# The working principle of the electronic thermometer

The thermoelectric thermometer uses a thermocouple as the temperature measuring element to measure the thermoelectromotive force corresponding to the temperature and the temperature value is displayed by the meter. It is widely used to measure the temperature in the range of -200℃~1300℃, and under special circumstances, it can measure the high temperature of 2800℃ or the low temperature of 4K. It has the characteristics of simple structure, low price, high accuracy, and wide temperature measurement range. Because the thermocouple converts temperature into electricity for detection, it is convenient to measure and control temperature, and to amplify and transform temperature signals. It is suitable for long-distance measurement and automatic control. In the contact temperature measurement method, the application of thermoelectric thermometers is the most common. (1) Thermocouple temperature measurement principle
The principle of thermocouple temperature measurement is based on the thermoelectric effect.
Connect the conductors A and B of two different materials in series into a closed loop. When the temperature of the two contacts 1 and 2 are different, if T>T0, a thermoelectromotive force will be generated in the loop, and there will be a certain amount in the loop. Large and small currents, this phenomenon is called pyroelectric effect. This electromotive force is the well-known “Seebeck thermoelectromotive force”, referred to as “thermoelectromotive force”, denoted as EAB, and conductors A and B are called thermoelectrodes. Contact 1 is usually welded together, and it is placed in the temperature measurement place to feel the measured temperature during measurement, so it is called the measurement end (or hot end of the working end). The junction 2 requires a constant temperature, which is called the reference junction (or cold junction). A sensor that combines two conductors and converts temperature into thermoelectromotive force is called a thermocouple.

The thermoelectromotive force is composed of the contact potential of two conductors (Peltier potential) and the temperature difference potential of a single conductor (Thomson potential). The magnitude of the thermoelectromotive force is related to the properties of the two conductor materials and the junction temperature.
The electron density inside the conductor is different. When two conductors A and B with different electron densities are in contact, electron diffusion occurs on the contact surface, and the electrons flow from the conductor with high electron density to the conductor with low density. The rate of electron diffusion is related to the electron density of the two conductors and is proportional to the temperature of the contact area. Assuming that the free electron densities of conductors A and B are NA and NB, and NA>NB, as a result of electron diffusion, conductor A loses electrons and becomes positively charged, while conductor B gains electrons and becomes negatively charged, forming an electric field on the contact surface. This electric field hinders the diffusion of electrons, and when dynamic equilibrium is reached, a stable potential difference is formed in the contact area, that is, the contact potential, whose magnitude is

(8.2-2)

Where k–Boltzmann’s constant, k=1.38×10-23J/K;
e–the amount of electron charge, e=1.6×10-19 C;
T–The temperature at the contact point, K;
NA, NB– are the free electron densities of conductors A and B, respectively.
The electromotive force generated by the temperature difference between the two ends of the conductor is called the thermoelectric potential. Due to the temperature gradient, the energy distribution of the electrons is changed. The high temperature end (T) electrons will diffuse to the low temperature end (T0), causing the high temperature end to be positively charged due to the loss of electrons, and the low temperature end to be negatively charged due to electrons. Therefore, a potential difference is also generated at the two ends of the same conductor and prevents electrons from spreading from the high temperature end to the low temperature end. Then the electrons diffuse to form a dynamic equilibrium. The potential difference established at this time is called the thermoelectric potential or Thomson potential, which is related to temperature For

(8.2-3) In the formula, σ is the Thomson coefficient, which represents the electromotive force value generated by a temperature difference of 1°C, and its magnitude is related to the material properties and the temperature at both ends.
The thermocouple closed circuit composed of conductors A and B has two contact potentials eAB(T) and eAB(T0) at the two contacts, and because T>T0, there is also a thermoelectric potential in each of conductors A and B. Therefore, the total thermal electromotive force EAB (T, T0) of the closed loop should be the algebraic sum of the contact electromotive force and the temperature difference electric potential, namely:

(8.2-4)

For the selected thermocouple, when the reference temperature is constant, the total thermoelectromotive force becomes a single-valued function of the measurement terminal temperature T, that is, EAB(T,T0)=f(T). This is the basic principle of thermocouple measuring temperature.

Post time: Jun-11-2021