is: a temperature scale based on measurements made with real gases using a constant-volume gas thermometer, and extrapolated to the limit of zero pressure in which the gas may be considered to be an ideal gas. [P7.2]
is defined: by the thermometric relation
where 
is the pressure of a fixed volume of gas at temperature 
and 
is the pressure of the same sample of gas, occupying the same volume, at the triple-point temperature of 
which is defined to be 
[P7.2]
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