Machinery's Handbook, 31st Edition
2870
Temperature Units of Temperature
There are two thermometer scales in general use: the Fahrenheit (°F), which is used in the United States and in other countries still using the English system of units, and the Celsius (°C) or Centigrade used throughout the rest of the world. In the Fahrenheit thermometer, the freezing point of water is marked at 32 degrees on the scale and the boiling point, at atmospheric pressure, at 212 degrees. The distance between these two points is divided into 180 degrees. On the Celsius scale, the freezing point of water is at 0 degree and the boiling point at 100 degrees. The following formulas may be used for converting temperatures given on any one of the scales to the other scale: ° F C Tables on the pages that follow can be used to convert degrees Celsius into degrees Fahrenheit or vice versa, and to convert the absolute temperatures of Kelvin and Rankine scales. In the event that the conversions are not covered in the tables, use those applicable portions of the formulas given above and in Table 52 for converting. Table 52. Temperature Conversion Fomulas To Convert To Use Formula To Convert To Use Formula 9 5 -- ° = C+32 5 9 -- ° F– 32 ( ) =
t K = t C + 273.15 t F = 1.8 t C + 32
K, t K ° F, t F ° R, t R K, t K ° C, t C ° R, t R
° C, t C ° F, t F ° R, t R K, t K ° C, t C ° F, t F
t C = t K − 273.15 t F = 1.8 t K − 459.67
Celsius, t C
Kelvin, t K
t R = 9( t C + 273.15)/5 t K = ( t F + 459.67)/1.8
t R = 9/5 3 t K t K = 5/9 3 t R
t C = ( t F − 32)/1.8 t R = t F + 459.67
t C = 5/9 3 t R − 273.15
Fahrenheit, t F
Rankine, t R
t F = t R − 459.67
Kelvin temperatures are expressed by K without use of the degree symbol (°). Absolute Temperature and Absolute Zero.— A point has been determined on the thermometer scale, by theoretical considerations, that is called the absolute zero and beyond which a further decrease in temperature is inconceivable. This point is located at − 273.15 degrees Celsius or − 459.67 degrees F. A temperature reckoned from this point, instead of from the zero on the ordinary thermometers, is called absolute temperature. The absolute temperature scale corresponding to Celsius is the Kelvin scale; the absolute scale corresponding to Fahrenheit is the Rankine. 0 K and 0 °R are both absolute zero. Conversions from Celsius and Fahrenheit are obtained as follows: K ° = C+273.15 ° R ° = F+459.67 Measures of the Quantity of Thermal Energy.— The unit of quantity of thermal energy used in the United States is the British thermal unit, which is the quantity of heat or thermal energy required to raise the temperature of one pound of pure water one degree F. (American National Standard abbreviation, Btu; conventional British symbol, B.Th.U.) The French thermal unit, or kilogram calorie, is the quantity of heat or thermal energy required to raise the temperature of one kilogram of pure water one degree C. One kilogram calorie = 3.968 British thermal units = 1000 gram calories. The number of foot-pounds of mechanical energy equivalent to one British thermal unit is called the mechanical equivalent of heat and equals 778 foot-pounds. In the modern metric or SI system of units, the unit for thermal energy is the joule (J); a commonly used multiple being the kilojoule (kJ), or 1000 joules. See page 2831 for an explanation of the SI System. One kilojoule = 0.9478 Btu. Also in the SI System, the watt (W), equal to joule per second (J/s), is used for power, where 1 watt = 3.412 Btu per hour.
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