Temperature

The SI unit of temperature is Kelvin. It is usually referred to as the thermodynamic temperature and is used primarily in scientific applications. The Kelvin scale begins at 0, which is referred to as “absolute zero”, that is, there are no negative values of Kelvin unlike Fahrenheit and Celsius, which is more commonly used throughout the world.

http://physics.nist.gov/cuu/Units/kelvin.html

The definition of the unit of thermodynamic temperature was given in 1954, which selected the triple point of water as the fundamental fixed point and assigned to it the temperature 273.16 K, and in so doing defined the unit. (See the Phase Diagram on the right. Consider that at the precise temperature and pressure where the “triple point” exists all three forms (phases) of water are in equilibrium at the same time. The name Kelvin (symbol K) instead of "degree Kelvin, (symbol °K)" was adopted in 1967 and defined the unit of thermodynamic temperature as follows: The Kelvin, unit of thermodynamic temperature, is the fraction 1/273.16 of the thermodynamic temperature of the triple point of water.

Because of the way temperature scales used to be defined, it remains common practice to express thermodynamic temperature, T, in terms of its difference from the reference temperature T_o = 273.15 K, the freezing/melting point. This temperature difference is called a Celsius temperature (symbol °C), t in the equation below, and is defined by the equation

t= T- T_o

t (^oC)= T(K) - 273.15K

The unit of Celsius temperature is the degree Celsius, symbol °C, which is by definition equal in magnitude to the Kelvin. A difference in temperature at two different conditions may be expressed in Kelvins or in degrees Celsius. These are equal to each other.

ΔT = T_final - T_initial = Δt = t_final - t_initial

The numerical value of a Celsius temperature t expressed in degrees Celsius is given by

t/°C = T/K - 273.15.

In 1989/1990 the Kelvin and the degree Celsius were adopted as the International Temperature Scale.