoyant. That upward force is counteracted by the weight of your body, the buoyancy compensator (a jacket containing weights with a means of holding the tank) and the other gear you carry. The numbers depend on your specific weight and that of the gear. But it also depends on the kind of water you dive in. Salt water is denser and therefore heavier, so you have to displace less of it in order to be positively buoyant.


A column of air as tall as the atmosphere would weigh about 14.7 lbs (6.7 kg). Thus, the air pressure per square inch acting on your head and shoulders is about 14.7 lbs per square inch. Water, since it’s much denser (and therefore heavier) exerts a much greater force. It only takes a column of water 33 feet (10m) high to weigh 14.7 lbs. Thus the water pressure at a depth of 33 feet is 2 atm. (1 atm from the air + 1 atm from the water. atm = atmosphere, a unit of pressure.) Common sense says the deeper you go, the greater the pressure. Now you know by how much. At 66 feet you experience 3 atm, at 99 feet 4 atm, and so forth. One final note. Gauge pressure – that shown on a depth gauge – will discount the 1 atm from the atmosphere.

Now with a little physics under your diving belt, you’re ready to learn about the fluids – both water and gases – that affect you under the water.

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