by Sandeep Johar

Created on: February 16, 2009

The delight of floating underwater, swimming with the fishes and escaping the day-to-day life are some of the pleasures of scuba diving. To enjoy this safe water sport it is however a good idea to be familiar with how physics is applied in scuba diving. In this particular article we will examine buoyancy and its relationship to safe diving.

The principles of buoyancy go back to Archimedes, who said: "A body immersed in a liquid, either wholly or partially, is buoyed up by a force equal to the weight of the liquid displaced by the body". A principle we all learned in high school physics. This means that when swimming underwater there is an upward force that is directly related to the volume of our bodies plus the entire dive gear that we are carrying. And of course there is a downward force related directly to the weight of our bodies plus the equipment. When two forces are exactly equal we are neutrally buoyant and have achieved a state of weightless suspension in the water.

How can we manipulate our buoyancy? Well the simplest is by controlling the amount of air in the jacket that we wear when diving. This jacket is called the Buoyancy Control Device (BCD). We add air to the BCD at the touch of a button from the tank on our backs and similarly release air with a second button. Adding air increases volume and tends to push us upwards and decreasing air decreases volume pushing us downwards. Quite simple really!

Next time you are on a dive and have achieved perfectly neutral buoyancy, you may notice that as you breathe you rise and fall in the water. The physics are exactly the same, as you breathe in your lungs expand and you are now positively buoyant and rise in the water and as you breathe out you fall in the water. Achieving neutral buoyancy and concentrating on the breathing are one of the joys of diving, and physics can explain it.

Experienced divers may have noted an apparent problem with this analysis. Starting with our swimming pool practice we are taught that to come up from a Scuba dive, we should never inflate the BCD and that in fact we need to constantly deflate our BCD. What is going on here? As a dive instructor myself I completely agree. To rise up in the water kick with your fins and be ready to deflate as you rise up in the water. Never ascend by inflating the BCD.

What is happening here? To understand this better we need to look at another law of physics called Boyle's Law. Robert Boyle showed that as the pressure on a gas in a closed flexible container falls the container expands in volume. If we add air to the BCD our total volume increases and by Archimedes principle we are pushed upwards. However, as we are pushed upwards the pressure of the surrounding water on the BCD (which is a flexible container) drops and by Boyle's law our total volume increases again and the upward force of buoyancy also increases. This causes our buoyancy to become even more positive and we rise faster. This cycle would cause us to rise at an ever increasing pace. This is called a runaway ascent. Runaway ascents can be fatal.

If we do as the instructors say and kick upwards, the force of the kick forces us up in the water. As we start to rise Boyle's law once again applies and the volume of the air in the BCD increases as the weight of the surrounding water drops. To maintain our buoyancy and avoid a runaway ascent we must actually release some of this expanded air. So somewhat counter-intuitively, to rise up in the water safely we must release air, just as we do when we descend.

Enjoy diving, as I do, and perhaps a little insight into the physics will make your dives safer and even more enjoyable. Controlling your buoyancy and controlling the rate of ascent makes for safe fun dives!

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