conducts heat very well, an unprotected diver loses heat to the water about 25 times faster than he or she would in the air, but also because water has a very high heat capacity, the water warms very slowly and the diver keeps losing heat. The surface temperature of your body is maintained at about 33°C/92°F. If the water temperature is any less than this, you will slowly give up heat to the water. At colder water temperatures heat loss is progressively more rapid. Becoming chilled in diving can be dangerous as it progressively affects your ability to function, make judgements, and think. A rule of thumb on the dangers of cold water that is sometimes heard in rescue work is: “In 50 degree Fahrenheit/10°C water, an unprotected person has a 50 percent chance of being alive after 50 minutes.” Heat is generated in many ways. Remember that heat is actually energy in transition, and it always moves from places of higher temperature into places of lower temperature. There are three modes of heat transfer: conduction, convection, and radiation. Conduction is the transfer of heat by direct contact in solids or fluids at rest. Energy is transferred between the molecules of the substance from areas of higher energy to areas of lower energy. Heat is conducted from a diver to the cooler water environment, and the diver loses heat to the water. As discussed above, this heat transfer is rapid in part because the water is very much denser than the air and conducts heat better, and it has a high heat capacity. The diver’s heat loss is very rapid and continuous. Divers also lose heat to the air that they breathe through their regulator. The air in the cylinder is cool, and the inspired air is cooled further by expansion from the cylinder. The diver must warm the air to body temperature as well as moisturize it as it moves through the airway to the lungs. Heat transmission in a gas is directly proportional to its density. In a denser gas, the molecules are closer together and react more frequently. Therefore, respiratory heat loss increases with a diver’s depth and the increasing density of the inspired air. The rate at which heat is transferred through an object or from warmer objects to cooler areas is its thermal conductivity. Thermal conductivity is expressed as SUBSTANCE CP (cal/g° C) Air 0.3439 Argon 0.1252 Helium 1.2420 Nitrogen 0.2477 Oxygen 0.2200 the amount of heat that will be transferred across a given distance of material in a certain amount of time. An object with a low thermal conductivity is a poor heat conductor and a good thermal insulator. A material with a high thermal conductivity transmits heat rapidly. Water is a very good heat conductor, so is metal; still air is a poor heat conductor and a very good thermal insulator. Foam neoprene, with its trapped nitrogen bubbles, is a very effective insulator and is used in wetsuits for that reason. The wetsuit forms an insulating barrier between the diver and the water and slows the loss of heat. Unfortunately, with increasing depth foam neoprene is compressed and the loss of thickness or loft results in a corresponding loss of insulating properties. Wetsuits do not offer as much thermal protection at depth as in shallow water. Convection is the transfer of heat in liquids or gases in a state of motion. As a liquid or gas is heated, the less dense, warmer fluid moves away from the heat source and is replaced with the colder fluid. A convection current results, and the warmer water, air, or other fluid circulates and gives up its heat to the cooler surroundings with which it comes into contact. An unprotected diver’s heat loss is in some part due to convection, for even if the diver remains still, the water that is warmed to any degree moves away from the diver and is replaced by cold water. A diver wearing a wetsuit or a dry suit is limiting heat loss by limiting direct contact with the water and also limiting convective flow of water. Radiation is the transfer of heat that takes place without any material carrier. It is the transmission of heat by electromagnetic waves of energy such as from the sun. Most of the radiant heat energy experienced by NAUI Master Scuba Diver 100 Diving Physics TABLE 3-1. EXAMPLES OF GAS HEAT CAPACITIES AT 25°C AND ONE ATM PRESSURE
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