In many parts of the world the metric system of measurement is used rather than the "English" system presently used in the United States. Table 1-1 gives conversion factors from metric to U.S. units.

1.2 PRESSURE

To a diver under the water, pressure is the result of two forces: the weight of the water over him, and the weight of the atmosphere over the water. Table 1-2 provides the factors required for converting various barometric pressure units. As a practical matter, the pressures experienced by a diver can be viewed as follows:

1.2.1 Atmospheric Pressure

Atmospheric pressure results from the weight of the atmospheric gases and acts on all bodies or structures in the atmosphere. Atmospheric pressure acts in all directions at any specific point. Since it is equal in all directions the effects are usually neutralized. At sea level atmospheric pressure is 14.7 psi or 1.03 kg/cm2. At higher elevations, this value decreases. Pressures above 14.7 psi are often expressed in atmospheres. For example, one atmosphere is 14.7 psi, 10 atmospheres is 147 psi, and 100 atmospheres is 1470 psi.

Figure 1-1 shows equivalent pressures in the most commonly used units for both altitude and depth.

1.2.2 Hydrostatic Pressure

Hydrostatic pressure results from the weight of water (or any fluid) and acts upon any body or structure immersed in the water. Like atmospheric pressure it is equal in all directions at a specific depth. Hydrostatic pressure is most important to a diver. It increases at a rate of 0.445 psi per foot of descent (1 kg/cm2 per 9.75 meters) in seawater and 0.432 psi per foot of descent (1 kg/cm2 per 10 meters) in fresh water. This is shown graphically in Figure 1-2.

1.2.3 Absolute Pressure

Absolute pressure exerted on a submerged body is the sum of the atmospheric pressure and the hydrostatic pressure. Absolute pressure is measured in "pounds per square inch absolute" (psia) or "kilograms per square centimeter absolute" (kg/cm2 absolute).

1.2.4 Gauge Pressure

Gauge pressure is the difference between absolute pressure and a specific pressure being measured. Pressures are usually measured with gauges that are balanced to read "0" at sea level when they are open to the air. Gauge pressure is therefore converted to absolute pressure by adding 14.7 if the dial reads in psi or 1.03 if the dial reads in kg/cm2.

1.2.5 Partial Pressure

In a mixture of gases, the proportion of the total pressure contributed by a single gas in the mixture is called the partial pressure. The partial pressure contributed by a single gas is in direct proportion to its percentage of the total volume of the mixture. (See Paragraph 1.5.1).

1.3 BUOYANCY

Archimedes' Principle explains the nature of buoyancy.

"A body immersed in a liquid, either wholly or partially, is buoyed up by a force equal to the weight of the displaced liquid."

Using Archimedes' Principle to establish the buoyant force, we can establish the buoyancy of a submerged body by subtracting the weight of the

submerged body from the weight of the displaced liquid.

If the total displacement, that is, the weight of the displaced liquid is greater than the weight of the submerged body, the buoyancy will be positive, and the body will float or be buoyed upward. If the weight of the body is equal to that of the displaced liquid, the buoyancy will be neutral, and the body will remain suspended in the liquid. If the weight of the submerged body is greater than the displaced liquid, the buoyancy will be negative, and the body will sink.

The buoyant force of a liquid is dependent upon its density, that is, its weight per unit volume. Fresh water has a density of 62.4 pounds per cubic foot or 1.01 grams per cubic centimeter. Seawater is heavier, having a density of 64.0 pounds per cu ft or 1.04 g/cc. Therefore, a body will be buoyed up by a greater force in seawater than in fresh water making it easier to float in the ocean than in a fresh water lake.

Lung capacity can have a significant effect on the buoyancy of an individual. With full lungs the diver displaces a greater volume of water and therefore is more buoyant than with exhaled lungs. Other individual differences include bone structure and bone weight and obesity or leanness. This explains why certain individuals float easily while others do not.

A diver wearing a wet suit is usually required to

Oxygen is a colorless, odorless, and tasteless gas. It is only slightly soluble in water. It can be liquefied at -183° C at atmospheric pressure and will solidify when cooled to -218.4° C. Oxygen is the only life supporting gas used by the human body. The other gases breathed from the atmosphere, or those breathed by a diver in a gas mixture, serve only as a vehicle and a diluent for the oxygen. Oxygen is dangerous when excessive amounts are breathed under pressure. This harmful effect is called oxygen poisoning (See Paragraph 2.3).

1.4.3 Nitrogen (N2)

Nitrogen is a colorless, odorless, and tasteless gas. It is chemically inert and is incapable of supporting life. Its boiling point is -196° C. Nitrogen is commonly used as a diluent with oxygen in a diving gas mixture, but has several disadvantages as compared

with some other gases. Nitrogen, when breathed under increasing partial pressure, has a distinct anesthetic effect, producing a disorder called "nitrogen narcosis" characterized by a loss of judg. ment and disorientation (See Paragraph 2.2.3.4).

1.4.4 Helium (He)

Helium is found in the atmosphere only in trace amounts. It has the lowest boiling point of any known substance, -268.9° C. Helium is colorless, odorless, and tasteless and is used extensively in deep diving gas mixtures as a diluent for oxygen. It has some disadvantages, but not of the severity of nitrogen. Breathing helium-oxygen mixtures causes temporary speech distortion (Donald Ducklike voice) which hinders communication. Helium also has a high thermal conductivity, which results in rapid loss of body heat. It is used because of its lower density and lack of narcotic effect upon the diver at depth.

1.4.5 Carbon Dioxide (CO2)

Carbon dioxide is produced by various natural processes such as animal metabolism, combustion. and fermentation. It is colorless, odorless, and tasteless. Although carbon dioxide is not generally considered poisonous, in excessive amounts it is harmful to the diver. While some CO2 is necessary, unconsciousness will result when it is breathed under increasing partial pressure (See Paragraph 1.5.1 and Paragraph 2.1.3.2). For example, a person should not breathe air containing more than 0.02 atmospheres partial pressure CO2 (U.S. Navy Diving-Gas Manual 1971). Divers must be concerned with the partial pressure of carbon dioxide in a breathing supply. In the case of closed- and semi-closed-circuit breathing systems, the removal of excess CO2 generated by breathing is essential to safety.

1.4.6 Carbon Monoxide (CO)

Carbon monoxide is a poisonous gas. It is described here because of its potential lethality. It is colorless, odorless, tasteless, and is difficult for the individual to detect. Carbon monoxide is produced by incomplete combustion of hydrocarbons, as in the exhaust systems of internal combustion engines. Carbon monoxide also may be produced in overheated oil-lubricated compressors. A level of 10 parts per million