Fahrenheit temperature can be converted to Celsius and vice versa with a pair of simple formulas: °F = (9/5 x °C) + 32 °C = (°F – 32) x 5/9 The third temperature scale is the Kelvin scale, named after William Thomson, 1st Lord Kelvin, a Scottish scientist who worked with J. P. Joule in the midnineteenth century to produce a temperature scale based on the laws of thermodynamics rather than phase transition points of water. Their work produced the concept of absolute zero, the temperature below which it was not possible to go, and absolute zero provides the zero point for the Kelvin scale. At absolute zero the molecules of a substance are at their minimal energy, and no energy is available for transfer out of the system. The Kelvin scale is the basic scale in International System, and the kelvin (K) is the basic SI unit of temperature (with lower case spelling and no degree sign is required since it is the SI defining unit). The Kelvin scale is related to the Celsius scale because it also has a 100-degree interval between the freezing point and boiling point of water. Absolute zero is – 273.15° on the Celsius scale; the freezing point of water is +273.15K on the Kelvin scale. Kelvin temperatures are used in science because many formulas, including those you will encounter later in this chapter, use “absolute temperature” in order to begin their reference at absolute zero. To convert a Celsius temperature to kelvin: K= °C + 273.15 (or 273 as an approximation) Also in the nineteenth century, William Rankine promoted an absolute temperature scale based on the Fahrenheit scale. In the Rankine scale the value of absolute zero is – 459.67°F. Water freezes at 491.67°R (459.67 + 32) and boils at 671.67°R (459.67 + 212). To convert from a Fahrenheit temperature to its Rankine equivalent: °R = °F + 459.67 (or 460 as an approximation) DENSITY Chapter 3- Diving Physics Kelvin Rankine Density is mass per unit volume. We often use “heaviness” when we mean density (“Iron is heavier than wood.”), although heaviness can be an expression of total weight of specific objects (“Johannes is heavier than Adriana.”). Expressed as a formula: Density = Mass Volume In the SI/metric system, density is usually expressed in kilograms per liter (kg/L), rather than kilograms per cubic meter (kg/m3). This produces more useful sized units and has the advantage of being numerically equal to grams per cubic centimeter (gm/cc). In the U.S./Imperial system density is normally expressed in pounds per cubic foot (lbs/ft3). Some useful values: Density of fresh water = 1 kg/L = 62.43 lbs/ft3 (usually 62.4 lbs/ft3 is used) Density of seawater = 1.025 kg/L = 64 lbs/ft3 Density of dry air (sea level) = 1.2929 kg/m3 = 0.0807 lbs/ft3 These values are not constant. The density of water and air vary somewhat with temperature. Virtually all liquids and gases become less dense as the temperature Diving Physics 75 Fahrenheit Celsius 212°F 100°C 373K 672°R 32°F 0°C 273K 492°R FIGURE 3-3. FREEZING AND BOILING POINT OF WATER ON FOUR DIFFERENT TEMPERATURE SCALES
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