Fundamental Physics

Two basic laws of physics which affect a diver and are so basic as to be almost forgotten are: AIR HAS WEIGHT and PRESSURE APPLIED TO A FLUID IS TRANSMITTED EQUALLY AND UNDIMINISHED IN ALL DIRECTIONS. Let's consider these in order.

Air Has Weight

If one would picture all of the air above the surface of the earth as composed of tiny blocks of one cubic inch dimension, stacked one on top of the other up to the limits of the atmosphere, he can visualize this weight. Each of these columns of air press upon the earth with a weight, or pressure, of 14. 7 pounds, at sea level. In other words, the figure 14. 7 pounds per square inch (psi) is referred to as one atmosphere of pressure.

A simple experiment will demonstrate the weight of air. If a gallon varnish can is placed over heat, with a small amount of water inside, soon steam will form, forcing the air from the can. If the can is then capped and allowed to cool the steam will condense into moisture, and so leave the can with a partial vacuum inside. As the can cools, because there is no longer any equalizing pressure inside, the can will collapse from the atmospheric pressure being exerted upon it.

Water, of course, is heavier than air. So much heavier, in fact, that it takes only a column of fresh water one inch x one inch x 34 feet, or salt water one inch x one inch x 33 feet to equal in weight the atmospheric pressure of air. For every foot of fresh water the pressure increases by .432 psi, and for every foot of salt water the pressure increases by .445 psi. For every 33 feet of salt water and every 34 feet of fresh water we increase the pressure by one atmosphere, or 14.7 psi.

Transmission Of Pressure

If, therefore, a diver immerses himself under 33 feet of sea water, or 34 feet of fresh water, the pressure of the water alone would be one atmosphere, or 14. 7 psi. However, there is also pressing upon the surface of the water one atmosphere of air pressure. It can readily be seen that the diver underwater is being subjected to the sum of both pressures, or 29.4 psi. (14. 7 plus 14. 7). This pressure, or combined weight of water and air is called absolute pressure.
Most gauges are corrected to not show the atmospheric pressure on the surface, hence read zero on the surface. Therefore, the pressure shown on an air gauge, etc. is called gauge pressure. Gauge pressure, then, is absolute pressure minus 14.7 psi.

Bouyancy

Another very important factor with which the diver is concerned is Archimedes principle which essentially states: An object immersed in a fluid is bouyed up by a force equal to the weight of the water it displaces. If an object weighs less than does the water it displaces, it will be positivily bouyant and will float. If an object weighs the same as does the water it displaces, it will be neutrally bouyant and will neither sink nor float, but will hover. If it weighs more than does the water it displaces it will be negatively bouyant and will sink.

Since one cubic foot of salt water weighs 64 pounds, it can readily be seen that an object which has a volume of one cubic foot and which weighs 54 pounds, will be positively bouyant by 10 pounds.

An excellent example of Archimedes principle as related to diving is afforded in the varying bouyancy of a diver wearing a wet suit. For instance, a diver wearing a wet suit at shallow depth displaces a given amount of water and weighs a fixed weight. He wears a weight belt so that he is neutrally bouyant at a given depth. However, if he descends much below that depth he becomes negatively bouyant because the nitrogen bubbles in the suit are compressed to the point where the diver displaces considerably less water, but his weight has remained constant. In other words, he now weighs more than does the water he displaces, and so is negatively bouyant.

Composition Of Air

Inasmuch as the atmosphere in which we live is air, it behooves us to know a little about its composition. Air, rather than being a homogeous gas is, rather, a mixture of gases. It is composed of approximately 80% Nitrogen and 20% Oxygen. Although these figures are rounded off, these are the gases important to us, the others being inert.

Oxygen is the life sustaining gas, without which life as we know it cannot exist. Nitrogen, the major component of air, serves no useful purpose, other than to take up space. Unfortunately, it has some harmful effects upon the diver under pressure, which will be discussed in detail later.

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