As a consequence of the changing relationships between the state parameters and the high degree of compressibility, all data must, particularly in the case of gases, be referred to an accurately specified and agreed state. Normal volume Vn is the volume referred to at normal temperature and normal pressure. It is that state which is characterised by
- normal temperature Tn = 273.15 °K
- normal pressure Pn = 1.01325 bar and is also defined as the standard physical volume
Formula 2.4.1.1
Differing from the above, ISO 2787 prescribed the following for indicating a volume of air
- reference temperature Tn = 293.15 °K
- reference pressure Pn = 1.00 bar
and this was accepted as the internationally valid normal state. This basic reference is generally used and known as “technical and atmospheric normal state”. Unfortunately, in practical applications it is not always made clear to which definition reference is made. In principle, the standard volume describes the state of air under normal temperature and normal (atmospheric) pressure.
