The convert(u, temperature, tempFrom, tempTo) function converts a value u from the tempFrom temperature scale to the TempTo temperature scale.

The temperature scales tempFrom and tempTo can be any unit label, for example, name or symbol, accepted by the Units package as one of kelvin, degrees Celsius, degrees Rankine, degrees Fahrenheit, degrees centigrade, or degrees Reaumur.

The convert(u*Unit(tempFrom), temperature, tempTo) function is similar to the first calling sequence. However, it accepts tempFrom as a unit.

Each temperature scale is set by fixing the values of two points.  Historically, the freezing and boiling points of water have been chosen as these points. In 1954, temperature scales were redefined in terms of absolute zero and the temperature of the triple point of water (the temperature and pressure at which water exists in all three states-solid, liquid and gas) because the measurements are much more precise than those of the transition temperatures of water.

All defined values are exact. However, for presentation they have been written as floating-point numbers. All temperatures that refer to the freezing and boiling points of water assume a standard pressure of 101.325 kilopascals.

Kelvin

In this system, named after William Thomson Kelvin (1824-1907), absolute zero is defined as 0 kelvin, and the temperature of the triple point of water is defined as 273.16 kelvin. Consequently, the freezing and boiling points of water are approximately 273.15 and 373.125 kelvin, respectively.

Celsius

In this system, named after Anders Celsius (1701-1744), absolute zero is defined as -273.15 degrees Celsius, and the temperature of the triple point of water is defined as 0.01 degrees Celsius. Consequently, the freezing and boiling points of water are approximately 0. and 99.975 degrees Celsius, respectively.

By definition, a temperature of n degrees Celsius is equal to n + 273.15 kelvin.

Rankine

In this system, named after William John Macquorn Rankine (1820-1872), absolute zero is defined as 0 degrees Rankine, and the temperature of the triple point of water is defined as 491.688 degrees Rankine. Consequently, the freezing and boiling points of water are approximately 491.67 and 671.625 degrees Rankine, respectively.

By definition, a temperature of n degrees Rankine is equal to 5/9*n kelvin.

Fahrenheit

In this system, named after Gabriel Daniel Fahrenheit (1686-1736), absolute zero is defined as -459.67 degrees Fahrenheit, and the temperature of the triple point of water is defined as 32.018 degrees Fahrenheit. Consequently, the freezing and boiling points of water are approximately 32 and 211.955 degrees Fahrenheit, respectively.

By definition, a temperature of n degrees Fahrenheit is equal to n + 459.67 degrees Rankine.

Before Fahrenheit, a proposed temperature scale by Ole Christensen Roemer defined 0 to be the freezing temperature of a saturated brine solution, and 50 to be the boiling point of water. Fahrenheit modified the limits so that the difference between the freezing and boiling points of water was exactly 180 degrees. He then used the freezing and boiling points of water to define 32 and 212 degrees Fahrenheit.

Centigrade

The temperatures of the freezing and boiling points of water are defined as exactly 0 and 100 degrees centigrade, respectively. Consequently, absolute zero is approximately -273.218 degrees centigrade and the temperature of the triple point of water is approximately 0.01 degrees centigrade.

Before the Celsius scale was redefined in 1954, it was identical to the centigrade scale.

Reaumur

In this system, named after Rene Antoine Ferchault de Reaumur (1683-1757), the temperatures of the freezing and boiling points of water are defined as 0 and 80 degrees Reaumur, respectively. Consequently, absolute zero is approximately -218.575 degrees Reaumur and the temperature of the triple point of water is approximately 0.008 degrees Reaumur.

By definition, a temperature of n degrees Reaumur is equal to 5/4*n degrees centigrade.

For more information about temperature scales, see the following references.

Barry N. Taylor. Guide for the Use of the International System of Units (SI). National Institute of Standards and Technology, Special Publication 811, 1995 Edition.

IEEE/ASTM SI 10-1997 Standard for Use of the International System of Units (SI): The Modern Metric System.