Boyle's Law
For SCUBA divers, the most important of the gas laws--and the most deadly one to ignore--is Boyle's law. Boyle's law says that as long as temperature is constant, pressure and volume are inversely proportional--when one goes up the other goes down and vice versa. If you have an inflatable container--such as your lungs-- decreasing the pressure increases the volume. So if you take a deep breath and then move toward the surface while holding your breath, your lungs expand. If you hold it too long your lungs will explode. This is why you should never hold your breath while SCUBA diving--not even briefly. If you breathe consistently the volume of air in your lungs will constantly adjust to the changing pressure.
Amonton's Law
Amonton's law says that if the volume of a gas is constant--as it is in a SCUBA tank--the pressure and temperature are directly proportional. If one increases so does the other, and vice versa. If you fill up your tank on a summer day and then jump in water that's 40 degrees cooler, you are going to have less pressure than you might think. Therefore, the tank will not last as long. If you fill up a tank and then put it in the trunk of the car where it is 60 degrees hotter, the pressure might rise to dangerous levels and damage your equipment.
Henry's Law
Henry's law describes how pressure determines a liquid's ability to absorb gasses. As pressure increases, more of a gas will dissolve in a liquid. When a diver dives, the pressure increases and more gasses dissolve in the diver's tissues. Most of Earth's atmosphere is nitrogen, which is relatively inert so it does no real harm. If the diver ascends slowly the nitrogen comes out of the tissues as the pressure decreases. If the diver with nitrogen-saturated tissues (too deep, too long) ascends too quickly, the nitrogen comes out too fast--in bubbles. This causes a very dangerous condition known as the "bends," which can cause permanent damage or even death.
Dalton's Law
Dalton's law says that if a gas is a mixture, the pressure of each component depends on the percentage of that component in the mixture. Earth's atmosphere is about 78% nitrogen, 21% oxygen, and one or two percent other gasses--depending on where you live. If your tank is filled using faulty equipment, you might be getting a lethal dose of something that is relatively safe at low pressures. Even oxygen can be toxic at high pressures. Nitrogen at high pressures can have a narcotic effect (known as "rapture of the deep") that can cause a deadly loss of judgement.
By Carlos Mano, eHow Contributor
Kathy Dowsett
www.kirkscubagear.com
