Mobile Diving and Salvage Unit One and Two, the modern-day descendants of the Vietnam era Harbor Clearance Units, have a proud and distinguished history of combat salvage operations. Diving records have been set and broken with increasing regularity since the early s:.

The fsw mark was exceeded. Three U. Navy divers, F. Crilley, W. Loughman, and F. Nielson, reached fsw using the MK V dress. A French dive team broke the open-sea record with 1, fsw. Commercial open water diving operations to over 1, fsw. Throughout the evolution of diving, from the earliest breath-holding sponge diver to the modern saturation diver, the basic reasons for diving have not changed.

National defense, commerce, and science continue to provide the underlying basis for the development of diving. What has changed and continues to change radically is diving technology.

Each person who prepares for a dive has the opportunity and obligation to take along the knowledge of his or her predecessors that was gained through difficult and dangerous experience. The modern diver must have a broad understanding of the physical properties of the undersea environment and a detailed knowledge of his or her own physiology and how it is affected by the environment. Divers must learn to adapt to environmental conditions to successfully carry out their missions.

Much of the divers practical education will come from experience. However, before a diver can gain this experience, he or she must build a basic foundation from certain principles of physics, chemistry and physiology and must understand the application of these principles to the profession of diving.

This chapter describes the laws of physics as they affect humans in the water. A thorough understanding of the principles outlined in this chapter is essential to safe and effective diving performance.

Humans readily function within the narrow atmospheric envelope present at the earths surface and are seldom concerned with survival requirements.

Outside the boundaries of the envelope, the environment is hostile and our existence depends on our ability to counteract threatening forces. To function safely, divers must understand the characteristics of the subsea environment and the techniques that can be used to modify its effects. To accomplish this, a diver must have a basic knowledge of physicsthe science of matter and energy. Of particular importance to a diver are the behavior of gases, the principles of buoyancy, and the properties of heat, light, and sound.

Matter is anything that occupies space and has mass, and is the building block of the physical world. Energy is required to cause matter to change course or speed. The diver, the divers air supply, everything that supports him or her, and the surrounding environment is composed of matter.

An element is the simplest form of matter that exhibits distinct physical and chemical properties. An element cannot be broken down by chemical means into other, more basic forms.

Scientists have identified more than elements in the physical universe. Elements combine to form the more than four million substances known to man. The atom is the smallest particle of matter that carries the specific properties of an element.

Atoms are made up of electrically charged particles known as protons, neutrons, and electrons. Protons have a positive charge, neutrons have a neutral charge, and electrons have a negative charge. Molecules are formed when atoms group together Figure Molecules usually exhibit properties different from any of the contributing atoms. For example, when two hydrogen atoms combine with one oxygen atom, a new substancewateris formed.

Some molecules are active and try to combine with many of the other molecules that surround them. Other molecules are inert and. Two similar atoms Figure The Three States of Matter. The presence of inert elements in breathing mixtures is important when calculating a divers decompression obligations. Matter can exist in one of three natural states: solid, liquid, or gas Figure A solid has a definite size and shape.

A liquid has a definite volume, but takes the shape of the container. Gas has neither definite shape nor volume, but will expand to fill a container. Gases and liquids are collectively referred to as fluids. The physical state of a substance depends primarily upon temperature and partially upon pressure. A solid is the coolest of the three states, with its molecules rigidly aligned in fixed patterns. The molecules move, but their motion is like a constant vibration.

As heat is added the molecules increase their motion, slip apart from each other and move around; the solid becomes a liquid. A few of the molecules will spontaneously leave the surface of the liquid and become a gas.

When the substance reaches its boiling point, the molecules are moving very rapidly in all directions and the liquid is quickly transformed into a gas. Lowering the temperature reverses the sequence. As the gas molecules cool, their motion is reduced and the gas condenses into a liquid. As the temperature continues to fall, the liquid reaches the freezing point and transforms to a solid state.

Physics relies heavily upon standards of comparison of one state of matter or energy to another. To apply the principles of physics, divers must be able to employ a variety of units of measurement. Two systems of measurement are widely used throughout the world. Although the English System is commonly used in the United States, the most common system of measurement in the world is the International System of Units.

The SI system is decimal based with all its units related, so that it is not necessary. The SI system changes one of its units of measurement to another by moving the decimal point, rather than by the lengthy calculations necessary in the English System.

Because measurements are often reported in units of the English system, it is important to be able to convert them to SI units. Measurements can be converted from one system to another by using the conversion factors in Table through While the English System of weights and measures uses the Fahrenheit F temperature scale, the Celsius C scale is the one most commonly used in scientific work.

Both scales are based upon the freezing and boiling points of water. The freezing point of water is 32F or 0C; the boiling point of water is F or C. Temperature conversion formulas and charts are found in Table Absolute temperature values are used when employing the ideal gas laws.

The absolute temperature scales are o R based upon absolute zero. Absolute F C K. One example of an absolute tempera ture scale is the Kelvin scale, which has the same Figure Temperature Scales. The Fahrenheit, Celsius, Kelvin, and Rankine freezing point of water is K and temperature scales showing the freezing boiling point of water is K. Use and boiling points of water. The Rankine scale is another absolute temperature scale, which has the same size degrees as the Fahrenheit scale.

The freezing point of water is R and the boiling point of water is R. When measuring gas, actual cubic feet acf of a gas refers to the quantity of a gas at ambient conditions. The most common unit of measurement for gas in the United States is standard cubic feet scf. Standard cubic feet relates the quantity measurement of a gas under pressure to a specific condition. The specific condition is a common basis for comparison. For air, the standard cubic foot is measured at 60F and Energy is the capacity to do work.

The six basic types of energy are mechanical, heat, light, chemical, electromagnetic, and nuclear, and may appear in a variety of forms Figure Energy is a vast and complex aspect of physics beyond the scope of this manual. Consequently, this chapter only covers a few aspects of light, heat, and mechanical energy because of their unusual effects underwater and their impact on diving.

The Law of the Conservation of Energy, formulated in the s, states that energy in the universe can neither be created nor destroyed. Energy can be changed, however, from one form to another. The two general classifications of energy are potential energy and kinetic energy. Potential energy is due to position. An automobile parked on a hill with its brakes set possesses potential energy.

Kinetic energy is energy of motion. An automobile rolling on a flat road possesses kinetic energy while it is moving. Refraction, turbidity of the water, salinity, and pollution all contribute to the distance, size, shape, and color perception of underwater objects. Divers must understand the factors affecting underwater visual perception, and must realize that distance perception is very likely to be inaccurate. Light passing from an object bends as it passes through the divers faceplate and the air in his mask Figure This phenomenon is called refraction, and occurs because light travels faster in air than in water.

Although the refraction that occurs Water. When a diver loses his face mask, his eyes are immersed in water, which has about the same refrac tive index as the eye.

Consequently, the light is not focused normally and the divers vision is reduced to a level that would be Figure Objects Underwater classified as legally blind on the surface.

Appear Closer. Refraction can make objects appear closer than they really are. A distant object will appear to be approximately three-quarters of its actual distance. At greater distances, the effects of refraction may be reversed, making objects appear farther away than they actually are.

Reduced brightness and contrast combine with refraction to affect visual distance relationships. Refraction can also affect perception of size and shape. Generally, underwater objects appear to be about 30 percent larger than they actually are. Refraction effects are greater for objects off to the side in the field of view.

This distortion interferes with hand-eye coordination, and explains why grasping objects under water is sometimes difficult for a diver. Experience and training can help a diver learn to compensate for the misinterpretation of size, distance, and shape caused by refraction. Water turbidity can also profoundly influence underwater vision and distance perception. The more turbid the water, the shorter the distance at which the reversal from underestimation to overestimation occurs. For example, in highly turbid water, the distance of objects at 3 or 4 feet may be overestimated; in moderately turbid water, the change might occur at 20 to 25 feet and in very clear water, objects as far away as 50 to 70 feet might appear closer than they actually are.

Generally speaking, the closer the object, the more it will appear to be too close, and the more turbid the water, the greater the tendency to see it as too far away. Light scattering is intensified underwater.

Light rays are diffused and scattered by the water molecules and particulate matter. At times diffusion is helpful because it scatters light into areas that otherwise would be in shadow or have no illumination. Normally, however, diffusion interferes with vision and underwater photography because the backscatter reduces the contrast between an object and its background. The loss of contrast is the major reason why vision underwater is so much more restricted than it is in air. Similar degrees of scattering occur in air only in unusual conditions such as heavy fog or smoke.

Object size and distance are not the only characteristics distorted underwater. A variety of factors may combine to alter a divers color perception.

Painting objects different colors is an obvious means of changing their visibility by enhancing their contrast with the surroundings, or by camouflaging them to merge with the background.

Determining the most and least visible colors is much more complicated underwater than in air. Colors are filtered out of light as it enters the water and travels to depth. Red light is filtered out at relatively shallow depths. Orange is filtered out next, followed by yellow, green, and then blue.

Water depth is not the only factor affecting the filtering of colors. Salinity, turbidity, size of the particles suspended in the water, and pollution all affect the color-filtering properties of water. Color changes vary from one body of water to another, and become more pronounced as the amount of water between the observer and the object increases.

The components of any underwater scene, such as weeds, rocks, and encrusting animals, generally appear to be the same color as the depth or viewing range increases. Objects become distinguishable only by differences in brightness and not color. Contrast becomes the most important factor in visibility; even very large objects may be undetectable if their brightness is similar to that of the background. Mechanical energy mostly affects divers in the form of sound.

Sound is a periodic motion or pressure change transmitted through a gas, a liquid, or a solid. Because liquid is denser than gas, more energy is required to disturb its equilibrium. Once this disturbance takes place, sound travels farther and faster in the denser medium. Several aspects of sound underwater are of interest to the working diver. In any body of water, there may be two or more distinct contiguous layers of water at different temperatures; these layers are known as thermoclines.

The colder a layer of water, the greater its density. As the difference in density between layers increases, the sound energy transmitted between them decreases. This means that a sound heard 50 meters from its source within one layer may be inaudible a few meters from its source if the diver is in another layer.

When swimming in shallow water, among coral heads, or in enclosed spaces, a diver can expect periodic losses in acoustic communication signals and disruption of acoustic navigation beacons. The problem becomes more pronounced as the frequency of the signal increases. Because sound travels so quickly underwater 4, feet per second , human ears cannot detect the difference in time of arrival of a sound at each ear. Consequently, a diver cannot always locate the direction of a sound source.

This disadvantage can have serious consequences for a diver or swimmer trying to locate an object or a source of danger, such as a powerboat. Open-circuit SCUBA affects sound reception by producing high noise levels at the divers head and by creating a screen of bubbles that reduces the effective sound pressure level SPL. When several divers are working in the same area, the noise and bubbles affect communication signals more for some divers than for others, depending on the position of the divers in relation to the communicator and to each other.

A neoprene wet suit is an effective barrier to sound above 1, Hz and it becomes more of a barrier as frequency increases. This problem can be overcome by exposing a small area of the head either by cutting holes at the ears of the suit or by folding a small flap away from the surface.

Sound is transmitted through water as a series of pressure waves. High-intensity sound is transmitted by correspondingly high-intensity pressure waves. A high-pressure wave transmitted from the water surrounding a diver to the open spaces within the body ears, sinuses, lungs may increase the pressure within these open spaces, causing injury. Underwater explosions and sonar can create high-intensity sound or pressure waves. Low intensity sonar, such as depth finders and fish finders, do not produce pressure waves intense enough to endanger divers.

However, anti-submarine sonar-equipped ships do pulse dangerous, high- intensity pressure waves. Diving operations must be suspended if a high-powered sonar transponder is being operated in the area. When using a diver-held pinger system, divers are advised to wear the standard -inch neoprene hood for ear protection. Experiments have shown that such a hood offers adequate protection when the ultrasonic pulses are of 4-millisecond duration, repeated once per second for acoustic source levels up.

An underwater explosion creates a series of waves that are transmitted as hydraulic shock waves in the water, and as seismic waves in the seabed. The hydraulic shock wave of an underwater explosion consists of an initial wave followed by further pressure waves of diminishing intensity.

The initial high-intensity shock wave is the result of the violent creation and liberation of a large volume of gas, in the form of a gas pocket, at high pressure and temperature. Subsequent pressure waves are caused by rapid gas expansion in a non-compress ible environment, causing a sequence of contractions and expansions as the gas pocket rises to the surface.

The initial high-intensity shock wave is the most dangerous; as it travels outward from the source of the explosion, it loses its intensity. Less severe pressure waves closely follow the initial shock wave.

Considerable turbulence and movement of the water in the area of the explosion are evident for an extended time after the detonation. Some explosives have characteristics of high brisance shattering power in the immediate vicinity of the explosion with less power at long range, while the brisance of others is reduced to increase their power over a greater area. Those with high brisance generally are used for cutting or shattering purposes, while high-power, low-brisance explosives are used in depth charges and sea mines where the target may not be in immediate contact and the ability to inflict damage over a greater area is an advantage.

The high-brisance explosives create a high-level shock and pressure waves of short duration over a limited area. Low brisance explosives create a less intense shock and pressure waves of long duration over a greater area. Aside from the fact that rock or other bottom debris may be propelled through the water or into the air with shallow-placed charges, bottom conditions can affect an explosions pressure waves. A soft bottom tends to dampen reflected shock and pressure waves, while a hard, rock bottom may amplify the effect.

Rock strata, ridges and other topographical features of the seabed may affect the direction of the shock and pressure waves, and may also produce secondary reflecting waves. Research has indicated that the magnitude of shock and pressure waves generated from charges freely suspended in water is considerably greater than that from charges placed in drill holes in rock or coral. At great depth, the shock and pressure waves are drawn out by the greater water volume and are thus reduced in intensity.

An explosion near the surface is not weakened to the same degree. In general, the farther away from the explosion, the greater the attenuation of the shock and pressure waves and the less the intensity. This factor must be considered in the context of bottom conditions, depth of. A fully submerged diver receives the total effect of the shock and pressure waves passing over the body.

A partially submerged diver whose head and upper body are out of the water, may experience a reduced effect of the shock and pressure waves on the lungs, ears, and sinuses. However, air will transmit some portion of the explosive shock and pressure waves. The head, lungs, and intestines are the parts of the body most vulnerable to the pressure effects of an explosion.

A pressure wave of pounds per square inch is sufficient to cause serious injury to the lungs and intestinal tract, and one greater than 2, pounds per square inch will cause certain death. Even a pressure wave of pounds per square inch could cause fatal injury under certain circumstances. There are various formulas for estimating the pressure wave resulting from an explosion of TNT. The equations vary in format and the results illustrate that the technique for estimation is only an approximation.

Moreover, these formulas relate to TNT and are not applicable to other types of explosives. The formula below Greenbaum and Hoff, is one method of estimating the pressure on a diver resulting from an explosion of tetryl or TNT. Sample Problem. Determine the pressure exerted by a pound charge at a distance of 80 feet. Substitute the known values. When expecting an underwater blast, the diver shall get out of the water and out of range of the blast whenever possible.

If the diver must be in the water, it is prudent to limit the pressure he experiences from the explosion to less than 50 pounds per square inch. To minimize the effects, the diver can position himself with feet pointing toward and head directly away from the explosion. The head and upper section of the body should be out of the water or the diver should float on his back with his head out of the water.

Heat is crucial to mans environmental balance. The human body functions within only a very narrow range of internal temperature and contains delicate mechanisms to control that temperature. Heat is a form of energy associated with and proportional to the molecular motion of a substance.

It is closely related to temperature, but must be distinguished from temperature because different substances do not necessarily contain the same heat energy even though their temperatures are the same.

Heat is generated in many ways. Burning fuels, chemical reactions, friction, and electricity all generate heat. Heat is transmitted from one place to another by conduction, convection, and radiation. Conduction is the transmission of heat by direct contact. Because water is an excellent heat conductor, an unprotected diver can lose a great deal of body heat to the surrounding water by direct conduction.

Convection is the transfer of heat by the movement of heated fluids. Most home heating systems operate on the principle of convection, setting up a flow of air currents based on the natural tendency of warm air to rise and cool air to fall. A diver seated on the bottom of a tank of water in a cold room can lose heat not only by direct conduction to the water, but also by convection currents in the water. The warmed water next to his body will rise and be replaced by colder water passing along the walls of the tank.

Upon reaching the surface, the warmed water will lose heat to the cooler surroundings. Once cooled, the water will sink only to be warmed again as part of a continuing cycle.

Radiation is heat transmission by electromagnetic waves of energy. Every warm object gives off waves of electromagnetic energy, which is absorbed by cool objects.

Heat from the sun, electric heaters, and fireplaces is primarily radiant heat. To divers, conduction is the most significant means of transmitting heat. The rate at which heat is transferred by conduction depends on two basic factors:. Not all substances conduct heat at the same rate. Iron, helium, and water are excellent heat conductors while air is a very poor conductor. Placing a poor heat conductor between a source of heat and another substance insulates the substance and slows the transfer of heat.

Materials such as wool and foam rubber insulate the human body and are effective because they contain thousands of pockets of trapped air. The air pockets are too small to be subject to convective currents, but block conductive transfer of heat. A diver will start to become chilled when the water temperature falls below a seemingly comfortable 70F 21C.

Below 70F, a diver wearing only a swimming suit loses heat to the water faster than his body can replace it. Unless he is provided some protection or insulation, he may quickly experience difficulties. A chilled diver cannot work efficiently or think clearly, and is more susceptible to decompression sickness.

Suit compression, increased gas density, thermal conductivity of breathing gases, and respiratory heat loss are contributory factors in maintaining a divers body temperature. Cellular neoprene wet suits lose a major portion of their insulating properties as depth increases and the material compresses. As a consequence, it is often necessary to employ a thicker suit, a dry suit, or a hot water suit for extended exposures to cold water. The heat transmission characteristics of an individual gas are directly proportional to its density.

Therefore, the heat lost through gas insulating barriers and respira tory heat lost to the surrounding areas increase with depth. The heat loss is further aggravated when high thermal conductivity gases, such as helium-oxygen, are used for breathing.

The respiratory heat loss alone increases from 10 percent of the bodys heat generating capacity at one ata atmosphere absolute , to 28 percent at 7ata, to 50 percent at 21 ata when breathing helium-oxygen. Under these circumstances, standard insulating materials are insufficient to maintain body temperatures and supplementary heat must be supplied to the body surface and respiratory gas.

Pressure is defined as a force acting upon a particular area of matter. Underwater pressure is a result of the weight of the water above the diver and the weight of the atmosphere over the water. There is one concept that must be remembered at all timesany diver, at any depth, must be in pressure balance with the forces at that depth.

The body can only function normally when the pressure difference between the forces acting inside of the divers body and forces acting outside is very small. Pressure, whether of the atmosphere, seawater, or the divers breathing gases, must always be thought of in terms of maintaining pressure balance.

Given that one atmosphere is equal to 33 feet of sea water or Thus, for every foot of sea water, the total pressure is increased by 0. Atmospheric pressure is constant at sea level; minor fluctuations caused by the weather are usually ignored. Atmospheric pressure acts on all things in all directions. Most pressure gauges measure differential pressure between the inside and outside of the gauge.

Thus, the atmospheric pressure does not register on the pressure gauge of a cylinder of compressed air. The initial air in the cylinder and the gauge are already under a base pressure of one atmosphere The gauge measures the pressure difference between the atmosphere and the increased air pressure in the tank. This reading is called gauge pressure and for most purposes it is sufficient. In diving, however, it is important to include atmospheric pressure in computa tions.

This total pressure is called absolute pressure and is normally expressed in units of atmospheres. The distinction is important and pressure must be identified as either gauge psig or absolute psia. When the type of pressure is identified only as psi, it refers to gauge pressure. Table contains conversion factors for pressure measurement units. Four terms are used to describe gas pressure:.

Essentially the same as atmospheric but varying with the weather and expressed in terms of the height of a column of mercury. Standard pressure is equal to Indicates the difference between atmospheric pressure and the pressure being measured. The total pressure being exerted, i. The water on the surface pushes down on the water below and so on down to the bottom where, at the greatest depths of the ocean approximately 36, fsw , the pressure is more than 8 tons per square inch 1, ata.

The pressure due to the weight of a water column is referred to as hydrostatic pressure. The pressure of seawater at a depth of 33 feet equals one atmosphere. The absolute pressure, which is a combination of atmospheric and water pressure for that depth, is two atmospheres.

For every additional 33 feet of depth, another atmosphere of pressure Thus, at 99 feet, the absolute pressure is equal. Table 21 and Figure 27 shows how pressure increases with depth. The change in pressure with depth is so pronounced that the feet of a 6-foot tall person standing underwater are exposed to pressure that is almost 3 pounds per square inch greater than that exerted at his head.

Buoyancy is the force that makes objects float. It was first defined by the Greek mathematician Archimedes, who established that Any object wholly or partly immersed in a fluid is buoyed up by a force equal to the weight of the fluid displaced by the object. This is known as Archimedes Principle and applies to all objects and all fluids. According to Archimedes Principle, the buoyancy of a submerged body can be established by subtracting the weight of the submerged body from the weight of the displaced liquid.

If the total displacement the weight of the displaced liquid is greater than the weight of the submerged body, the buoyancy is 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 is neutral and the body will remain suspended in the liquid. If the weight of the submerged body is greater than that of the displaced liquid, the buoyancy is negative and the body will sink. The buoyant force on an object is dependent upon the density of the substance it is immersed in weight per unit volume.

Fresh water has a density of Sea water is heavier, having a density of Thus an object is 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 has a significant effect on buoyancy of a diver. A diver with full lungs displaces a greater volume of water and, therefore, is more buoyant than with deflated lungs. Individual differences that may affect the buoyancy of a diver include bone structure, bone weight, and body fat.

These differences explain why some individuals float easily while others do not. A diver can vary his buoyancy in several ways. By adding weight to his gear, he can cause himself to sink. When wearing a variable volume dry suit, he can increase or decrease the amount of air in his suit, thus changing his displacement.

Divers usually seek a condition of neutral to slightly negative buoyancy. Negative buoyancy gives a diver in a helmet and dress a better foothold on the bottom. Knowledge of the properties and behavior of gases, especially those used for breathing, is vitally important to divers. The most common gas used in diving is atmospheric air, the composition of which is shown in Table Any gases found in concentrations different than those in Table or that are not listed in Table are considered contaminants.

Depending on weather and location, many industrial pollutants may be found in air. Carbon monoxide is the most commonly encountered and is often present around air compressor engine exhaust. Care must be taken to exclude the pollutants from the divers compressed air by appropriate filtering, inlet location, and compressor maintenance. Water vapor in varying quantities is present in compressed air and its concentration is important in certain instances.

Carbon Dioxide 0. Nitrous Oxide 0. For most purposes and computations, diving air may be assumed to be composed of 79 percent nitrogen and 21 percent oxygen. Besides air, varying mixtures of oxygen, nitrogen, and helium are commonly used in diving. While these gases are discussed separately, the gases themselves are almost always used in some mixture.

Air is a naturally occurring mixture of most of them. In certain types of diving applications, special mixtures may be blended using one or more of the gases with oxygen. Oxygen O2 is the most important of all gases and is one of the most abundant elements on earth. Fire cannot burn without oxygen and people cannot survive without oxygen.

Atmospheric air contains approximately 21 percent oxygen, which exists freely in a diatomic state two atoms paired off to make one molecule. This colorless, odorless, tasteless, and active gas readily combines with other elements. Members Current visitors. Log in Register. Search titles only. Search Advanced search…. New posts. Search forums. Log in. Install the app. JavaScript is disabled. For a better experience, please enable JavaScript in your browser before proceeding.

You are using an out of date browser. It may not display this or other websites correctly. You should upgrade or use an alternative browser. US Navy Manual, Rev 7. Thread starter Akimbo Start date Dec 17, Please register or login Welcome to ScubaBoard, the world's largest scuba diving community.

Benefits of registering include Ability to post and comment on topics and discussions. A Free photo gallery to share your dive photos with the world. You can make this box go away Joining is quick and easy. Akimbo Just a diver Staff member. For up to two hours during the post-dive observation period, subjects were monitored at approximately minute intervals for VGE.

The actual examination times were at mean range 16 10—25 , 35 27—45 , 56 48—67 , 76 69—87 , 96 87— , and — minutes post-dive. For each examination, the subject reclined in the left decubital position while the heart chambers were imaged apical long-axis four-chamber view with a trans-thoracic two-dimensional 2-D echocardiogram.

VGE in the right heart chambers were graded according to an ordinal scale adapted from Eftedal and Brubakk[ 10 , 11 ] and defined in Table 2. At each examination, VGE in the right heart chambers were graded three times: after the subject had been at rest for approximately one minute and then after forceful limb flexions of the right elbow and the right knee.

For the flexion conditions, the grades were the maximum sustained for the following periods: grades 2 and 3 for at least four cardiac cycles; and grades 4a, 4b, and 5 for at least 0. The four cardiac cycle period follows from the grade 2 definition and the 0. Ninety-six man-dives were completed with no diagnosed incidents of DCS. Since stopping criteria were not reached, all dives were conducted on the 20 fsw last stop schedule.

It is conventional to express VGE as the peak grade of any examination time. The median interquartile range peak VGE grade at rest was 3 2, 3 , and the median peak VGE grade with movement was 3 3, 3. Figure 1 illustrates the VGE grades at each examination time and shows that the highest VGE measurements at rest typically occurred at the minute post-dive examination. VGE typically were detected throughout the two-hour post-dive observation period. At the end of the two-hour post-dive observation period, the median VGE grades remained elevated at grade 1 with rest and grade 3 with movement.

Figure 1 illustrates the significant inter-subject variability in VGE grades. The maximum VGE grade modified Eftedal Brubakk scale grades, y-axis of any exam for the rest condition and the movement condition. The box and whisker plots indicate the median, interquartile range, and the range. Five subjects had no observable VGE at the and minute examination times, therefore, in accord with the study protocol, they were not examined at the and minute examination times.

These five individuals were given scores of 0 for the last two exam times to calculate the medians and interquartile ranges in Figure 1. Three of the divers in the DNC study were treated with normobaric oxygen because of KM grade IV maximum grade that was still present one-hour post-dive. However, the DNC study reported suspicious symptoms of heaviness and unease in one diver that occurred and resolved overnight between medical examinations.

This latter difference motivates an examination of differences in the two study designs. The major differences were the total stop time, the number of dives in the preceding 48 hours, the water temperature and dress of the divers, and the diver activity levels on bottom and during ascent. While the NEDU study was conducted with a total stop time of 9 minutes at 20 fsw per the VVal Thalmann algorithm , the DNC study conducted 20 dives with a total stop time of 7 minutes at 6 msw in accord with USN-Rev6 and an additional 12 dives with a total stop time of 10 minutes 7 minutes at 6 msw and 3 minutes at a 3 msw safety stop.

The NMRI probabilistic-model-estimated P DCS of the three schedules are very similar see Table 1 , indicating that the differences in total stop time between the schedules are trivial. To participate in the NEDU study, all subjects had to refrain from hypo- or hyperbaric exposure for 48 hours preceding their experimental dive.

The only exceptions to this rule were three divers who participated in a study dive approximately 24—27 hours after surfacing from a preceding study dive that was aborted during descent. No cases of DCS were diagnosed following the first diving day. The DNC study reported no significant difference in VGE grades between the dive series group and the single dive group. While it has long been held that multi-day diving is a risk factor for DCS, there is also evidence that dives conducted during the preceding days actually lead to acclimatization and decrease DCS risk.

A large series of experimental air decompression dives established that colder temperature during decompression significantly increases the risk of DCS.

For the other 10 minutes of their bottom time, the DNC divers solved a jigsaw puzzle. The NEDU divers exercised on bottom and rested during decompression. Exercise during bottom time has been shown to increase the risk of DCS, possibly due to increased blood flow to and increased gas uptake by exercising muscles. In a departure from previous air decompression procedures, USN-Rev6 and USN-Rev7 introduced a 20 fsw last decompression stop instead of a 10 fsw last decompression stop due to the operational advantages of a deeper last decompression stop.

The 20 fsw last stop was promulgated without man-testing but on the basis of evaluation with probabilistic models of DCS, which showed no difference in the PDCS of the 20 fsw and 10 fsw last decompression stops. Testing of a short, deep air decompression schedule computed with the VVal Thalmann algorithm, tested under diving conditions similar to earlier US Navy dive trials, resulted in low incidence of DCS.

The views presented are those of the authors and do not necessarily represent the views of the US Department of the Navy. Conflict of interest: nil. National Center for Biotechnology Information , U. Journal List Diving Hyperb Med v. Diving Hyperb Med.