Temperature is a dominant factor affecting the physical properties of fluids. It is of particular concern when calculating changes in the state of gases.
The three temperature scales used extensively are the centigrade, the Fahrenheit, and the absolute or Kelvin scales. The centigrade scale is constructed by using the freezing and boiling points of water, under standard conditions, as fixed points of zero and 100, respectively, with 100 equal divisions between. The Fahrenheit scale uses 32° as the freezing point of water and 212° as the boiling point, and has 180 equal divisions between. The absolute or Kelvin scale is constructed with its zero point established as -273° C, or -459.4° F below the freezing point of water. The relationships of the other fixed points of the scales are shown in B of figure 7-5.
Absolute zero, one of the fundamental constants of physics, is commonly used in the study of gases. It is usually expressed in terms of the centigrade scale. If the heat energy of a given gas sample could be progressively reduced, some temperature would be reached at which the motion of the molecules would cease entirely. If accurately determined, this temperature could then be taken as a natural reference, or as a true "absolute zero" value.
Experiments with hydrogen indicated that if a gas were cooled to -273.16° C (used as -273° for most calculations), all molecular motion would cease, and no additional heat could be extracted from the substance.
When temperatures are measured with respect to the absolute zero reference, they are expressed as zero in the absolute or Kelvin scale. Thus, absolute zero may be expressed as 0° K, as -273° C, or as -459.4° F (used as -460° for most calculations).
When working with temperatures, always make sure which system of measurement is being used and know how to convert from one to another. The conversion formulas are shown in B of figure 7-5. For purposes of calculations, the Rankine scale illustrated in A of figure 7-5 is commonly used to convert Fahrenheit to absolute. For Fahrenheit readings above zero, 460° is added. Thus, 72° F equals 460° plus 72°, or 532° absolute. If the Fahrenheit reading is below zero, it is subtracted from 460°. Thus -40° F equals 460° minus 40°, or 420° absolute. It should be stressed that the Rankine scale does not indicate absolute temperature readings in accordance with the Kelvin scale, but these conversions may be used for the calculations of changes in the state of gases.
The Kelvin and centigrade scales are used more extensively in scientific work; therefore, some technical manuals may use these scales in giving directions and operating instructions. The Fahrenheit scale is commonly used in the United States, and most people are familiar with it. Therefore, the Fahrenheit scale is used in most areas of this text.