November 1, 2021

Thermometers are devices used with the objective of knowing the temperature of a substance, body or environment on a scale known or managed by the user.

What is it?

The term comes from the Greek “thermos” which means “heat” and “metron” which means measure. As these words indicate, the thermometer is nothing more than an instrument used to measure temperature. Since it was invented to the present day it has evolved into high precision electronic thermometers.

In the past they were made taking advantage of the phenomenon of dilation. Elements were used that expanded to a great extent with temperature, in order to notice their stretching. One of the most used materials for this application is mercury and colored alcohols.

Its best known origin dates back to 1592, the year in which Galileo Galilei built the thermoscope, a device that used the contraction of air as it cooled to displace water in a tube that had a hollow glass ball welded together. The open end of the tube was immersed in a liquid and its temperature was judged by analyzing whether it rose or fell within it.

Between 1611 and 1613 Francesco Sagredo and Santorio Santorio added a numerical scale to this instrument.

Today the most common thermometers are composed of a glass bulb that contains a capillary loaded with mercury or alcohol. This expands or contracts depending on the temperature and allows us to measure on a graduated scale.

Mercury thermometers are banned in Spain, although they are still being used in Latin American homes.

Temperature scales

The most used scale is the Celsius (° C) in honor of Anders Celsius. On this scale, the temperature ranges from 0º to 100º C, the freezing and boiling points of water at a pressure of 1 atmosphere.

Another widely used scale is the Fahrenheit (° F) , made by Daniel Gabriel Fahrenheit. It is used in the English system of units.

The third most used scale is the Kelvin (K) known as absolute temperature and used in the International System of Units. Absolute zero is set at -273.15 ° C and is an unattainable value according to the third law of thermodynamics.

Types of thermometers

There are many types of thermometers:

Mercury thermometer: it was invented by Gabriel Fahrenheit in the year 1714 and consists of a capillary tube with mercury, its volume changes according to the temperature in a uniform range, which allows the application of a graduated scale.

Thermistor: it is an instrument that varies its electrical resistance according to temperature.

Wet bulb thermometer: measures the influence of ambient humidity on the thermal sensation. Its bulb or tank has a cotton cloth soaked in water that is placed to one side and much lower than the bulb.

Gas thermometer: they can be of two types at constant pressure or constant volume. They are accurate and are used to calibrate other thermometers.

Globe thermometer: used to measure average radiant temperature. It is a traditional mercury thermometer whose bulb is surrounded by a hollow black metal sphere. It absorbs radiation from hotter objects in the environment and therefore emits thermal radiation. It is used to evaluate the comfort of people in environments.

Bimetallic foil thermometer: it has two metal foils with different coefficients of expansion.

Maximum and minimum thermometer: it is used in meteorology to know the lowest and highest temperatures of the day. It is composed of two instruments installed in a single computer.

Resistance thermometer: it is made of a metal wire whose resistance to electricity varies with temperature.

Clinical thermometers: they are used to measure the temperature of a living being. They can be mercury or digital.

Digital thermometers: they have transducers or thermistors that transmit their information to electronic circuits that translate the temperature on a screen.

Thermocouple: allows to measure temperature using the electromotive force generated when heating the weld between two different metals.

Pyrometers: They are thermometers used to measure high temperatures, they are widely used in functions, factories and ovens. They measure temperatures without being in contact with the substance or the surface. They may be:

  • Infrared pyrometer: they record infrared radiation, which is converted into an electric current that is translated by a circuit which calculates the temperature. It measures temperatures below 0ºC and above 2,000 ° C.
  • Total radiation pyrometer: it is based on the Stefan-Boltzmann law, which establishes that the intensity of energy emitted by a black body is directly proportional to the fourth power of its absolute temperature.
  • Photoelectric pyrometer: it uses the photoelectric effect, according to which electrons are released from crystalline semiconductors when they are affected by thermal radiation.
  • Optical pyrometer: works based on Wien’s law of distribution of thermal radiation, which states that the color of radiation from a surface varies with temperature. It measures temperatures ranging from 700 ° C to 3,200 ° C.

Thermograph: it is a thermometer attached to a piece of equipment that records, graphically or digitally, the temperature on time scales or continuously.

Dr. Loony Davis5
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Born and raised in Brussels in an English family, I have always lived in a multicultural environment. After several work experiences in marketing and communication, I came to Smart Water Magazine, which I describe as the most exciting challenge of my career.
I am a person with great restlessness and curiosity to learn, discover what I do not know, as well as reinvent myself daily, someone who is curious about life and wants to know. I enjoy sharing knowledge.
This is my personal project but I also collaborate in other blogs, it is the case, the most important web on water currently exists in the US, if you are interested you can read my articles here.

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