Units, Symbols, and Measuring Scales

Proceedings of the International Conference on Thermodynamics, CardiflF, 1—4th April, 1970, Pure Appl. Chem., 1970, 22, No. 3—4. 

163 j jjg International System of Units SI, translation approved by the International Bureau of Weights and Measures of its publication Le Systeme International d Unites , prepared jointly and published independently by the National Physical Laboratory, U.K. (Her Majesty s Stationery Office, London, 1970) and the National Bureau of Standards, U.S.A. (N.B.S. Special Publication 330,1970, U.S. Government Printing Office, Washington, D.C.). 

Of the derived units, it may be noted that the unit for force is the newton (N), which, expressed in terms of SI base units, is mkgs the unit for density or mass density is kilogram per cubic metre (kgm ) the unit for concentration (of amount of substance) is mole per cubic metre (mol m ) the unit for specific volume is cubic metre per kilogram (m kg ). Some other derived units have special names for example, the name for the unit of energy, work, or quantity of heat is the joule (J), equal to the newton metre (N m). The imit of power is the watt (W), equal to J s. The unit for heat capacity and entropy is the joule per kelvin (JK ) the unit for molar energy is joule per mole (J mol ) and for molar entropy and molar heat capacity is joule per kelvin mole (J mol ). 

The document cited above is the definitive statement of SI, but many other publications have appeared suggesting the style of usage of SI in different fields, and a few examples may be listed.  

The situation with respect to pressure in SI units and thermodynamics is, however, not so happy. The SI pressure unit is the newton per square metre (N m ) or pascal (Pa) and the quantity previously called the standard atmosphere is equal to 101.325 kNm . The difficulty arises because for many purposes (for example in the definition of enthalpies of formation) the standard pressure has been chosen as the standard atmosphere. Thus not only is the number 101.325 involved between the old and new systems, but no universal factor can be applied to change the value of a thermodynamic quantity for a real vapour from a pressure of 101.325 kN m to say 100kNm . Therefore it appears likely that for many years in the future thermodynamicists will have to head their tables, Standard pressure 101.325 kPa (formerly known as 1 standard atmosphere) . 

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