The 7 most important types of thermometers

Classification of the types of thermometers used in fields such as health or industry.

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

These thermometers can come with different scales, such as Celsius, Kelvin and Fahrenheit, as well as having special mechanisms to take temperatures in underwater or moving objects.

The following are the seven main families we are going to see the seven main families of thermometer types, their operating mechanism and their uses.Their mechanism of operation and their uses, both in the domestic, sanitary and industrial fields.

Types of thermometers, classified and explained.

Although there are infinity of different types of thermometers, in essence, these can be classified in seven great familiesdepending on their operating mechanism. Some of them are very well known in the domestic field, such as the classic glass thermometers and digital thermometers, although there are others, such as pyrometers, which are very popular in areas such as health care.

Glass or liquid thermometer

The glass thermometer, also known as liquid thermometer, is the most well-known and prototypical type of thermometer. It is also called a hand-held or mercury thermometer, since it was traditionally filled with this liquid metal and did not require batteries to operate. 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 systems for measuring temperature, the Fahrenheit scale..

How do they work?

Glass thermometers consist of a sealed glass tube called a capillary. At one end of the capillary is a metal part, called a bulb, which contains a liquid, either mercury or alcohol with a red dye.

When the metal part comes into contact with the object whose temperature is to be measured, the part can either expand or expand. As a result of this, the liquid inside it rises or falls along the capillary, and the thermal value is displayed on the capillary. and marking the thermal value on the scale written along the tube.

Uses

The glass thermometer is widely used in the daily domestic environment, because it is easy to handle.It is simple to operate, does not require batteries and is relatively easy to obtain.

The most common use is to measure body temperature, especially when there is suspicion that a person may have a fever. To measure body temperature, the thermometer is placed in the patient's mouth, armpit or rectum, the bulb is heated and the temperature on the scale is observed.

It is because of this that for a decade pharmaceutical companies have been manufacturing these instruments with alcohol instead of mercury, since they are glass tubes that are placed in body orifices, There was a risk that, by accident, they could break and ingest this substance, known to cause Minamata disease..

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

2. Pyrometers or non-contact thermometers

The pyrometer, or non-contact thermometer, is a type of thermometer that works without touching the object whose temperature is to be measured, measuring the temperature by means of 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 the body.. These radiations vary in intensity depending on the temperature of the object itself. Thus, the higher the temperature, the greater the radiation.

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

Uses

Pyrometers have several uses, both in the domestic environment 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 disturb them, making the measurement very difficult. Thanks to the fact that pyrometers can be used without touching the baby, they can also be used with newborns.can be used while you are asleep. It is enough to project the light of the device and record its temperature.

In industry, pyrometers are used to measure the temperature 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 break. because they would either burst or break down. In fact, pyrometers can record very high temperatures, some reaching 700 °C or even up to 3,200 °C.

3. Thermometers with bimetallic foil

Thermometers with bimetallic foil measure the temperature by means of a mechanism that contains two different types of metal, which, depending on how they are used, can be used to measure the temperature.which, depending on how they contract or expand, will help to indicate the temperature of the object being measured.

How do they work?

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

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

Uses

The bimetallic reed thermometers are not usually used in domestic applications, but they are used in industrial processes where aggressive or hazardous substances have to be measured..

Some examples of 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 be measured. They can record temperatures from -70ºC to more than 600ºC..

4. Gas thermometers

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

How do they work?

Gas thermometers consist of several parts. The first is a pressure measuring element, which is connected to a capillary and, at the same time, this element is connected to an ampoule, which is the part that is exposed to the temperature to be measured.

The mechanism of this instrument starts to work when the device is filled with pressurized gas, the most commonly used being nitrogen.. The gas is in the ampoule 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.

Uses

Gas thermometers are especially used to check that other thermometers are working properly, because they have a high accuracy and measuring range. The problem is that, as they are very complex instruments to use, they take a long time to measure the temperature, and they are difficult to use in a domestic environment.They are also difficult to use in the domestic environment.

Their 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 the object's temperature.. This is why, in 1871, Wilhelm Siemens made the proposal to use platinum to measure temperature. Later, in the 20th century, the resistance thermometer was invented, using essentially the same mechanism proposed by Siemens.

Today the material used in the resistance thermometer can vary. While there are models that still 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 measure temperature by evaluating the behavior of a wire made of platinumor other metals, which is integrated inside the thermometer. This wire is linked to an electrical resistance, which changes as a function of temperature.

Uses

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

6. Thermal couple or thermocouple

They are similar to resistance thermometers, in that they they measure the temperature from an electrical resistance that produces voltage, which varies according to the temperature of the measured object..

How do they work?

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

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

Uses

The thermocouples give the temperature quickly and efficiently. For this reason they are often used in laboratoriesFor this reason they are often used in laboratories, especially in processes where the temperature or the electromotive force required to heat up the welding of two different metals must be measured.

7. Digital thermometers

Digital thermometers are thermometers very similar to glass thermometers, since they are used as an electronic substitute for glass thermometers. They have been snatching popularity from the more classic ones for quite some time, and they have their advantages and disadvantages compared to glass ones.

How do they work?

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

Uses

This type of thermometers are very practical and easy to use, in addition to being economical. They are safer than the classic glass tube thermometers, especially in comparison with those using mercury..

They have different sizes and utilities. There are digital thermometers specialized in taking the temperature of newborns, made of soft and flexible materials that do not hurt babies' gums when introduced orally.

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

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

A disadvantage is that need batteries to operatewhich can run out of power. However, these batteries can be quite durable, readily available and inexpensive.

Bibliographical references:

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