The 7 most important types of thermometers

There are many different types of thermometers, many of them with very specific functions that we are all familiar with.

These thermometers can be presented with different scales, such as Centigrade, Kelvin and Fahrenheit, in addition to having special mechanisms to take temperatures in underwater objects or that are in movement.

Then let's see the seven main families of types of thermometers, its operating mechanism and its uses, both in the domestic, sanitary and industrial fields.

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The types of thermometers, classified and explained

Although there are countless different types of thermometers, in essence, these can be classified into seven large families, depending on its operating mechanism. There are well-known ones in the domestic sphere, such as the classic glass and the although there are others, such as pyrometers, widely used in areas such as sanitary.

1. Glass or liquid thermometer

The glass thermometer, also known as a liquid thermometer, is the best known and most prototypical type of thermometer. It is also called a manual or mercury thermometer, since, traditionally, it was filled with this liquid metal and does not require batteries to function. Eventually, mercury was replaced by other substances, since this element is toxic.

A curiosity about this thermometer is that was invented by Daniel Fahrenheit, who invented one of the temperature measurement systems, the Fahrenheit scale.

How do they work?

Glass thermometers consist of a sealed glass tube called a capillary. At one end of the capillary there is a metallic piece, called a bulb, which contains a liquid, which may be mercury or alcohol with a red tint.

When the metal part comes into contact with the object whose temperature is to be measured, the part can either expand or expand. Following this, the liquid inside it goes up or down along the capillary and marking the thermal value on the scale written along the tube.

Applications

The glass thermometer is widely used in the everyday household., thanks to the fact that it is simple to handle, does not require batteries and is relatively easy to obtain.

The most common use is to measure body temperature, especially when it is suspected that a person may have a fever. To measure body temperature, the thermometer is placed in the mouth, armpit or rectum of the patient, it is waited for the bulb to heat up and it is observed that the temperature marks the scale.

It is because of this that pharmaceutical companies have been manufacturing these instruments with alcohol instead of mercury for a decade, given that, as they are glass tubes that are placed in body orifices, there was a risk that, by accident, this substance, known to cause Minamata disease, would be broken and ingested.

Other uses of glass thermometers are in culinary preparations, industrial processes and measuring the temperature of aquariums.

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2. Non-contact pyrometers or thermometers

The pyrometer, or non-contact thermometer, is a type of thermometer that works without having to touch the object whose temperature you want to measure, measuring the temperature using infrared.

How do they work?

Compared to glass thermometers, pyrometers work in a much more sophisticated way. They have a lens that captures the infrared radiation emitted by bodies. These radiations vary in intensity as a function of the temperature of the object itself. Thus, the higher the temperature, the higher the radiation.

The lens has a sensor that transforms this infrared radiation into electrical current, which travels through a circuit that, finally, will cause a small screen to indicate the temperature of the object evaluated.

Applications

There are several uses for pyrometers, both in the home and in more specialized places.

They can be used with newborn babies, especially if we take into account that they are very sensitive and that placing a glass thermometer on them can bother them, making measurement very difficult. Thanks to pyrometers can be used without touching the baby, can be used while you are asleep. All you have to do is project the light from the device and record its temperature.

As for the industry, pyrometers They are used to measure the temperature of surfaces that are very hot and that other thermometers could not evaluate because they would either burst or they would go bad. In fact, pyrometers can register very high temperatures, some reaching 700 ° C or even up to 3,200 ° C.

3. Bimetallic foil thermometers

Bimetallic foil thermometers measure temperature by means of a mechanism that contains two different types of metal, which, depending on how they contract or expand, will help indicate the temperature of the object whose temperature is being taken.

How do they work?

Bimetallic foil thermometers have a mechanism with two metallic foils. One is made of a metal with a high coefficient of expansion, while the other has a low coefficient..

This bimetallic sheet is found forming a spiral inside a tube. The spiral is welded from one end to the other of this capillary, attached to a transmission rod. In turn, the transmission rod is welded to a needle which will indicate the temperature of the measured object.

Applications

Bi-metal foil thermometers They are not usually used in the home, but they are used in industrial processes in which aggressive or dangerous substances have to be measured.

Some examples of the uses of these instruments are in the pharmaceutical, food, chemical, textile and petrochemical industries.

These thermometers, unlike pyrometers, establish direct contact with the substance to take its temperature. They can register temperatures from -70ºC to more than 600ºC.

4. Gas thermometers

Gas thermometers are instruments little used in the domestic sphere, but they are used in the industrial sector. They contain inside a gas, usually nitrogen, with which it is possible to measure the precision and reliability of other thermal instruments..

How do they work?

Gas thermometers have several parts. The first is an element that is responsible for measuring pressure, which is connected to a capillary and, at the same time, this element is connected to an ampoule, part of which is exposed to the desired temperature measure.

The mechanism of this instrument begins to work when the device is filled with gas under pressure, nitrogen being the most used.. The gas is in the vial and, depending on the temperature of what is being measured, this gas will push some handles, with which the temperature of the gas is indicated.

Applications

Gas thermometers are used especially to check that other thermometers work correctly, thanks to their high precision and measuring range. The problem is that, as they are instruments whose use is very complex, they take a long time to measure the temperature, in addition to the fact that they are difficult to apply in the domestic sphere.

Its temperature measurement range goes from -450ºF to 1000ºF (-268 ºC to + 538 ºC).

5. Resistance thermometers

In the 19th century, it was discovered that the electrical resistance of an object varied as a function of its temperature.. This is why, in 1871, Wilhelm Siemens made the proposal to use platinum to measure temperature. Later, into the 20th century, the resistance thermometer would be invented, which would use, in essence, the same mechanism proposed by Siemens.

Today the material used in the resistance thermometer can vary. While there are models that continue to use platinum, there are also copper and tungsten models. However, platinum is considered the ideal material for temperature measurement.

How do they work?

Resistance thermometers take the temperature by evaluating the behavior of a platinum wire, or other metals, which is built into the thermometer. This wire is linked to an electrical resistance, which changes depending on the temperature

Applications

Their measurement capabilities are extremely wide, ranging from 200 ° C to 3568 ° C, and they take time to measure temperature. They are often used to take outdoor temperatures.

6. Thermal torque or thermocouple

They resemble resistance thermometers in that measure the temperature from an electrical resistance that produces voltage, which varies as a function of the temperature of the measured object.

How do they work?

This device consists of two metal wires that are joined at one end. The joint becomes the measurement point, while the ends are identified as a hot joint and a cold joint.

The ends of this mechanism must be placed on the object to be measured. This will cause the measurement point to heat up, causing an electrical voltage, generating a voltage that is proportional to the object temperature.

Applications

Thermal couples give the temperature quickly and efficiently. For this reason are often used in laboratories, especially in processes in which the temperature or electromotive force necessary to heat the weld of two different metals has to be measured.

7. Digital thermometers

Digital thermometers are very similar to glass thermometers, since they are used as an electronic substitute for them. They have been taking away the popularity of the most classic ones for a long time, and they have their advantages and disadvantages compared to glass.

How do they work?

Digital thermometers measure temperature through a mechanism that captures energy through a resistance. The resistance generates an electric current that travels through a circuit, which is responsible for transform electricity into a value that appears on a screen, indicating the temperature of the measured body.

Applications

These types of thermometers are very practical and easy to use, as well as being inexpensive. They are safer than the classic glass tube, especially compared to those that use mercury.

They have different sizes and uses. There are digital thermometers specialized in taking the temperature of newborns, made of soft and flexible materials that do not hurt the gums of the babies when they are introduced by route oral.

As for other areas, we have very diverse applications of digital thermometers in industry, home, aquariums, baking, gardening and veterinary.

On average, these thermometers give the result after two to three minutes. Some have memory, storing the last temperature measurement results, as well as light and sound indicators that tell us when the temperature has already been measured.

One downside is that need batteries to work, which may run out. However, these batteries can be quite durable, readily available, and inexpensive.

Bibliographic references:

• Creus Solé, A. (2005). Industrial instrumentation. Marcombo. ISBN 84-267-1361-0. P. 283-296.