In addition to the thermodynamic temperature, the unit of Celsius temperature (symbol
°C) is defined by the equation t = T - T_{0}, where T_{0} = 273.15K (kelvin) by definition.
The Celsius temperature scale first came from the Swedish astronomer
Anders Celsius (1701 – 1744) who invented his Celsius thermometer, with 0 for the boiling point of water and 100 for the freezing point. In 1948, the 9th CIPM
adopts the name degree Celsius for the Celsius temperature scale and the zero of
Celsius temperature to be 0.01 degree below the triple point. The degree Celsius is equal in magnitude to the kelvin. An interval
of difference of temperature may be expressed in kelvins or degrees Celsius. In
this case, degree Celsius is a special name used in place of kelvin.

Celsius has the relationships with other temperature units,

F=(9/5*t)+32

T=t+273.15

R=(9/5)*t+491.67

T=t+273.15

R=(9/5)*t+491.67

Fahrenheit is named for the German physicist Daniel Gabriel Fahrenheit (1686–1736) who invented the mercury thermometer and created the scale in 1724

Each point on the scale is 5⁄9 the size of a Celsius degree. In Fahrenheit, 32°F degrees represents the freezing point of water and 212°F for the boiling point. The human body is about 98.6 °F.

Fahrenheit was widely used in weather, science and medical in the majority of English speaking countries until the late 1960s and early 1970s when governments, trying to standardize the unit with the metric system switched over to Celsius.

Fahrenheit has the equations with other temperature units,

t=5/9*(F-32)

T=5/9*(F-32)+273.15

R=F+459.67

T=5/9*(F-32)+273.15

R=F+459.67

The SI base unit of thermodynamic temperature (symbol K), named after the Irish physicist and engineer William Thomson, 1st Baron Kelvin (1824 – 1907), is the fraction 1/273.16 of the thermodynamic temperature of the triple point of water. The temperature of the triple point of water is exactly 273.16 kelvins (where 0 K is the absolute zero of temperature). This definition was given in the 13th CGPM (1967, Resolution 3). The Thermodynamic Kelvin Temperature Scale (TKTS) is the only temperature scale with a real basis in nature. According to the early temperature scales the atmospheric boiling point of water is a fixed point of 373.15 K, or 100 degrees Celsius). In 1989, the International Committee on Weights and Measures accepted a recommendation of its Consultative Committee on Thermometry for a new “practical” temperature scale, i.e. the International Temperature Scale of 1990 (ITS 90), to replace the International Practical Temperature Scale of 1968. In ITS-90, the atmospheric boiling temperature of water turns out to be approximately 373.124 K (99.974 degrees Celsius).

Kelvin has the relationships with other temperature units,

t=K-273.15

F=(9/5*(T-273.15))+32

R=9/5*T

F=(9/5*(T-273.15))+32

R=9/5*T

Rankine is an thermodynamic (absolute) temperature scale in which zero is defined as absolute zero -273.15°C (the point where all spontaneous molecular motion ceases). The scale divisions are the same as those in Fahrenheit, 0°F = approximately 459.67 R.

It is named for the Scottish engineer and physicist William John Macquorn Rankine, who proposed it in 1859.

Rankine has the relationships with other temperature units,

t=(R-491.67)*5/9

F=R-459.67

T=5/9*R

F=R-459.67

T=5/9*R

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