Page 88

20047TC

increases and more dense as the temperature decreases, all the way down to the point where they change phase–from liquid to solid or from gas to liquid (or sometimes directly to solid). Water is the single notable exception to this rule. As fresh water is cooled, it becomes more dense until it reaches its maximum density at 4°C (39.2°F). As it is further cooled, its density decreases, and as a solid (ice) it is less dense than as a liquid. This is why ice floats on water rather than sinking to the bottom. The density of seawater is also affected by its salinity, which varies around the globe. The most marked departures from seawater density are actually in saline lakes, such as the Dead Sea and Great Salt Lake. Standard air density is measured at 0°C/32°F. Problems: SI/metric: What is the mass (weight) of one cubic centimeter of ocean water given the fact that ocean water has a density of 1.025 kg/L? Mass = 1 cc x 1.025 kg x 1 L x 1,000 g = 1.025 g 1 L 1,000 cc 1 kg U.S./Imperial: If 302 one-inch ice cubes weigh 10 pounds what is the density of the ice in pounds per cubic foot? Density = 10 lbs x 1,728 in3 = 57.2 lbs/ft3 302 in3 1 ft3 Mixed: What is the weight in kilograms of the air in a fully charged 80 cubic foot scuba cylinder? Mass = Density x Volume = 0.0807 lbs/ft3 x 80 ft3 = 6.46 lbs and converting from U.S. to SI: 6.46 lbs x 1 kg = 2.94 kg 2.2 lbs Sometimes, you will see the value for specific gravity instead of density. The specific gravity of an object is the ratio of the density of the object to the density of water. In the SI/metric system the number is the same as the density because the density of fresh water is 1 kg/L. In the U.S./Imperial system you must divide the density of the object by the density of fresh water (62.43 or 62.4 lbs/ft3) to find the specific gravity. Two phenomena of water density that are encountered by divers are thermoclines and haloclines. A thermocline is a zone of rapidly changing temperature. Warmer, less dense water floats on top of colder, denser water. In the absence of some mixing force, a thermocline can be quite stable, and the transition boundary can be very abrupt. Thermoclines are a common feature of fresh water lakes in which summer temperatures and solar energy warm the surface layer, while the colder layer remains underneath. A halocline is a similar layering of waters of differing salinity in which lighter fresh water floats on top of denser salt water. Haloclines are a very common occurrence in the underwater caves of the Yucatan peninsula and in places where gentle rivers or freshwater springs flow into sheltered salt water lagoons. When the boundary layer of a halocline is disturbed, as by a diver swimming through the interface, the visual effect as the layers begin to mix is similar to frosted glass. BUOYANCY You are already familiar with buoyancy. As a beginning diver, you learned that things that are lighter than water float, and things that are heavier than water sink. You, as a responsible as well as a comfortable diver, were to strive for what was called neutral buoyancy. You also learned that you would have to add more weight when you moved from fresh water to salt water to compensate for increased buoyancy. Everyone understands something of buoyancy. We know that a piece of wood floats. We know that a boat floats unless it gets a hole in its bottom. Then it will fill with water and sink. You may also have noticed that any object you pick up underwater does not weigh as much in the water as it does when you lift it out of the water into the air. These are all examples of the phenomenon of buoyancy. Buoyancy is the force that allows lighter objects to float and that causes heavy objects to weigh less when immersed in water. It was first defined by the Greek mathematician Archimedes, who lived in Syracuse (on Sicily) in the third century BCE. The story is that he used the principle of buoyancy to determine the proportion of gold in a gold and silver wreath crafted for Hieron II, the king of Syracuse. The story also is that the con- NAUI Master Scuba Diver 76 Diving Physics


20047TC
To see the actual publication please follow the link above