little can cause an uncomfortable sensation of dehydration in a diver's mouth, throat, nasal passages. and sinus cavities (U.S. Navy Diving Manual 1973). Air or other breathing gases supplied from surface compressors or tanks can be assumed to be dry. This dryness can be reduced by removing the mouthpiece and rinsing the mouth with water or introducing a small amount of water inside a full face mask. The use of gum or candy to reduce dryness while diving can be dangerous. The mouthpiece should not be removed in water which may be polluted.

1.7.1 Condensation in Breathing Tubes or Mask

Expired gas contains moisture that may condense in the breathing tubes or mask. This water is easily blown out through the exhaust valve and, in general. presents no problem. In very cold water freezing of the condensate can occur. Should the freezing of the condensate become serious enough to block the regulator mechanism, the dive should be aborted.

1.7.2 Fogging of the Mask

Condensation of expired moisture or evaporation from the skin may cause fogging of the face mask glass. Moistening of the glass with saliva, liquid soap, or commercially available anti-fog compounds will reduce or prevent this difficulty.

1.8 LIGHT AND UNDERWATER VISIBILITY

To function effectively in an underwater environment, a diver must understand the factors affecting his visual perception. The principal physical factor affecting visibility in the water is that light behaves differently in the denser water medium than it does in air. As light passes from a medium of one density to another, the light rays are bent and the effect known as "refraction" occurs. Refraction can cause an object to appear larger or smaller than its true size, and to appear in other than its true position. Refraction occurs at the surface, between the water and air interface, as well as at the interface between the water and the diver's mask.

With the eye functioning normally, refraction between air and water causes nearby objects to appear larger and closer than they actually are. This accounts for the ditheulty often experienced by novice divers in attempting to grasp objects

under water. The object appears to lie at a distance which is, in reality, approximately 3/4 of its actual distance. This is shown in Figure 1-5. Beyond a distance of approximately 4 feet, however, this phenomenon partially tends to reverse itself. Reduced brightness and contrast, combined with the lack of normal visual distance relationships, cause faraway objects to again appear larger, but to appear farther away than they actually are. This distortion occurs. even under optimum conditions of clear water and good lighting. Conditions of reduced lighting and/or increased turbidity will further impair the accurate perception of size and distance. Depending on the severity of the conditions involved, misinterpretation of size and distance due to refraction can be overcome or compensated for with experience and training (Kinney et al. 1968a).

"Diffusion" while occurring in air is further intensified under water. Light rays are diffused and scattered by the water molecules and particulate matter. At times diffusion is helpful in that it scatters light into areas that would otherwise be in shadow or have no illumination, at other times it is annoying because of the backscatter it produces which interferes with vision and underwater photography, particularly when artificial lighting is required.

1.8.1 Visibility of Colors

Distortion of underwater visibility is not limited to size and distance. A variety of factors may combine to alter the accurate perception of color. The use of colored paints on objects is an obvious means of changing their visibility either by enhancing their contrast with the surroundings or by camouflaging them to merge with their background. The problem of determining which colors will be most and least visible under water is, however, much more complicated than it is in air. Transmission of light through air does not appreciably change its spectral composition, but transmission through water can alter the appearance beyond recognition. Certain conditions of lighting and water hue may, for example, cause the color red to be perceived as black. This is readily explained when one considers that we see red objects on the surface because of the reflected red light. Because sea water absorbs red, no red light reaches the object, hence if it is a red reflector it appears as black. In the same way, a blue object in green water will also appear black, and a blue-green object in

ocean water and a yellow object in green water will resemble an object that was white or pale grey on the surface. Red and blue-green objects in green water and yellow and dark blue objects in clear ocean water will retain their color to considerable depths. Some substances with more than one peak in their spectral reflectance curve may appear quite different in color on land than under water. Blood is a good example: at the surface the reflectance maximum in the green is swamped by the red, but at depth the water absorbs the long wavelength red light and blood appears green. Both the quantity and quality of such changes depend on the particular body of water involved and the light source used. For this reason, the selective use of specific colors for underwater identification should result from actual on-site experimentation. Some experimental work has been done on the visibility of colors under water (Kinney et al. 1968b). Table 1-3 gives the results of these experiments, which were conducted by divers using spherical floats painted with different colors and illuminated by different types of light sources. The diver, the colored float, and the light source were at a depth of about 2 meters.

To summarize, as sunlight enters the water and travels to depth, red light is filtered out at relatively shallow depths in clear water. Orange is filtered out

next, then yellow, green, and blue. The filtering of various colors is not solely dependent on depth. Other factors, such as salinity, turbidity, or the degree of pollution have effects on the color filtering properties of the water. The above order could even be reversed in turbid water (Kinney et al. 1968b, Mertens 1970).

In general, the components of any underwater scene (weeds, rocks, encrusting animals, etc.) all tend to approach the same color as the depth or viewing range increases and are distinguished by differences in brightness and not of color. However, some colors (deep blue and yellow in clear blue water, or red and blue-green in yellow-green water) do tend to contrast strongly in color against the general scenery, and objects painted in these colors would be relatively easy to find. Fluorescent colors are always conspicuous because they emit light at wavelengths which are rarely present under water. Thus, not only do fluorescent objects retain their colors at all depths where there is light enough for color vision, but they are extremely unlikely to be displayed against a background of similarly fluorescent objects.

1.8.2 Dark Adaptation

Anytime there is a significant decrease in the

level of ambient light the eye must adapt according ly. This can occur for the diver when he enters the water following exposure to bright sunlight especially if the water is dirty or as he descends to greater depths. As the light level decreases there is a switch from the day vision system where there is color vision, to the night vision system where there is not. While a substantial degree of dark adaptation occurs in the first ten minutes, the complete process may require over 30 minutes if the ambient brightness level just prior to entering the water was very high. This adaptation process accounts in part for the apparent loss of perceived color as the diver goes deeper. It also accounts for the feeling that the light level has increased as the diver remains on the bottom.

Although the diver's descent rate far exceeds the rate of dark adaptation, certain precautions may be taken prior to making a dive where the underwater light level is significantly below that on the surface. This is especially important during dives where bottom time is short and visual observation is important.

The night vision system of the eye is relatively insensitive to red light. If a red filter is worn over the faceplate for 30 minutes or so prior to diving, the eye reacts as though it were dark and adapts accordingly. Once the light becomes dim, the red filter is

removed. Because high visual sensitivity is reached sooner when this procedure is used, visual tasks can be performed effectively at the beginning of a dive instead of 20-30 minutes later. If it is necessary to momentarily return to the surface the red filter should be worn, as exposure to bright sunlight will instantly destroy the dark adapted state of the eye.

1.9 ACOUSTICS

Sound is the result of vibrations being transmitted through some form of matter to the ear. Sound travels more rapidly through denser substances. Water is denser than air and, for this reason, is a better conductor of sound. A diver will hear sounds more clearly and at a greater distance than in air.

Underwater hearing is affected by several conditions not encountered in the atmosphere. These include:

1. Reverberations of sound resulting from reflections from the bottom.

2. Gradients and discontinuities in the water resulting from thermal and salinity conditions and microorganisms.

3. Noises caused by water movement and passing ships and marine life.

4. Type of head covering.

With the head submerged, the auditory localization ability of a diver is seriously impaired. In air sound travels at the rate of about 1,100 feet/second. The slight amount of time that elapses between the instant the sound wave hits one ear and then the other ear permits an individual to judge the direction from which the sound came. Sound travels in water at the rate of about 4,900 feet/second. The difference in time between the instant the sound wave hits one ear and then the other is so slight that the sound seems to surround the diver. However, recent studies have shown that humans possess some ability to localize sound under water. Localization is best for low frequency or broad band signals (less than 6000 HZ) (Feinstein 1973).

Sound is transmitted through water as a series of pressure waves. High intensity sound is transmitted by correspondingly high intensity pressure waves. A diver may be affected by a high intensity pressure wave that is transmitted from the water surrounding him to the open spaces within the body (ears, sinuses, lungs). The pressure wave may create increased pressure within these open spaces, which

could result in injury.

The sources of high intensity sound or pressure waves include underwater explosions and, in some cases, sonar. Low intensity sonars, such as depth finders and fish finders do not produce pressure waves of an intensity dangerous to a diver. However, some military anti-submarine sonar equipped ships do pulse high intensity pressure waves dangerous to a diver. It is prudent to suspend diving operations if a high powered sonar transponder is being operated in the area. Underwater explosions are discussed in the following paragraph. The use of explosives is discussed in Paragraph 4.12.

1.10 UNDERWATER EXPLOSIONS

An underwater explosion produces a pressure wave that emanates outward from the source of the explosion in all directions. The pressure wave characteristics of an underwater explosion consist of an initial shock wave followed by a less pronounced pressure wave. The initial high intensity shock wave is the result of the violent liberation of high pressure gas produced by the explosion. The subsequent pressure wave is caused by the collapse of the gas pocket produced by the explosive charge.

The initial high pressure shock wave is the most dangerous and always travels away from the source of the explosion. Less severe pressure waves follow the high pressure wave very closely. Considerable turbulence and movement of water in the area of a high-order explosion is in evidence for an extended period of time after the explosion.

A number of factors affect the intensity of the shock wave and the pressure waves that follow the shock wave (Greenbaum and Hoff 1966). Each should be evaluated in terms of the particular circumstance in which the explosion occurs.

1. The size of the explosive charge and the type of explosive utilized. Some explosives produce a high-order, short duration explosion, resulting in a high level pressure wave of short duration. Other explosives produce lower order explosions that result in less intense but longer duration shock and pressure waves, which do more damage at a longer range. It is imperative that the characteristics of the explosive to be utilized be carefully evaluated prior to use, to estimate the type and duration of the shock and pressure waves.

2. The character of the bottom. Aside from the

*Robert D. Workman, M.D., 1973: personal communication.

fact that rock or other bottom debris may be propelled through the water by the explosion, the bottom conditions may have a dampening or amplifying effect on the shock and pressure waves. A soft bottom may tend to dampen the effect, while a hard bottom may amplify the effect. The contour of the bottom is an important consideration, as rock strata, ridges, and other topographical features may affect the direction of the shock and pressure waves, as well as produce secondary reflecting waves.

3. The depth of the water. At great depth, the pressure waves are attenuated through a greater water volume and thus reduced in intensity. An explosion near the surface is not attenuated to the same degree.

4. The distance a diver is from the explosion. In general, the farther away from the explosion, the greater the attenuation of the pressure wave and the less intensity. This factor must be considered in the context of the bottom conditions, depth of water, and reflection of shock and pressure waves from underwater structures and topographical features.

5. The degree of submersion of the diver. A fully submerged diver receives the total effect of the shock and pressure wave passing across his body. If a diver is only partially submerged with his head and upper parts of the body out of the water, the effect of the shock and pressure wave on his lungs, ears, and sinuses may be reduced; however, air will transmit some portion of the explosive shock and pressure wave. The head, lungs, and intestines are the parts of the body most vulnerable to the pressure effects of an explosion.

The following formula (Greenbaum and Hoff 1966) provides a means of estimating the pressure on a diver resulting from an explosion of tetryl or TNT. Note that this formula is not applicable to other explosives and should be used only to esti

mate the level of pressure on the diver in conjunction with factors described above.

where

P=

= pressure on the diver in pounds per square inch

W = weight of the explosive (tetryl or TNT) in pounds

r = range of the diver from the explosion in yards

A sample calculation shows that a 600-pound charge at a distance of 50 feet exerts a pressure of 2,180 pounds per square inch. Since a pressure wave of 500 pounds per square inch is sufficient to cause serious injury to the lungs and intestinal tract, a pressure wave over 2,000 pounds per square inch would certainly be fatal. Even a pressure wave of 500 pounds per square inch could cause fatal injury under certain conditions.*

It is prudent practice to limit the pressure a diver experiences from an explosion to a pressure less than 50 to 70 psi or best, to remove the diver from the water at the time of the explosion.

For scientific experimental work, very low order explosions are sometimes used to blast loose samples or create pressure waves through substrata. Each of these cases must be evaluated in terms of diver protection. Bottom conditions, degree of submersion of the diver, or protective measures afforded the diver can modify the effects of an explosion and are to be considered in planning a dive in which explosive experimentation is involved. Also, divers should be cautioned against diving nearby when subbottom profiling is being conducted using highpressure air or high electrical discharges.

*Robert D. Workman, M.D., 1973: personal communication.