Temperature and pressure scales

Remember that all equations in thermodynamics use the absolute or kelvin temperature scale, so that if you are given temperatures in °C, you must convert them to the kelvin scale before using them. The standard temperature of 25 °C for example is 298.15 K. 

Force is measured in newtons (N), where 1 newton will give a mass of 1 kg an acceleration of 1 msec . Pressure is defined as force per unit area, and a pressure of 1 newton per square meter (IN m ) is called 1 pascal (1 Pa). This is a very small pressure, and older, larger pressure units are still in use. 

The bar, for example, is 10 Pa and is almost equal to the standard atmosphere (1 atm = 1.01325 bar). Weather reports in many countries give the atmospheric pressure in kilopascals (kPa), and it is usually close to 101 kPa, or 1 atm, or 1 bar. These units are summarized in Appendix A. 

The standard temperature and pressure chosen for reporting values of thermodynamic variables is now 25 °C and 0.1 MPa. A pressure of 0.1 MPa is 100 kPa and 10 Pa, or 1 bar. It is convenient to use bars instead of pascals, because the bar is essentially the same as atmospheric pressure, and the notation is slightly simpler. 

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Suppose, for example, an engineer wants to plot process yield contours as a function of temperature and pressure. He can hardly say that, for instance, the experiment (200°F., 3 atm.) is farther from (400°F., 1 atm.) than from (500°F., 4 atm.). For this reason he will probably select temperature and pressure scales to fit whatever graph paper happens to be in his desk. 

Paluch, M., Patkowski, A., and Fischer, E. W. (2000) Temperature and Pressure Scaling of the a Relaxation Process in Fragile Glass Formers A Dynamic Light Scattering StudyRev. Lett. 85, 2140-3143. 

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