The Temperature is Such and Such. What is Temperature?

Temperature is, Celsius, no, Fahrenheit, no, Kelvin, no, urgh. Yes, I have been in that position myself. I thought that temperature is any of those three, but I was proven wrong.

It is the measurement of degree of hotness or coldness, in laymen term.

Human being, from the beginning tried to measure the degree of hotness or the degree of coldness of an object.

Fire is hotter than a roasted lamb.

Roasted lamb is hotter than the river’s water.

Everything is a relative measure.


Birth of its measurement:

It was not until 1701 when Mr Rømer first measured degree of "hotness" using liquid.

The systematic use of its measurement using expansion and contraction of liquid due to heat addition or removal was born.

Then in 1714, the famous Mr Fahrenheit used the mercury thermometer. However, his scale was from the coldest (0 oF), to the hottest degree (100 oF) in Western Europe.

However, this type of measurement is quite inconsistent, in my honest opinion. What if the hottest condition the following year is higher than the previous year?

In 1744, Mr Linnaeus used the scale design by Mr Celsius in 1742, to develop the, also commonly used scale – the Celsius scale.

The scale was designed based on a single substance, water. It was designed from the freezing point (0 oC) to the boiling point (100 oC) of water.

This scale is more accurate, as it based on temperature change of a single substance, from one phase, to another phase. This will be explained, later in this page.

Accurate as it may be, the measurement is still a relative measure. It is based on water.

What if?

What if the scale design was based on cough syrup? What if the scale design was carried out on Everest where the pressure will be low, and effect the scale? Yes, the scale will be different.

And then, in 1848, Mr Thomson suggested a scale that is so powerful, that Thermodynamic-"ists" love it a lot. He suggested the Kelvin scale.

He only suggested one number, and that number is 0 oK. He didn’t relate it to any substance.


Because he really understood the behaviour of particles:

He understood that, the degree of "coldness" or "hotness" of a matter is a unique behaviour of its atoms or molecules. Those two are proportional to each other.

Be it water, steel, or refrigerants. Water at 100 oC will "feel" the same as refrigerant at 100 oC. Only the phase will be different. Therefore, the measurement is independent of the substance.

Hence, he only suggested the number 0, as that is when the kinetic energy of any material will also be 0. Any number above this goes up in a linear increment.

The scale is now absolute.

The method to find absolute zero is quite simple, although achieving it is quite improbable.

It is achieved by doing an experiment to find the relationship between absolute pressure, and temperature (T).

Why absolute pressure?

Imagine a closed container filled with gas. The pressure of the gas is directly proportional to the temperature. As T is increased, the gas particles’ kinetic energy will increase proportionally.

The gas particles will increasingly hit the walls of the container, and hence resulting in increased pressure.

The reverse will happen if T is reduced. Particles’ kinetic energy will be reduced, and hence the pressure.

How to reduce the pressure to 0? By reducing the T, to absolute 0.

We need to do graph extrapolation, since absolute 0 (i.e 0 oK) is quite improbable to achieve.

How? First,

  • take a sample of gas

  • heat it from one point, to another point

  • measure the absolute pressure from these two points

  • draw a straight line graph

  • extrapolate the graph’s line, until it meets x-axis (i.e. the temperature scale)

  • you’ll "find" absolute 0, that is at -273.15 oC

Finding absolute temperature


Conversion between Fahrenheit, Celsius, and Kelvin:

  • 0 oC is 0 + 273.15 oK

  • 1 oC is [(1 oC x 1.8) + 32] oF

This is why I don’t feel that the Fahrenheit measurement is quite inconsistent. It because Fahrenheit’s scale is not directly proportional to Kelvin’s scale.

Celsius scales, on the other hand, do increase, at a similar increment as Kelvin’s scale. The only difference is the reference point. Celsius’ 0 reference is at water’s freezing point.

However, Fahrenheit’s scale is still very much applicable, if the conversion is understood.


How do you build a thermometer for measurement?

Many ways.

The principle is, to use expansion of either gas, liquid, or solid. It is preferred that the expansion do occur uniformly with increase in T. However, it is not a ‘must’ rule.

Gas, liquid and solids are all made of particles that are continuously in motion. The motion is related to T.

As T is increased, particles’ distances will increase due to the increased energy. The vice versa also happens.

Temperatures will be recorded from a known point, to another known point. Say, from freezing point to boiling point of water?

The freezing point is set with a value. Temperature will be increased and liquid’s length inside tube is measured until boiling is reached. In this case, a uniformly expanding liquid is preferred, so that the scale could be uniformly distributed from 0 oC to 100 oC.

Liquid such as mercury or alcohol will be commonly used in thermometers. Water is not preferred as there is limitation to measure air temperatures in cold winter.

Non-uniform expanding substance may be used to measure T, but adjustment to the scale has to be made, to reflect true temperature of the subject under measurement.

Other temperature measuring tools are, thermocouples, bi-metallic strip thermometers, fixed gas thermometer, and wire resistance thermometers. My favourite is infrared thermometers.

Infrared thermometers are used to measure T using radiation heat from a body. This is a versatile tool, but quite expensive.


Phase change:

“Hang on a minute, you’re telling me that Celsius scale is in the same line as Kelvin’s scale, but I didn’t see any temperature change when the water is boiling, and when it is freezing. How so?

Not just Celsius. Even Fahrenheit and Kelvin, and any other scale won’t show change in T, during phase change.

It is simply because of one reason. The T of the subject is not changing.

There are two phase changes for any material. From

  • solid to liquid or the reverse, and from

  • liquid to gas or the reverse (boiling and condensation)

As explained in heat transfer section, every matter's particle will have a specific internal energy.

Let us consider a liquid. As heat is provided, the internal energy, T and the particles’ distances will increase.

This is true until the boundary of liquid-vapour is reached. At this point, the heat provided will be used for breaking the particles’ bond between each other.

Hence, heat is used to break the bond, rather than increasing its kinetic energy, which is related to the temperature.

Temperature of the liquid will not rise, until everything is converted into vapour form.

Similar phenomenon will happen for condensation from vapour. This time, the heat removal will be used by the particles, to "re-bond". Here, T will not drop until all is changed from vapour, to liquid.

Boiling and condensation will not raise or lower temperature of a substance


Temperature and air conditioners:

The understanding of phase change and "hot-cold" measurement is used by air conditioner designers, to design air conditioner units. Without this, designing an air conditioner will be more of a guessing thing.



Top of Temperature Page

Back to How Air Conditioners Work