Celsius Scale and Kelvin Scale

Temperature scales are defined by the numerical value assigned to a standard fixed point. By
international agreement the standard fixed point is the easily reproducible triple point of water:
the state of equilibrium between steam, ice, and liquid water (Sec. 3.2). As a matter of convenience, the temperature at this standard fixed point is defined as 273.16 kelvins, abbreviated as
273.16 K. This makes the temperature interval from the ice point1 (273.15 K) to the steam point2
equal to 100 K and thus in agreement over the interval with the Celsius scale discussed next,
which assigns 100 Celsius degrees to it. The kelvin is the SI base unit for temperature.
The Celsius temperature scale (formerly called the centigrade scale) uses the unit degree
Celsius (C), which has the same magnitude as the kelvin. Thus, temperature differences are
identical on both scales. However, the zero point on the Celsius scale is shifted to 273.15 K,
as shown by the following relationship between the Celsius temperature and the Kelvin
temperature
T(°C)=  T(K) -  273.15

From this it can be seen that on the Celsius scale the triple point of water is 0.01C and that
0 K corresponds to 273.15C

In the other hand

In view of the limitations of empirical means for measuring temperature, it is desirable
to have a procedure for assigning temperature values that does not depend on the properties of any particular substance or class of substances. Such a scale is called a thermodynamic temperature scale. The Kelvin scale is an absolute thermodynamic temperature scale
that provides a continuous definition of temperature, valid over all ranges of temperature.
Empirical measures of temperature, with different thermometers, can be related to the
Kelvin scale.
To develop the Kelvin scale, it is necessary to use the conservation of energy principle
and the second law of thermodynamics; therefore, further discussion is deferred to Sec. 5.5
after these principles have been introduced. However, we note here that the Kelvin scale has
a zero of 0 K, and lower temperatures than this are not defined.
The Kelvin scale and the gas scale defined by Eq. 1.18 can be shown to be identical in
the temperature range in which a gas thermometer can be used. For this reason we may
write K after a temperature determined by means of constant-volume gas thermometry.
Moreover, until the concept of temperature is reconsidered in more detail in Chap. 5, we
assume that all temperatures referred to in the interim are in accord with values given by
a constant-volume gas thermometer