Diving physics
The term diving physics any special conditions of non-living nature are grouped under water, where a diver , particularly a scuba diver , is exposed.
Knowledge of special laws or properties under water is important for danger-conscious diving. Diving physics is part of the basic training of divers. The rules of conduct derived from this must be observed by divers in order to minimize the risk of accidents for themselves and their diving partners .
Properties of solids
boost
The static buoyancy of a body surrounded by water counteracts the weight force, the downforce, and corresponds to the weight force of the displaced water ( Archimedes principle ). If the buoyancy force is less than the weight, the object sinks. If it is bigger, it rises. Due to the equipment carried , the diver has a greater weight than a swimmer in swimwear, for example. Its volume is also considerably larger. The volume of the equipment is usually not balanced by its weight, so additional lead weights are used to increase downforce. In order to be able to control the buoyancy under water, the diver must have a controllable volume with air available with which buoyancy or downforce can be achieved. Such a controllable air bubble is usually represented by a buoyancy compensator that can be filled with the compressed air bottle . A second controllable bladder, more suitable for fine-balancing, is the diver's lungs. The buoyancy compensator can be used to achieve an unstable balance . A balance between buoyancy and downforce can only be achieved by continuously fine-tuning the lung volume, so the diver hovers at a constant depth. Only an unstable equilibrium is achieved, since gas volumes are compressed with increasing pressure (i.e. greater depth) and thus generate less lift. An initially weak downforce grows more and more, the buoyancy behaves analogously, an initially weak uplift increases itself. If a diver did not counterbalance with downforce, he would sink faster and faster to the bottom, with uplift he would ultimately with increasing speed shoot like a cork to the surface of the water. That is why the buoyancy control has to be adjusted very frequently during a dive so that one does not unintentionally ascend or descend when the depth changes. The volume of air in the BC, and thus its buoyancy, changes depending on the pressure of the surrounding water. If the diver wears a wetsuit , this also changes its volume with the depth, as the air bubbles enclosed in the material are more or less compressed. Finally, during the course of the dive, the weight of the diving cylinder also changes because the diver uses up the breathing gas and is therefore significantly lighter at the end of the dive than at the beginning.
Properties of water
Hydrostatic pressure
The water pressure increases by about 1 bar per 10 m depth. Theoretically, it is 0.98 bar at 10 m in pure fresh water and up to about 1.03 bar in sea water because of the salinity . For the scuba diver, it is usually sufficient to calculate with a value of 1 bar. The diving depth, and therefore the water pressure, is one of the most important criteria that the diver must consider when planning and performing a dive .
Thermal conductivity
The thermal conductivity of water (0.6 W / (m · K) at 20 ° C) is significantly higher than that of air. The human body gives off more heat to the water around it than to the air, which is why a diver cools down quickly if he is not protected by a diving suit . Possible consequences are cramps in the limbs and hypothermia .
sound
The speed of sound under water (1483 m / s at 20 ° C) is 4.4 times as great as in air. On land, humans can locate the source of the sound because the sound is received by each ear at minimally different times. The brain develops an idea of the spatial location of the sound source from the transit time difference . In water, this spatial perception is difficult or impossible due to the higher speed of sound. That is why every noise underwater sounds like it is in your own body. The diver must therefore rely on their eyes and experience to find the source of a sound.
Assuming that the human ear can perceive a transit time difference of at least 300 microseconds, a distance from the eardrum of at least 44.5 millimeters would be necessary for directional hearing. That is why it is not infrequently stated in the specialist literature that the difference in transit time in water is so small that it can no longer be perceived by the sense of hearing. In some cases, however, a minimal perceptible transit time difference of 10 microseconds is assumed, which means that a distance from the eardrum of just 1.4 millimeters would be sufficient. This would mean that directional hearing underwater is significantly impaired by the fact that the brain has only learned to hear in the air and therefore misinterprets the changed transit time differences in the water.
The conductivity of water for sound is greater than that of air, but also more dependent on the frequency . Low frequencies are conducted many times better than high frequencies. The diver may therefore hear noises that are generated several kilometers away, for example the engine noises of large ships.
viscosity
Due to the internal friction ( viscosity ) of the water, moving under water requires significantly more effort than moving on land. However, higher energy consumption also means higher air consumption, which is why divers strive to move as efficiently as possible in order to avoid overexertion. The viscosity and the mass of the water also ensure that it is difficult to counter a stronger current. Swimming at right angles to the direction of the current or moving close to the bottom can help with strong currents because the currents there are not as strong as in open water due to the friction.
density
The density of the water changes with its temperature (water is most dense at +4 ° C), but this has no practical significance for the diver. However, the salt content ( salinity ) has a noticeable effect: salt water with up to 1350 kg / m³ has a higher density than fresh water with 1000 kg / m³. This is why a certain pressure is already reached in salt water at a shallower water depth than in fresh water. Since all common depth gauges are based on water pressure, many must be set up for use in seawater or freshwater before they can be used.
Influencing the light
refraction
When passing from an optically thinner into an optically denser medium, light waves are refracted towards the perpendicular , and when passing from the denser to the thinner medium, they are diverted away from the perpendicular. When diving, this refraction of light is noticeable in the fact that objects in the water are apparently larger and closer than they actually are, because the light enters the air space between the diving mask and the eyes from the water through the glass of the diving mask . Sometimes the phenomenon can also be observed the other way round: If you dive in a lake with a relatively calm water surface and you look up to the water surface, you can see details on the shore, but optically distorted and apparently at a greater distance. The angle from which objects can be seen outside the water is limited. If the angle of refraction is more than 48.3 ° to the perpendicular, total reflection occurs: Objects that are in the water are reflected on the surface of the water.
The refractive index of water changes with its density , and the density is dependent on temperature and salinity . If water of different densities mixes, a separating layer or streaks can be observed, similar to the optical effects in the heated air above a candle flame. In this way, for example, a submarine fresh water source or a "cold water lake" can be identified.
Absorption and scattering
Depending on its wavelength, water absorbs light to different degrees, the greater the wavelength , the more so. Red light loses 50% of its intensity per meter. The colors are so strongly reduced by absorption that red from 3 m, orange from 5 m, yellow from 8 m, purple from 18 m, green from 35 m and blue from 60 m can no longer be recognized. The particularly short-wave violet is an exception here, as it is particularly strongly scattered. The scattering of light in (clean) water decreases as the wavelength increases. Blue and violet are therefore scattered the most. If the water is also clouded by suspended matter (e.g. plankton ), the scattering increases and the color green penetrates the deepest, as it is least affected by the combined effect of light scattering and absorption. In order to still be able to see all the colors in greater depth, the use of a diving lamp helps .
Properties of compressed gases
volume
According to Boyle-Mariotte's law , the product of volume and pressure is constant for gases. This means that the two parameters pressure and volume are inversely proportional to each other: If you double the pressure of a given amount of gas, its volume is reduced by half. Since the pressure is increased at depth, but the maximum lung volume always remains the same, the diver needs more air to fill his lungs (at a depth of 10 m, twice as much as on land at sea level). One consequence of this is that the air supply carried in the compressed air cylinder runs out faster the greater the diving depth. The pressure must be equalized not only in the lungs, but also in the cavities of the head. ( Middle ear , frontal sinuses, etc.) This applies to increasing pressure, i.e. when descending, as well as decreasing pressure during ascending. An important behavior when surfacing is: “Never hold your breath!” If you violate this rule, there is a risk of barotrauma , e. B. a ruptured lung , because the expanding air cannot escape when ascending. As a result of the pressure increase in the outer ear when diving, a relative negative pressure arises in the middle ear. The eardrum is stretched towards the middle ear. The diver equalizes the pressure by holding his nose, closing his mouth and at the same time lightly pressing his breath. If he descends without pressure compensation, the eardrum threatens to tear.
Partial pressures
The air we breathe normally is a mixture of different gases - it contains 78% N 2 (nitrogen), 21% O 2 (oxygen) and a small proportion of other gases. Only the oxygen content is physiologically effective above the water surface. In compressed air, the proportions of the gases do not change, but their amount of substance does. The Dalton's law states that the total pressure of a gas is made up of the partial pressures existing in this gas individual gases. For example, there is a pressure of about 5 bar at a depth of 40 m. With an oxygen content of 21% in the breathing air, this means a partial pressure of this gas of 5 × 0.21 bar = 1.05 bar. At sea level, this corresponds to breathing pure oxygen above the surface of the water. Since oxygen is an aggressive gas, there may be a great depth in oxygen poisoning occur. The first damage to the lung tissue occurs when pure oxygen with a partial pressure of 1.6 bar is inhaled for more than 45 minutes. Also due to the higher partial pressure, the danger of nitrogen anesthesia increases with increasing depth (compare deep intoxication ).
Solution in liquids
The higher the gas pressure, the more gas molecules are dissolved in a liquid. This law was discovered by William Henry and is named after him. The consequence for the diver is that in the depths - when he breathes air under higher pressure - the nitrogen contained in it in particular accumulates in the blood, in the muscle tissue, in nerve cells, in fat and in the bones. If the pressure is then reduced again when surfacing, the ability of the human body tissue to dissolve nitrogen also decreases. It is slowly released and exhaled through the lungs. It can take more than 24 hours for a diver to exhale all nitrogen from their body after one or more dives. It is very important to consider the nitrogen content of the body depending on the depth and time of the dive. If too much nitrogen is dissolved in the body and / or if the pressure is relieved (surfacing) too quickly, the excess gas cannot be completely released through the lungs, and microscopic bubbles form in the blood. When these bind together to form larger vesicles, embolism leads to a life-threatening condition called decompression sickness . In order to give the body time to desaturate nitrogen and to prevent the formation of bubbles, one or more breaks, so-called decompression stops , must be observed when surfacing from a certain level of saturation , during which the depth is kept constant. Since air is still being breathed from the compressed air cylinder that you have brought with you, you must take these decompression stops into account when planning the dive so that you do not have to return prematurely due to a lack of air. Recreational divers try to dive often within the no-decompression limit to minimize the risks. However, decompression tables and dive computers allow you to calculate the nitrogen saturation level before or during the dive and to adapt your diving behavior accordingly. Even changing diving depths and surface breaks are taken into account by modern dive computers.
temperature
The pressure of a tightly enclosed volume of gas increases when it is heated and falls when it is cooled ( Gay-Lussac's law ). Conversely, one can conclude from this: The temperature of a tightly enclosed gas volume increases with increasing pressure and decreases with decreasing pressure. Since a compressed air cylinder for scuba diving typically has a pressure of 200 bar or 300 bar when filled, but only 4 bar even at a depth of 30 m, the breathing air is greatly relieved when it is withdrawn from the cylinder and therefore cools down. This encourages the regulator to freeze up . Especially when diving in cool and cold waters, this can lead to the uncontrolled release of air or, in the worst case, to the blockage of any air supply. Carrying a second regulator ( called an octopus ) with you and diving in the buddy system greatly reduces the risk of suffocation that comes from frozen machines.
Thermal conductivity
The thermal conductivity of gases increases with their density. In the depths, the diver breathes compressed, denser air that is warmed up in the lungs. That is why he loses more heat than usual when he breathes: the compressed air cools the inner surface of the lungs more than uncompressed air. In addition, the inhaled air is comparatively cold due to the pressure relief that took place shortly beforehand when it was withdrawn from the high-pressure bottle (see also valve icing ). This effect is not compensated by any of the usual equipment used by recreational divers.
density
The higher the gas pressure, the greater the viscosity of the breathing gas, which causes a “slower” flow of the gas and thus an increase in breathing resistance . This can lead to exhaustion of the respiratory muscles and thus to breathing problems.
Individual evidence
- ^ Archimedes. (No longer available online.) Archived from the original on November 25, 2010 ; Retrieved May 19, 2011 . Info: The archive link was inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice.
- ↑ a b The density of sea water. (No longer available online.) Archived from the original on October 14, 2008 ; Retrieved May 20, 2011 . Info: The archive link was inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice.
- ↑ a b pressure. (No longer available online.) Archived from the original on August 11, 2011 ; Retrieved May 19, 2011 . Info: The archive link was inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice.
- ↑ a b Properties of water in tables. Retrieved May 20, 2011 .
- ↑ a b temperature and thermal conductivity. (No longer available online.) Archived from the original on February 25, 2014 ; Retrieved May 19, 2011 . Info: The archive link was inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice.
- ↑ a b Sound and Hear. (No longer available online.) Archived from the original on July 23, 2010 ; Retrieved May 19, 2011 . Info: The archive link was inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice.
- ↑ Ortwin Khan: Preparation for the medical exam for TL1 (PDF; 12.2 MB). htsv.de, February 11, 2012, p. 69.
- ↑ Thomas Kromp , Hans J. Roggenbach, Peter Bredebusch: Practice of diving. 3. Edition. Delius Klasing Verlag, Bielefeld 2008, ISBN 978-3-7688-1816-2 , p. 191.
- ^ Thomas Görne: Tontechnik Hanser Verlag, page 118
- ↑ Hubertus Bartmann: Diver Manual , 74 Erg.-Lfg., Chapter II-1.7.2.2
- ↑ a b Seeing underwater. Retrieved May 19, 2011 .
- ↑ partial pressure. (PDF; 322 KB) (No longer available online.) Archived from the original on December 3, 2012 ; Retrieved May 19, 2011 . Info: The archive link was inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice.
- ^ Dalton. (No longer available online.) Archived from the original on July 23, 2010 ; Retrieved May 19, 2011 . Info: The archive link was inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice.
- ↑ Henry. (No longer available online.) Archived from the original on July 23, 2010 ; Retrieved May 19, 2011 . Info: The archive link was inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice.
- ↑ Gay-Lussac. (No longer available online.) Archived from the original on August 19, 2011 ; Retrieved May 19, 2011 . Info: The archive link was inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice.
