There are many different types of thermometers, many of which have 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 for taking temperatures in underwater or moving objects.
Then we will examine the main families of types of thermometers, Its operating mechanism and its uses, both in the domestic, sanitary and industrial fields.
Types of thermometers, classified and explained
Although there are countless types of thermometers, they can be classified into seven major families., Depending on its operating mechanism. There are a few that are well known in the home field, such as glass classics and digital ones, although there are others, such as pyrometers, which are widely used in fields such as health.
1. Glass or liquid thermometer
The glass thermometer, also known as a liquid thermometer, is the most well-known and prototype type of thermometer. It is also called a manual or mercury thermometer because it was traditionally filled with this liquid metal and does not require batteries to operate. Eventually, mercury was replaced by other substances because this element is toxic.
A curiosity about this thermometer is that was invented by Daniel Fahrenheit, who invented one of the temperature measuring systems, the Fahrenheit scale.
How do they work?
Glass thermometers are made of a sealed glass tube called a capillary. At one end of the capillary is a piece of metal, called a bulb, which contains a liquid, which can be mercury or alcohol with a red tint.
When the metal part comes in contact with the object at which its temperature is to be measured, the part can either expand or expand. Therefore, the liquid inside rises or falls along the capillary and mark the thermal value on the scale written along the tube.
The glass thermometer is widely used in the daily home environment, Thanks to the fact that it is easy to use, does not require batteries and is relatively easy to obtain.
The most common use is to measure body temperature, especially when a person is suspected of having a fever. To measure body temperature, the thermometer is placed in the patient’s mouth, armpit or rectum, the bulb is waited to warm up, and the temperature is observed to mark the scale.
It is for this reason that for a decade now, pharmaceutical companies have been manufacturing these instruments with alcohol instead of mercury, as they are glass tubes placed in body holes, there was a risk that this substance, known to cause Minamata disease, would be broken and accidentally ingested..
Other uses of glass thermometers are in food preparations, industrial processes, and temperature measurement in aquariums.
2. Non-contact pyrometers or thermometers
The pyrometer, or non-contact thermometer, is a type of thermometer that works without needing to touch the object whose temperature is to be measured, by 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 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 higher the radiation.
The lens has a sensor that transforms this infrared radiation into electrical current, which travels through a circuit that will eventually cause a display to indicate the temperature of the object being assessed.
The uses of pyrometers are diverse, both at home and in more specialized places.
They can be used with babies, especially taking into account that they are very sensitive and placing a glass thermometer on them can disturb them, making measurement very difficult. Thanks to the fact that pyrometers can be used without touching the baby, Can be used while sleeping. All you need to do is project the light from the device and record its temperature.
As for industry, pyrometers they are used to measure the temperature of very hot surfaces that other thermometers have not been able to assess because they would burst or be damaged. Indeed, pyrometers can record very high temperatures, some reaching 700 ° C or even 3200 ° C.
3. Bimetallic foil thermometers
Bimetallic foil thermometers they measure temperature by means of a mechanism containing two different types of metals, Which, depending on how they contract or expand, will help indicate the temperature of the object at which the temperature is being taken.
How do they work?
Bimetallic 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.
This bimetallic sheet is found forming a spiral inside a tube. The spiral is welded from one end to the other of this capillary, fixed to a transmission rod. In turn, the transmission rod is welded to a needle that will indicate the temperature of the measured object.
Bimetallic foil thermometers they are usually not used at home, but in industrial processes in which aggressive or dangerous substances need to be measured.
We have some examples of the use of these instruments in the pharmaceutical, food, chemical, textile and petrochemical industries.
These thermometers, unlike pyrometers, make direct contact with the substance to take its temperature. They can record temperatures from -70 ° C to over 600 ° C.
4. Gas thermometers
Gas thermometers are a little used instrument in the domestic field, but in the industrial field. They contain a gas, 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 an element responsible for measuring the pressure, which is connected to a capillary and, at the same time, this element is connected to a bottle, part of which is exposed to the temperature that is to be measured.
The mechanism of this instrument begins to function when the apparatus is filled with pressurized gas, being the nitrogen most used. The gas is in the bottle, and depending on the temperature of what is being measured, this gas will push handcuffs, which indicate the temperature of the same.
Gas thermometers are mostly used to check that other thermometers are working properly, thanks to the fact that they have high accuracy and measuring range. The problem is that, as they are instruments the use is very complex, they take some time to measure the temperature, In addition to the domestic are difficult to apply.
Its temperature measurement range is -450 ° F to 1000 ° F (-268 ° C to + 538 ° C).
5. Resistance thermometers
In the 19th century, it was discovered that the electrical resistance of an object varied with its temperature.. This is why, in 1871, Wilhelm Siemens proposed to use platinum to measure temperature. Later, in the twentieth century, the resistance thermometer will be invented, which would use, in essence, the same mechanism proposed by Siemens.
Today, the material used in resistance thermometer may 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 take 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 with temperature
Their measuring capability is extremely large, ranging from 200 ° C to 3568 ° C, and they take their time measuring temperature. They are generally used to take the outside temperature.
6. Thermal couple or thermocouple
They look like resistance thermometers because they measure the temperature from an electrical resistance which produces a voltage, which varies according to the temperature of the object being measured.
How do they work?
This device consists of two metal wires which are joined at one end. The seal becomes the measuring point, while the ends are identified as hot and cold seal.
The ends of this mechanism should be placed on the object to be measured. This will make the measuring point hot, causing an electrical voltage, generating a voltage proportional to the temperature of the object.
Thermal pairs give temperature quickly and efficiently. For this reason they are generally used in laboratories, Particularly in processes in which the temperature or electromotive force required to heat the welding of two different metals is to be measured.
7. Digital thermometers
Digital thermometers are very similar to glass thermometers as they are used as electronic substitutes for them. They have long attracted the popularity of the more classics and have their advantages and disadvantages compared to glass glasses.
How do they work?
Digital thermometers they measure the temperature thanks to a mechanism that captures energy through a resistance. Resistance generates an electric current that flows through a circuit, which is responsible for transforming electricity into a value that appears on a screen, indicating the measured body temperature.
These types of thermometers are very convenient and easy to use, as well as being economical. They are safer than conventional glass tubes, especially compared to those that use mercury.
They have different sizes and utilities. There are digital thermometers that specialize in taking baby temperatures, made from soft, flexible materials that don’t hurt babies’ gums when introduced orally.
As for other areas, we have very diverse applications of digital thermometers in industry, home, aquariums, confectionery, gardening and veterinary.
On average, these thermometers give the result after two or three minutes. Some have a memory, storing the latest temperature measurement results, as well as light and sound indicators that notify us when you have already measured the temperature.
A downside is that they need batteries to function, Who can run out. However, these batteries can be quite durable, easy to obtain, and inexpensive.
- Creus Solé, A. (2005). Industrial instrumentation. Marcombo. ISBN 84-267-1361-0. Pg. 283-296.