Chances are, if you are a diver, you probably already have some awareness of the way the basic laws of physics affect you while you’re underwater. There are six basic laws and principles you should be aware of, and while the basics we discuss quickly here will help you grasp a basic understanding, there is no substitute for proper training and experience. This being, said, let’s dive right in.
Boyle’s Law
The Law: “Given a constant temperature, the volume of a gas varies inversely as the absolute pressure.”
As pressure increases, gas volume decreases, and as pressure decreases, gas volume increases. As you go deeper into water, the pressure increases at a rate of one atmosphere (ATA) per 10 meters (33 feet.) If you increase altitude, then atmospheric pressure decreases, causing gases to expand. You can use Boyle’s law to help increase your understanding of why controlled ascent and safety stops are so important. You can also use it to understand why you may feel sinus pressure, and to understand why you should never hold your breath while diving.
Charles’ Law
The Law: “Given a constant pressure, the volume of a gas varies directly as the absolute temperature.”
Also known as Gay-Lussac’s Law, this simple law has to do with expansion and contraction. When air is heated, it expands, and when it is cooled, it contracts, so long as pressure remains stable. When your SCUBA cylinder is filled, it heats up slightly, which is why most shops overfill tanks by about 10%; once the heat caused by the friction from filling is eliminated, pressure drops. The same thing can happen if your tanks are sitting in the sun, then you dive into a chilly lake. Putting your tanks in the water for about ten minutes before you dive can help you to more accurately track your own air consumption, since temperature change is actually responsible for consuming air volume at the beginning of a dive.
Dalton’s Law
The Law: “The total pressure exerted by a mixture of gasses is the sum of the pressures exerted by each of the gasses.”
Anyone who fills SCUBA cylinders uses this law to obtain target working pressure of 3000 psi or 206 bar. While standard air averages 79% Nitrogen and 21% Oxygen, Nitrox contains a higher Oxygen level, usually between 22 and 40%. To get the Oxygen level right, qualified technicians use a number of variables, including working pressure, percentage of Oxygen desired, and cylinder volume to mix Nitrox properly. The same law applies to compounding Trimix, Heliox, and other mixed gases.
Henry’s Law
The law: “The amount of gas that will dissolve in a liquid is directly proportional to the partial pressure of the gas.”
During any dive, this law is used to measure the amount of nitrogen the pressure around you forces into your bloodstream and tissues. The longer you stay in the water, and the deeper you go, the more saturated your tissues become. To reverse the law, you must ascend very slowly to allow excess gas to work its way out of your system, or you could suffer the effects of decompression sickness. If you’ve ever felt the effects of nitrogen narcosis, then you have felt Henry’s Law in action.
Archimedes Principle
The law: “An object wholly or partially immersed in liquid is buoyed up by the force equal to the weight of the liquid displaced.”
This principle is used to calculate specific buoyancy, ensuring certain things can float; it is the reason your body, BC, and other items, including massive ships, are capable of flotation. When you’re negatively buoyant, you’ll sink, and when you are positively buoyant you will float. When you have achieved neutral buoyancy, you do neither. Anytime you change your buoyancy level, you are using Archimedes Principle.
Now that you know a bit more about gases and the physics of diving, you may feel inspired to increase your level of knowledge even more by taking additional courses. Remember to always dive within your own limits, and never try to use your own body as a test subject; embolisms and decompression sickness can kill you or cause permanent damage. Be safe, and you’ll find your enjoyment of diving increases along with your understanding.