Learn about temperature, temperature scales, temperature units and temperature unit conversions.
Explanation of the picture
The above infographics illustrates a quick comparison between temperature units in a few common temperatures. Sure a room temperature varies and the human body temperature is not always 37°C (98.6°F). Also, the boiling point of water depends on air pressure and is not always exactly 100°C. But a real Finnish sauna is always at least 100°C (212°F) when properly heated… :-)
A while ago I wrote a blog post about pressure units and I was thinking it would be a good idea to write a similar one on temperature units and their conversions. Let’s first take a short look at what temperature really is, then take a look at some of the most common temperature units and finally the conversions between them.
We developed a Temperature Unit Converter into our web site,
click the below link to visit it:
What is temperature?
Temperature is an intensive quantity and describes the energy state of the matter. All materials have atoms and molecules that are in constant movement, vibrating or rotating. A difficult subject simplified, the more they move, the more temperature the material will have. The temperature of an object can be defined by the average kinetic energy of its atoms and molecules, a definition for temperature that we can understand relatively easily. Kelvin is the unit of fundamental physical quantity called thermodynamic temperature (T) and is currently defined as the fraction 1/273.16 of the thermodynamic temperature of the triple point of water (exactly 0.01 °C / 32.02 °F).
So, what is hot and what is cold?
It is all pretty relative, so these terms hot and cold are not very accurate or scientific. So we need more specific way to indicate temperature. Several different temperature scales and units have been developed during the recent centuries. And since different scales have been used in different parts of the world, there are still several different scales in use. The actual specifications of some of the old temperature scales were not initially very accurate (such as a human’s body temperature), but later on specific and accurate reference points and specifications were created.
For high temperatures, there is not really any limit, and it is possible to go to a very high temperature. For example, the temperature at sun’s surface is 5800 kelvin, while the temperature inside the sun is up to 13.6 million of kelvins.
But for the low end of temperature, there is a very specific limit, being the absolute zero temperature, which is the lowest possible temperature. Absolute zero is a theoretical state that possibly cannot ever be achieved. Theoretically all the movement of atoms would cease almost completely, retaining only quantum mechanical zero-point energy. Absolute zero temperature equals 0 kelvin, -273.15 °Celsius or -459.67 °Fahrenheit. In outer space the temperature is pretty cold and the average temperature of universe is less than 3 kelvin.
But let’s next take a look at some of the most common temperature scales and units.
International temperature scales
Thermodynamic temperature is very difficult to measure and several international temperature scales for practical measurements have been published:
- ITS-27; International Temperature Scale of 1927
- IPTS-48; International Practical Temperature Scale of 1948
- IPTS-68; International Practical Temperature Scale of 1968
- ITS-90; International Temperature Scale of 1990
Some additional scales have also been used, for example PLTS-2000 for improved measurements of very low temperatures in the range 0.9 mK...1 K (Provisional Low Temperature Scale of 2000).
By international agreement, the current ITS-90 scale is based on the before mentioned thermodynamic temperature (T). The scale defines the methods for calibrating a specified kind of thermometers in a way that the results are precise and repeatable all over the world. Also the numerical values are believed to be as close to the actual thermodynamic temperature (T) as possible at the time. The methods for realizing the ITS-90 temperature scale include fixed points and functions for interpolating the temperatures in between the fixed values.
The fixed points in the ITS-90 scale are the following:
Kelvin is the base unit of temperature in the SI system (International System of Units). Kelvin unit’s abbreviation is K (no degree or degree sign). Kelvin unit was first presented by William Thomson (Lord Kelvin) in 1848.
As mentioned earlier, the kelvin is currently defined as the fraction 1/273.16 of the thermodynamic temperature of the triple point of water, absolute zero point being 0 K. The size of one kelvin is the same as a Celsius degree. The temperature of melting ice is 273.15 K (the triple point of water is 273.16 K).
The kelvin is often used in science and technology. It is anyhow not that much used in everyday life. The symbol of kelvin temperature in terms of ITS-90 is the upper case letter T90.
A new definition of kelvin is expected to be published in the near future linking kelvin to the Boltzmann constant, continuing the work for defining all the SI units by fundamental physical constants.
Celsius is currently a derived unit for temperature in the SI system, kelvin being the base unit. The abbreviation of Celsius is °C (degree Celsius) and the size of one Celsius degree is the same size as one kelvin. The unit and the actual Celsius scale was first presented by a Swede Andreas Celsius in 1742. The two main reference points of the Celsius scale were the freezing point of water (or melting point of ice) being defined as 0 °C and the boiling point of water being 100 °C.
The melting point of ice is a relative accurate specification (assuming you have purified ice and it is properly stirred), but the boiling temperature of water is not such an accurate temperature in practice as the boiling temperature depends a lot on the atmospheric pressure. As the Celsius is SI unit derived from kelvin, it’s also linked to ITS-90 and its symbol is lower case letter t90. In terms of ITS-90 the melting point of the ice is slightly below 0 °C and the boiling point of the water at the normal atmospheric pressure is approximately 99.974 °C.
The Celsius unit is better suited for everyday use than kelvin and is very popular globally, although not so much used in the USA. A Celsius degree is sometimes also called Centigrade.
Fahrenheit unit’s abbreviation is °F. The Fahrenheit scale was first introduced by a Dutchman named Gabriel Fahrenheit in 1724. The two main reference points of the scale are the freezing point of water being specified as 32°F and the temperature of human body being 96°F.
In practice it is easy to see that the temperature of a human body is not a very precise definition.
Nowadays the Fahrenheit scale is redefined in a way that the melting point of ice is exactly 32 °F and the boiling point of water exactly 212 °F. The temperature of the human body is about 98 °F on the revised scale.
In many areas the Fahrenheit has been replaced with Celsius as a temperature unit, but Fahrenheit is still is use in USA, in Caribbean and also in parallel use with Celsius in Australia and in UK.
Rankine (°R, °Ra)
Rankine scale is abbreviated as °R or °Ra. Rankine scale was presented by a Scottish William Rankine in 1859, so a few years after Kelvin scale. The reference point of Rankine scale is absolute zero point being 0 °R, like in Kelvin scale. The size of one Rankine degree is the same as the size of one Fahrenheit degree, but the zero point is very different. The freezing point of water equals 491.67 °Rankine.
Rankine is not a widely used scale. It was used in some fields of technology in USA, but NIST does not recommend the use of Rankine anymore.
Réaumur (°Ré, °Re)
Réaumur scales were introduced by Réne de Réaumur in 1730. It has the reference points being the freezing point of water 0 °Ré and boiling point of water being 80 °Ré.
The Réaumur scale was used in some parts of Europe and Russia, but it has mainly disappeared during the last century.
Conversions between temperature units
The table below provides calculation formulas for converting temperature readings from one unit to another unit.
Temperature unit converter
I know that the above presented conversion table may not be the easiest one to use…
We developed a free and easy to use temperature unit converter on our web site that converts between the above listed 5 different temperature units. Hopefully you will find this converter helpful.
Beamex temperature calibration products
I hope you found this post useful.