Thermometers provide readings of temperature. They must rely on some physical property of the system , which depends on the temperature.

We will study two main classes of thermometers, those based on on thermal expansion (type alcohol / mercury or bimetallic) and those based on electric/ eçectronic phenomena ( thermocouple and thermistor)

An ordinary thermometer, based on alcohol or mercury, has a range of about -15°C to 100°C. This is enough for most domestic or even medical applications (and that is why it is so well known). 

In scientific and industrial processes, however,  we need to work with more extreme temperatures,well beyond the range of these thermometers. To achieve this it is necessary to apply  physical principles other than thermal expansion, and we will examine these.

 

RANGE

What limits the range of an alcohol thermometer?

The alcohol moves through a thin glass tube to allow for a reading, and it must be in the liquid state to be able to do so.

It is only liquid in the range between its boiling and freezing points , which are for  ethanol (the most common alcohol)   78 °C and -114 °C respectively. 

Because the liquid in this kind of  thermometer is not pure ethanol, its boiling point can be slightly higher than 100°C and that is why we can use it to check the boiling point of water. Non-pure substances have melting and boiling points different from those of the pure substance. That is why salted water does not freeze in polar waters, which have temperatures below 0 °C.

Another type of thermometer that depends on the phenomenon of thermal expansion is the bimetallic strip. Two strips of different metals are joined. WHen the temperature rises they expand but to different extents. The result is that the device bends to one side.

The thermometers discussed above depend on the phenomenon of thermal expansion: alcohol and metals expand in proportion to their temperatures. The next class of thermometers we will discuss depend on the Seeberg effect: a small voltage is produced when a piece of metal is heated so that each extremity is at a different temperature. This voltage can be measured and it is proportional to the temperature. If two different metals are used the effect is stronger. Such a device is called a thermopair (pair of metals) or thermocouple. Because the boiling point of metals is high, thermopairs can be used to measure high temperatures, which can be many hundreds of degrees celsius.

 

Thermopairs

Because the boiling point of metals is high, thermopairs can be used to measure high temperatures, which can be many hundreds of degrees celsius. Thermopairs are also more durable. The thin glass tube of alcohol thermometers can break easily. This is all you need to know about thermopairs in this course. But I am providing more details in case you are curious about it. 

Particle model explanation for thermopairs: it should not be a surprise the fact that thermal and electrical phenomena in metals are linked. Metals have lots of free electrons that can carry electricity and also heat. We observe that the metals that are excellent electric conductors, are also excellent heat conductors. Gold is an example.

When a piece of metal has an extremity heated, the electrons in this region will move faster and will go further away. As a result, there will be more electrons in the cold extremity. This imbalance in the amount of electrons on each side produces a voltage. It is similar to the hot air balloon, when the air molecules inside the balloon are heated and move away (escape the interior of the balloon) causing it to be lighter than the surrounding air and as a result it moves up.

Using Electronics

A thermistor is an electronic device (made of semiconductor material) that functions based on the variation of electrical resistance with temperature. It is a delicate electronic device, so tat its range is small, approximately between 10 and 65 °C  An easy and cheap way of connecting a thermistor to your computer is to use an arduino board . Watch this video about Arduino

Particle model explanation for the thermistor:The electrical resistance of conductors is reduced when the temperature increases, because  particles are jiggling fast inside the material and that makes it difficult for  other particles (the electrons) to move across. 

But with certain semiconducting devices, the resistance decreases when it is heated. That is because electrons that are bound to the atoms are released when they gain heat energy, and as a result they become free to move around and conduct electricity.

One big advantage of electrical or electronic thermometers is that the data they produce can be fed directly into a computer, without human mediation.

  

In the following examples I will refer to the liquid inside the thermometer as mercury, but it could also be alcohol or many other substances. In fact, mercury is highly toxic (it attacks the brain and the nervous system, among other places) and because of that it is no longer being used for this purpose. But it is well known as a thermometer liquid. It is a metal, the only liquid metal at room temperature. 

 RESPONSIVITY

Responsivity is how fast the thermometer responds to a change in temperature. The smaller the bulb, and as result the smaller the amount of mercury presents the, faster the response. That is quite intuitive. If more mercury is present it will take longer for all of it to get to a higher temperature. For instance, a medical thermometer must have a quick response, because we need to know as soon as possible if somebody has a fever. 

 

SENSITIVITY

Sensitivity is related to how much the mercury column rises. That is, how far the thread goes. The further it goes, the more sensitive the thermometer. To better understand it, we need to look at some concepts of thermal expansion. 

The thermal expansion of an object depends on its size. The larger it is, the more it will expand. There are formulas for expansions that are in one dimension, 2 or 3. They are all similar. For instance, the volumetric expansion (which is the relevant one for the thermometer bulb) depends on the volume and the coefficient of volumetric expansion. So, a substance with a high coefficient  of volumetric expansion will make a very sensitive thermometer. Because of the same formula, a larger bulb will provide a larger expansion, and as a result the thread will move more. This is another way of making a very sensitive thermometer. 

 

 
 

LINEARITY 

The thermometer must have a linear relationship between the temperature it measures and the amount by which the thread moves, to provide a reading. That is, the same distance must be covered for the same temperature difference. If it doubles, the distance also doubles. And so on… 

The term linear comes from the graph that represents such a situation? It is a straight line graph.