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Temperature Scales & Fixed Points

The Origins of Fixed Temperature Points
When the thermometer was first invented there was no clear understanding of fixed temperature points. To further confuse the situation the skills required to make thermometers were not available: it was difficult to make a thermometer with a bore of consistent diameter along its length, meaning that no two thermometers were alike. It was only when people travelled with individual thermometers that they were able to confirm that the fixed points of water boiling and freezing were the same in different locations and the effects of altitude and air pressure were recognised. The boiling and freezing points of water were chosen as fixed points because they were easily achievable.
Once the fixed points had been recognised and thermometer making skills had improved, the way was clear for a widely recognised temperature scale. Many were proposed and the main ones are outlined below.

The Fahrenheit Scale
This was the first widely used temperature scale and it is still in use today, though it has lost popularity as people have moved towards metric measurements.
Fixed Points: Freezing Point of Water
Boiling Point of Water
Number of Divisions: 180 degrees
Notes: 32 was chosen as the the figure for the lower fixed point as this produced a scale that would not fall below zero even when measuring the lowest possible temperatures that he could produce in his laboratory - a mixture of ice, salt and water.
It is sometimes suggested that Fahrenheit divided his scale into 100 degrees using blood temperature and his lowest possible temperature as fixed points - this is not true.

The Réamur Scale
This scale is not in use today, but its development was significant as it was the first scale to use 0° as the freezing point of water.
Fixed Points: Freezing Point of Water
Boiling Point of Water
Number of Divisions: 80 degrees
Notes: The Réamur scale is not in use today.

The Celsius Scale
While Celsius's original metric scale had to be inverted before it came into common use, it still bears his name today. The Celsius scale is widely used today for general, engineering and meteorological purposes.
Fixed Points: Freezing Point of Water
Boiling Point of Water
Number of Divisions: 100 degrees
Notes: In 1742 Swedish scientist Anders Celsius chose 0 degrees for the boiling point of water, and 100 degrees for the freezing point. A year later, the Frenchman Jean Pierre Cristin (1683-1755) inverted the Celsius scale to produce the Centigrade scale used today (freezing point 0°, boiling point 100°). By international agreement in 1948 Cristin's adapted scale became known as Celsius and is still in use today.

The Kelvin Scale
The Kelvin is the standard SI unit of thermodynamic temperature in use today.
It is most commonly used by physicists.
Fixed Points: Triple Point of Water
Boiling Point of Water
Number of Divisions: 100 degrees
Notes: In 1848 Sir William Thomson, Baron Kelvin of Largs, Lord Kelvin of Scotland (1824 - 1907) proposed the absolute temperature scale with zero degrees being the theoretical lowest temperature possible where molecular motion ceases. Kelvin defined 1 Kelvin degree as being equal to one Celsius degree.

Fixed points in use today

The practical temperature scale in use today is maintained by the General Conference on Weights & Measures (Paris). The most recent temperature scale was devised in 1990: The International Temperature Scale of 1990 or ITS-90 for short.
ITS-90 covers 16 fixed points, being the melting, freezing or triple points of various substances: Water, Mercury, Gallium, Indium, Tin, Zinc, Aluminium, Silver Hydrogen, Neon, Oxygen, Argon, Copper and Gold. Water apart, all of these are elements. These fixed points give a range of temperatures at which a thermometer can be calibrated from, for example, the triple point of Hydrogen at -259.3467°C (13.8033 K ) to the freezing point of Gold at 1064.16°C ( 1337.33K ).

Triple Points
As well as the boiling or melting point of a substance, the triple point of a substance can be use as a fixed point. This is the temperature at which that substance exists in its solid, liquid and gaseous state all at the same time.

Water Triple point cell in use in the Brannan Calibration Laboratory

The triple point of water is the most important fixed point on ITS-90 as it is the sole fixed point which is common to ITS-90 and the Kelvin Thermodynamic Temperature Scale.
In the Brannan Calibration Laboratory we regularly use the triple point of water to recalibrate thermometers for our customers - its temperature is 0.01°C ( 273.16K ) and it can be recreated with great accuracy.
To recreate the triple point conditions we use a high powered refrigeration unit and a triple point cell which has been made in accordance with the ITS-90 specifications.

Using a triple point cell the triple point of water can be reproduced with an accuracy of +/- 0.00001°C