Temperature and temperature scales

## Temperature scales and some useful temperatures

### Fahrenheit

The first person to make an accurate and reliable thermometer that could be produced in quantity and devise a satisfactory temperature scale is usually considered to be Gabriel Fahrenheit (1686-1736), although individual scientists had been experimenting with thermometers for at least a hundred years before him.

On his thermometer Fahrenheit took 0oF to be what he believed to be the coldest temperature that could exist, a mixture of ice and salt, and 96oF to be the only other temperature that he knew was constant, the temperature of the human body. (He chose 96 because it can be divided into eight and twelve parts.) In fact he was wrong on four counts: The coldest possible temperature (see below) is about -460oF; the coldest ice/salt mixture is about -4oF; and body temperature is about 98.4oF, but is not constant.

Once he had made his thermometer it was discovered that water always boiled at 212oF and froze at 32oF, but of course until a reliable thermometer had been invented and was widely available few people had understood what boiling point or freezing point were - it was obvious that if you heated water it got hotter and that if you cooled it it got colder. See Note below.

### Celsius

Once a reliable thermometer had been invented and the boiling and freezing points of water had been accurately measured it was possible to make a much simpler temperature scale. This is usually, but probably wrongly, attributed to Celsius (1701-1744). On the Celsius scale (originally and still sometimes but incorrectly called the Centigrade scale, from the Latin for one hundred divisions) 0oC is the freezing point of water and 100oC is its boiling point.

The Celsius scale is now used by almost all scientists and by most people throughout the world except in the USA.

### Kelvin

We now think that the coldest temperature that can exist is the temperature at which the atoms in all matter stop moving. This is Absolute Zero, about -273oC (or about -460oF). Thermodynamicists and a few other scientists use a temperature scale named after Lord Kelvin (1824-1907), with Absolute Zero at 0 K. The Kelvin (K) scale is sometimes referred to as the Absolute (A) Scale. To convert degrees Celsius to Kelvin add 273.

Note that on the Kelvin scale temperatures are in Kelvin not degrees Kelvin, so the boiling point of helium is 4 K not 4oK.

The Fahrenheit equivalent of the Kelvin Scale is the Rankine Scale. To convert from degrees Fahrenheit to Rankine add 460.

### The Reaumur Scale

There are also other temperature scales, including the Reaumur scale, introduced in 1730 and named after René de Reaumur (1683 - 1757). This scale was used by scientists in France and Germany before eventually being replaced by the Celsius scale. Prince Albert, Queen Victoria’s husband, introduced the Reaumur scale to Britain, and even today the thermometers in Osborne House, the house he built for Queen Victoria and himself on the Isle of Wight, are calibrated in degrees Reaumur. The freezing point of water is 0oR and its boiling point 80oR.

### Some useful temperatures

Here are some useful temperatures (to the nearest degree Celsius).

-273oC Absolute zero (0 K)
-269oC Boiling point of helium (4 K)
-196oC Boiling point of nitrogen
-183oC Boiling point of oxygen
-114oC Freezing point of ethanol
-79oC Subliming point of carbon dioxide
-39oC Freezing point of mercury
-20oC Coldest ice/salt mixture; temperature of deep freezer
0oC Freezing point of water

5oC Very cold day
10oC Cold day
15oC Cool day
20oC Warm day
25oC Hot day
30oC Very hot day

37oC Normal body temperature
78oC Boiling point of ethanol
100oC Boiling point of water
356oC Boiling point of mercury

Never give a temperature without saying what temperature scale you are using - a temperature of 30o could be a very cold night or a very hot day, depending upon whether you mean 30oF or 30oC!

You can find an Easy Converter between all the different temperature scales on the Easy Unit Converter Web Site

### A note on boiling point

When we say that water boils at 100oC we really mean that pure water boils at 100oC at normal atmospheric pressure at sea level (1013 mb). If the water is not pure it will not boil at 100oC. If the pressure is higher than 1013 mb the water will boil at a higher temperature, and at a lower pressure the water will boil at a lower temperature. Inside a pressure cooker the pressure is about twice atmospheric pressure and the water will boil at about 120oC (which is why food cooks more quickly inside a pressure cooker of course); on the top of Mount Everest the pressure is about 260 mb so the boiling point of water is about 69oC.

Similarly with ethanol. Although pure ethanol boils at 78oC at normal atmospheric pressure, we can use (coloured) ethanol in thermometers to measure temperatures up to 100oC because we pressurise the space above the ethanol with nitrogen. The higher pressure raises the boiling point of the ethanol to above 100oC so it remains a liquid.