add weights to his body to provide the negative buoyancy that allows him to descend. At the working depth, buoyancy should be adjusted so that the diver is neutral and can accomplish work without the additional physical effort of counteracting positive (upward) or negative (downward) buoyancy.

1.4 DIVING GASES

A diver is totally dependent on a supply of breathing gas while under water. Two methods of providing breathing gases are generally employed. The diver may be supplied with gas from an umbilical, or may carry the breathing gas supply with him. The second method is called SCUBA (scuba) for "Self-Contained Underwater Breathing Apparatus."

Many combinations of breathing gases are used. Air is the most common, but the use of other mixtures for special diving situations is increasing. The following paragraphs describe the gases most commonly used in diving operations.

1.4.1 Air

Air is a mixture of gases (and vapors) containing nitrogen (78.084), oxygen (20.946), argon (0.934), carbon dioxide (0.033) and other rare gases (0.003). Compressed air is the most commonly used breathing gas for diving applications (See Section 10).

1.4.2 Oxygen (O2)

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 judgment 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

In a gas mixture, the proportion of the total pressure contributed by a single gas is called the partial pressure of that gas. An easily understood example is that of a container at atmospheric pressure (14.7 psi). If the container were filled with oxygen alone, the partial pressure of the oxygen would be 1 atmosphere. Now, if the same container at 14.7 psi (1) atmosphere) were filled with dry air the partial pressures of all the constituent gases would contribute to the total pressure, as shown in the following tabulation:

Observe that while the partial pressures of some constituents of the gas, particularly CO2, were fairly small at atmospheric pressure, they increased significantly at higher pressures. The implications of Dalton's Law are highly significant and should be understood by the diver (See Section 2).

1.5.2 Boyle's Law

Boyle's Law states:

"For any gas at a constant temperature, the volume will vary inversely with the absolute pressure while the density will vary directly with the absolute pressure."

The temperature of the body is the measurement of the intensity of its heat and is produced by the average kinetic energy, or speed of its molecules. Temperature is measured by means of a thermometer, and is expressed in degrees Centigrade. (°C) or Fahrenheit (°F). The quantity of heat in the body is the total kinetic energy of all its molecules and is measured in calories or British Thermal Units (BTU).

Temperature must be converted to absolute for use with the gas laws. Absolute zero is a hypothetical

1.1.3 Heat

Heat is energy that causes an increase in the temperature of matter to which it is added and a decrease in the temperature of matter from which it is removed, provided that the matter does not change state during the process. Quantities of heat are measured in calories or BTU.