This chapter explains why we can't stay underwater for a long time even with an unlimited supply of air and why we should surface slow and follow the SSI diving tables to plan and do dives
Water is denser and heavier than air because in an equal space, water contains more molecules than air. A cubic foot of salt water weights 64 pounds (a cubic foot of fresh water weights 62.4 pounds) and a cubic foot of air weights 0.0881 pounds. We know that a square inch the height of the entire atmosphere exerts the same pressure as a one-inch square column of water 33 feet deep into the ocean. Therefore a diver under water will feel the pressure of the atmosphere and the pressure of the water above his body. Water is much better heat conductor than air so a diver under water will loose more heat than in air. Air temperatures of 80º F will feel comfortable but under water it will seem cold. Water absorbs body heat 25 times faster than air. This "pulling away" of heat is due to the property of heat energy transmission known as conduction. Conduction can be slowed or stopped by placing a pour heat conductor between the body and the water. Convection and radiation are the other two methods of heat energy transmission but they have little or no effect on the divers. The pressure felt by a diver (absolute pressure) is the sum of the hydrostatic pressure (the pressure of the water which results from the weight of the water) and the atmospheric pressure. If you are under water at a depth of 33 ft. the hydrostatic pressure is 1 ATM, the atmospheric pressure is 1 ATM so the absolute pressure is 2 ATM. A depth gage will only measure the hydrostatic pressure so at 33 ft. the depth gage will measure 1ATM. Here is a chart that shows the comparison between the pressure in bars, ATM and psi.
If a gas if formed from a mixture of gases, the partial pressure of one of the component gases is defined as the pressure of that gas in the mixture.
John Dalton, an English chemist and physicist, lived from 1776 to 1884.
In addition to discovering a law of physics, he also made the first scientific
account of color blindness, mostly because he was afflicted with it. He
formulated his law of partial pressure while trying to solve a problem
with Newtonian physics. His law states:
For example at the sea level the atmospheric pressure is 14.7 psi. Air,
which is a mixture of gases, consist of 78.08% nitrogen and 20.95% oxygen,
0.03% carbon dioxide and 0.94% other gases.
For divers this means that at any depth or pressure the proportion of the nitrogen and oxygen in air will remain constant. However, when the pressure increases the partial pressure increases in proportion to the change in pressure.
Robert Boyle was an English scientist who lived from 1627 to 1691 and
was a friend and contemporary to Isaac Newton. He developed his law of
pressure to show that air consisted of particles that behaved like tiny,
coiled springs. In 1662 he published his finding that states:
This means that if pressure increases volume decreases and if pressure decreases volume increases. The change in volume is predictable and can be calculated. For example an 80 cubic feet flexible container (at 1 ATM) will decrease to 40 cubic feet at 2 ATM. For divers, this is the explanation for the need of equalization on descend and on ascent and for the other pressure-related occurrences such as squeezes. It also explain why air is consumed faster at depth and the danger of the air expansion injuries.
Jacques Charles was a French scientist who showed the how the temperature
influence the volume of a gas under pressure. His discovery elaborated
on Boyle's work that assumed a constant temperature. His law states:
When air is cooled molecules come closer together, which makes the air denser. Dense air takes up less space so its volume decreases. If the air is inside of a balloon, heating the air would make the air to expand causing the balloon to increase in size. For divers Charles' law has implication in the SCUBA tank filling process. If a tank is filled at an ambient temperature of 70º F and after that the ambient temperature rise to 80º F the pressure inside the tank increases since the volume of the tank is constant.
William Henry, an English chemist and physician, lived from 1774 to 1836
and conducted concurrent investigations into the solubility of the gases
with John Dalton, who was researching the partial pressure of gases. In
1802 he developed Henry's law, which states:
According to Henry's law, pressure will force a gas into solution. Conversely, if the pressure is removed, the gas will come out of solution and return to its original form. The classic example of Henry's law is the soda bottle. For divers the concept of a gas coming out of solution has major implications. During a dive, nitrogen is absorbed into the diver's tissues. On ascent, pressure decreases and the nitrogen will come out of the solution as bubbles if the pressure is decreased to quickly. If this occurs while the nitrogen is still in the diver's tissues, decompression sickness (DCS) results. The rat with which the nitrogen is coming out of the solution can be controlled by slow ascents (30 ft. per minute).
|Updated July 1, 2002 3:49 PM by Vlad Pambucol|