A pressure of -1 bar g corresponds closely to a perfect vacuum. Conversely a vacuum or negative pressure is the pressure below that of the atmosphere.
Pressures above atmospheric will always yield a positive gauge pressure. Gauge pressure = Absolute pressure - Atmospheric pressure This is the average pressure exerted by the air of the earth’s atmosphere at sea level. Although in reality the atmospheric pressure will depend upon the climate and the height above sea level, a generally accepted value of 1.013 25 bar a (1 atm) is often used. This is the pressure measured from the datum of the atmospheric pressure. a perfect vacuum has a pressure of 0 bar a. This is the pressure measured from the datum of a perfect vacuum i.e. Internal energy (specific internal energy) Hertz, the unit of frequency (number of cycles per second) Specific heat capacity at constant volumeĭiameter of the circular cross section of a conduit Specific heat capacity at constant pressure Table 2.1.5 Symbols and units of measure used in The Steam and Condensate Loop SymbolĬross sectional area of a conduit,for the operating condition These quantities have all been assigned special names after famous pioneers in the development of science and engineering. Table 2.1.1 shows the derived units that are relevant to this subject, all of which should be familiar to those with any general engineering background. These may be familiar to readers with a background in electronics, chemistry and physics respectively, but have little relevance to steam engineering nor the contents of The Steam and Condensate Loop. The other SI base units are electric current (in amperes), amount of substance (in moles) and luminous intensity (in candela). The first three will hopefully need no further explanation, while the latter will be discussed in more detail later. The SI base units include length (in metres), mass (in kilograms), time (in seconds) and temperature (in kelvin). In the SI system there are seven well-defined base units from which the units of other properties can be derived, and these will be used throughout this publication. The problems this caused led to the development of an agreed international system of units (or SI units: Système International d’Unités).
Throughout the engineering industries, many different definitions and units have been proposed and used for mechanical and thermal properties.
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