8 junio, 2024

Thermometric scales: concept, formulas, examples, exercises

What are thermometric scales?

The thermometric scales They are those that are used in the measurement of temperature, a scalar magnitude that serves to quantify the thermal energy of a system. The device used to measure temperature, that is, a thermometer, must incorporate a scale in order to perform the reading.

To construct an appropriate scale, one must take two reference points and divide the interval between them. These divisions are called degrees. In this way, the temperature of the object to be measured is compared, which can be the temperature of the coffee, the bath or the body temperature, with the reference marked on the instrument.

The most commonly used temperature scales are the Celsius, Fahrenheit, Kelvin, and Rankine scales. All are equally suitable for measuring temperature, since the points selected as reference are arbitrary.

Both on the Celsius scale and on the Fahrenheit scale, the zero of the scale does not indicate the absence of temperature. For this reason they are relative scales. On the other hand, for the Kelvin scale and the Rankine scale, 0 represents the cessation of molecular activity, hence they are considered absolute scales.

celsius scale

This scale was invented by the 18th century Swedish astronomer Anders C. Celsius (1701–1744), around 1735. Very intuitive, this scale uses the freezing point and boiling point of water at normal atmospheric pressure (1 atm). as reference points.

Water is a very suitable universal substance for this, and its values ​​are easy to obtain in the laboratory.

On the Celsius scale, the freezing point of water is the one that corresponds to 0 °C and the boiling point to 100 °C, although originally Celsius had proposed them the other way around and the order was later reversed. Between these two reference values ​​there are 100 identical divisions, which is why it is sometimes known as the centigrade scale.


To establish an equivalence between degrees Celsius and other temperature scales, two aspects must be taken into account:

The relationship between the Celsius scale and the other scale is linear, therefore it is of the form:

y = mx +b

-You have to know the reference points of both scales.

Example: equivalence between the Celsius and Fahrenheit scales

Let TºC be the temperature on the Celsius scale and TºF the temperature on the Fahrenheit scale, therefore:

TºC = m. TºF + b

It is known that 0 ºC = 32 ºF and 100 ºC = 212 ºF. We substitute these values ​​in the previous equation and obtain:

0 = 32m + b

100 = 212m + b

This is a system of two linear equations with two unknowns, which can be solved by any of the known methods. For example, by reduction:

100 = 212m + b

0 = -32m – b


100 = 180m

m = 100 /180 = 5/9

knowing m, we obtain b by substitution:

b = -32m = -32. (5/9) = -160/9

Now we substitute the values ​​of m and b in our equivalence equation to obtain:

ºC = (5/9). TºF – (160/9)= (5TºF -160)/9

Equivalently: ºC = (5/9). (TºF – 32)

This equation allows you to convert Fahrenheit degrees to Celsius degrees directly, just by writing the value where TºF appears.

Example: equivalence between the Celsius and Kelvin scales

Many experiments have been carried out to try to measure the absolute zero temperature, that is, the value for which all molecular activity in a gas disappears. This temperature is close to -273 ºC.

Be T.K. the temperature in kelvin -the word «degree» is not used for this scale-, the equivalence is:

TºC = TK – 273

That is, the scales differ in that the Kelvin scale does not have negative values. In the Celsius – Fahrenheit relationship, the slope of the line is 5/9 and in this case it is 1.

Kelvins and degrees Celsius have the same size, except that the Kelvin scale, as can be seen from the above, does not include negative temperature values.

Fahrenheit scale

Daniel Fahrenheit (1686–1736) was a Polish-born German physicist. Around 1715, Fahrenheit made a thermometer with a scale based on two arbitrarily chosen reference points. Since then it has been widely used in English-speaking countries.

Originally Fahrenheit chose the temperature of a mixture of ice and salt for the lower reference point and set it as 0°. For the other point he selected the human body temperature and set it to 100 degrees.

Unsurprisingly, he had some trouble determining what «normal» body temperature is, because it changes throughout the day, or from one day to the next without the person necessarily being sick.

It turns out that there are totally healthy people with a body temperature of 99.1 ºF, while for others it is normal to have 98.6 ºF. The latter is the average value for the general population.

So the reference points on the Fahrenheit scale had to change for the freezing point of water, which was set at 32ºF and the boiling point at 212ºF. Finally, the scale was divided into 180 equal intervals.

Convert degrees Fahrenheit to degrees Celsius

From the equation shown above, it follows that:

TºF = (9/5) TºC + 32

In the same way we can consider it like this: the Celsius scale has 100 degrees, while the Fahrenheit scale has 180 degrees. So, for every increase or decrease of 1ºC, you have an increase or decrease of 1.8ºF = (9/5)ºF


Using the above equations, find a formula that allows you to go from Fahrenheit degrees to the Kelvin scale:

Knowing that: TºC = TK – 273 and substituting in the equation already derived, we have:

TºC = TK – 273

Therefore: TºF = (9/5)(TK – 273) + 32 = (9/5) TK – 459.4

Kelvin scale

William Thomson (1824–1907), Lord Kelvin, proposed a scale without arbitrary reference points. This is the absolute temperature scale named after him, proposed in 1892. It has no negative temperature values, since absolute 0 is the lowest possible temperature.

At the temperature of 0 K any movement of the molecules has completely ceased. This is the scale of the International System (SI), although the Celsius scale is also considered as an accessory unit. Remember that the Kelvin scale does not use «degrees», so any temperature is expressed as the numerical value plus the unit, called «kelvin».

So far absolute zero has not been reached, but scientists have come pretty close.

Indeed, in laboratories specializing in low temperatures, they have managed to cool sodium samples to 700 nanokelvin or 700 x 1010-9 kelvin. On the other hand, towards the other end of the scale, it is known that a nuclear explosion can generate temperatures of 100 million kelvins or more.

Each kelvin corresponds to 1/273.16 parts of the temperature of the triple point of water. At this temperature the three phases of water are in equilibrium.

Kelvin scale and Celsius and Fahrenheit scales

The relationship between the Kelvin and Celsius scales is -rounding 273.16 to 273-:

TK = TºC + 273

Similarly, by substitution, a relationship between the Kelvin and Fahrenheit scales is obtained:

TK =5(TºF + 459.4)/9

Rankine scale

The Rankine scale was proposed by William Rankine, a Scottish-born engineer (1820-1872). Pioneer of the Industrial Revolution, he made great contributions to thermodynamics. In 1859 he proposed an absolute temperature scale, setting zero at −459.67 °F.

On this scale the size of the degrees is the same as on the Fahrenheit scale. The Rankine scale is denoted as R and as with the Kelvin scale, its values ​​are not called degrees, but rankine.


0K = 0R = −459.67 °F = – 273.15 ºC

In short, here are the conversions needed to go to the Rankine scale from any of the ones already described:

Reaumur scale

Another previously used temperature scale is the Réaumur scale, which is denoted as degrees or ºR. It is currently out of use, although it was widely used in Europe until it was displaced by the Celsius scale.

It was created by René-Antoine Ferchault de Réaumur (1683-1757) around 1731. Its references are: 0 °R for the freezing point of water and 80 °R for the boiling point.

As can be seen, it coincides with the Celsius scale at zero, but certainly not at the other values. It is related to the centigrade scale by:

TºR=(4/5) TºC

In addition, there are other equivalences:

TºR= (4/5) (TK–273) = ( 4/9)(TºF-32)= (4/5)(5.TR/9 – 273) = (4/9)TR – 218.4

solved exercises

Exercise 1

Find the numerical value in which the centigrade scale coincides with the Fahrenheit scale.


As we have seen in the previous sections, these scales do not coincide, since the reference points are different; however it is possible to find a value x, such that it represents the same temperature on both scales. Therefore we take the equation:

ºC = (5/9). TºF – (160/9)= (5TºF -160)/9

And since the temperatures must match, then TºC = TºF = x, it follows that:

x = (5x – 160)/9

9x = 5x -160

4x = -160

x = –40

When TºC = -40ºC, also TºF = -40ºF

Exercise 2

The steam that comes out of a boiler is at a temperature of 610 ºR. Find the temperature in degrees Fahrenheit and in degrees Celsius.


The equivalences found in the section of the Réaumur scale are used, therefore: TºC=(5/4) TºR =(5/4). 610ºC = 762.5ºC.

You can then convert this found value to degrees Fahrenheit, or use one of the other conversions mentioned:

TºF = (9/5) TºC + 32= (9/5) 762.5 + 32 ºC = 1404.5 ºF

Or this other one, which gives the same result: TºR =( 4/9)(TºF – 32)

Clears: TºF =( 9/4) TºR + 32 =( 9/4) 610 + 32 ºF= 1404.5 ºF.

Conversion Summary

In summary, the following table provides conversions for all scales described:


Temperature Scales. Retrieved from: thales.cica.es.
Knight, R. 2017. Physics for Scientists and Engineering: a Strategy Approach. pearson.
Tillery, B. 2012. Physical Science. McGraw Hill.
Wikipedia. Celsius degree. Retrieved from: es.wikipedia.org
Wikipedia. Fahrenheit degree. Recovered from: es.wikipedia.org.
Wikipedia. rankine. Recovered from: es.wikipedia.org.

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