Temperature Sensors



Temperature is a physical quantity of matter that expresses quantitatively the common notions of heat and cold. The objects of low temperature are cold, while objects of higher temperatures are considered warm or hot. The temperature is measured quantitatively with thermometers. The thermometers can be calibrated with respect to different temperature scales.

Scales to measure temperature

The three most common scales to measure the temperature are:

  • The Celsius scale (degrees centigrade)
  • The Kelvin scale
  • The Fahrenheit scale

Almost everyone uses the Celsius scale (° C) for the measurement of most temperatures. The temperature variation between one degree and the next in a Celsius scale is the same variation as in a Kelvin scale. The difference between the Celsius and Kelvin scales is in the fixation of their null point: in the Celsius range the 0ºC corresponds to the freezing point of water. This temperature exfoliated on the Kelvin scale corresponds to 273.15 kelvins (273.15 K). The nucleus point of the Kelvin scale, the 0 kelvins, corresponds to the minimum temperature at which a body could theoretically reach. Colder than the 0 kelvins is impossible.

However, there are a few countries, especially the United States, where the Fahrenheit scale is still used in daily life. It is a historical temperature scale in which the freezing point of water is at 32 ° F and the boiling temperature of water is at 212 ° F.

The unit of measurement of the temperature in the International System of Units (SI) is Kelvin. The Kelvin is, therefore, the unit used by scientists.

For practical purposes of measuring the temperature within the fields of science, the International System of Units (SI) defines a scale and a unit for the thermodynamic temperature based on the triple point of water. The triple point is that in which the solid state, the liquid state and the gaseous state of a substance coexist in equilibrium. It is defined with a temperature and a vapor pressure. The triple point of water is a second easily reproducible reference point.

For historical reasons, the triple point of water has been set at 273.16 units of the measuring range. This interval is called kelvin (in lowercase) represented by the symbol K (capitalized) in honor of the Scottish physicist William Thomson (Lord Kelvin) who defined the scale for the first time.

Temperature and thermodynamics

The heat transfer is related in the change of temperature if there is no phase change. Thermodynamics.

One of the main properties studied in the field of thermodynamics is temperature. In thermodynamics, temperature differences between different regions of matter are especially important. These differences are those that allow the movement of heat from one region to another. The heat is that it is the transfer of thermal energy.

Spontaneously, the heat flows only from the regions of higher temperature in the regions of lower temperature. As stated in the second law of thermodynamics in the Clausius statement. So if heat is not transferred between two objects it is because both objects have the same temperature.

According to the classical thermodynamic approach, the temperature of an object varies proportionally to the velocity of the particles it contains. It does not depend on the number of particles (of the mass) but on its average speed: the higher the temperature, the higher the average speed. Therefore, the temperature is directly linked to the average kinetic energy of the particles that move relative to the object's center of mass.

Temperature is an intensive variable, since it is independent of the quantity of the particles contained inside an object, be they atoms, molecules or electrons. Temperature is a property that does not depend on the amount of substance or the type of material.

Temperature and thermal energy

The molecules of all material substances (solids, liquids and gases) are always in a continuous state of vibration or agitation, due to the multiple interactions that they suffer within the body. By virtue of this random agitation, the atoms and molecules of matter possess some internal energy, since they have kinetic energy in the form of movement and also potential energy due to the forces exerted between the particles.

The internal energy is also known as the thermal energy of the bodies.

On the other hand, the temperature is the magnitude that allows recording the average value of the thermal energy of a body.

(111)Medición de la temperatura(112) [Para poder determinar la temperatura de un sistema, éste debe estar en equilibrio termodinámico. Se puede considerar que la temperatura varía con la posición sólo si para cada punto hay una pequeña zona a su alrededor que se puede tratar como un sistema termodinámico en equilibrio. En la termodinámica estadística, en vez de partículas se habla de grados de libertad.]

In thermodynamics, a system is said to be in a state of thermodynamic equilibrium, if it is unable to spontaneously experience any change of state or thermodynamic process when it is subjected to certain boundary conditions.

In a more fundamental approach, the empirical definition of temperature is derived from the conditions of thermal equilibrium, which are expressed at the zero principle of thermodynamics. When two systems are in thermal equilibrium they have the same temperature. The extension of this principle as an equivalence relation between several systems basically justifies the use of the thermometer and establishes the principles of its construction to measure temperature. Although the zero law of thermodynamics would allow the empirical definition of many temperature scales, the second law of thermodynamics selects a single definition as the preferred one: absolute temperature, known as thermodynamic temperature.

This function corresponds to the variation of the internal energy with respect to changes in the entropy of a system. Its natural, intrinsic or zero point is absolute zero, where the entropy of any system is minimal. Although this is the absolute minimum temperature described by the model, the third law of thermodynamics postulates that absolute zero can not be reached by any physical system.

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Last review: November 9, 2016