The difference between thermocouple and thermal resistance1

Thermocouple

The thermocouple is one of the most commonly used temperature detection components in the industry. The working principle of the thermocouple is based on the Seeback effect, that is, two conductors of different components are connected to form a loop. If the temperature of the two connecting ends is different, the loop The physical phenomenon that generates thermal current inside. The advantages are:

①High measurement accuracy. Because the thermocouple is in direct contact with the measured object, it is not affected by the intermediate medium.

②Wide measuring range. Commonly used thermocouples can measure continuously from -50 to +1600°C. Some special thermocouples can measure as low as -269°C (such as gold, iron, nickel and chromium), and up to +2800°C (such as tungsten-rhenium).

③Simple structure and convenient use. Thermocouples are usually composed of two different metal wires, and are not limited by size and beginning, with a protective sleeve outside, which is very convenient to use.

1. The basic principle of thermocouple temperature measurement

Solder two conductors or semiconductors A and B of different materials to form a closed loop, as shown in Figure 2-1-1. When there is a temperature difference between the two attachment points 1 and 2 of the conductors A and B, an electromotive force is generated between the two, thus forming a large current in the loop. This phenomenon is called the thermoelectric effect. Thermocouples use this effect to work.

2. Types and structure of thermocouples

(1) Types of thermocouples

Commonly used thermocouples can be divided into two categories: standard thermocouples and non-standard thermocouples. The called standard thermocouple refers to the thermocouple whose thermoelectric power and temperature are stipulated in the national standard, the allowable error, and there is a unified standard graduation table. It has a matching display instrument for selection. Non-standardized thermocouples are inferior to standardized thermocouples in terms of use range or order of magnitude, and there is generally no unified indexing table, which is mainly used for measurement in some special occasions.

Standardized thermocouples. Since January 1, 1988, thermocouples and thermal resistances have all been produced in accordance with IEC international standards, and seven standardized thermocouples of S, B, E, K, R, J, and T are designated as my country’s unified design. Thermocouple.

(2) The structure of the thermocouple In order to ensure the reliable and stable operation of the thermocouple, its structure requirements are as follows:

①The welding of the two thermoelectrodes forming the thermocouple must be firm;

②The two hot electrodes should be well insulated from each other to prevent short circuit;

③The connection between the compensation wire and the free end of the thermocouple should be convenient and reliable;

④The protective sleeve should be able to ensure that the hot electrode is fully isolated from the harmful medium.

3. Temperature compensation of cold junction of thermocouple

Since the materials of thermocouples are generally more expensive (especially when precious metals are used), and the distance between the temperature measurement point and the instrument is very long, in order to save thermocouple materials and reduce costs, compensation wires are usually used to connect the cold end of the thermocouple (free End) extend into the control room where the temperature is relatively stable and connect to the terminal of the instrument. It must be pointed out that the function of the thermocouple compensation wire is only to extend the hot electrode and move the cold end of the thermocouple to the instrument terminal of the control room. By itself, it cannot eliminate the influence of the temperature change of the cold end on the temperature measurement and cannot compensate. Therefore, other correction methods are needed to compensate for the influence of cold junction temperature t0≠0℃ on temperature measurement.

When using the thermocouple compensation wire, you must pay attention to the matching of the model, the polarity can not be wrong, and the temperature of the compensation wire and the thermocouple connection end cannot exceed 100°C.

Thermal resistance

Thermal resistance is the most commonly used temperature detector in the medium and low temperature areas. Its main features are high measurement accuracy and stable performance. Among them, platinum thermal resistance has the highest measurement accuracy. It is not only widely used in industrial temperature measurement, but also made into a standard reference instrument.

1. The principle and material of thermal resistance temperature measurement

Thermal resistance temperature measurement is based on the characteristic that the resistance value of a metal conductor increases with temperature. Thermal resistors are mostly made of pure metal materials. At present, platinum and copper are the most widely used. In addition, materials such as nickel, manganese and rhodium have been used to make thermal resistors.

2. Type of thermal resistance

1) Ordinary thermal resistance

From the temperature measurement principle of the thermal resistance, the change of the measured temperature is directly measured by the change of the resistance of the thermal resistance. Therefore, the change of the resistance of various wires such as the lead wires of the thermal resistance will affect the temperature measurement.

2) Armored thermal resistance

Armored thermal resistance is a solid body composed of temperature sensing element (resistor body), lead wire, insulating material, and stainless steel casing. Its outer diameter is generally φ2--φ8mm, and the smallest can reach φmm. Compared with ordinary thermal resistance, it has the following advantages: ①Small size, no internal air gap, thermal inertia, small measurement hysteresis; ②Good mechanical performance, vibration resistance, impact resistance; ③Bendable, easy to install ④Long service life .

3) End surface thermal resistance

The end-face thermal resistance temperature-sensing element is wound by a specially treated resistance wire, which is closely attached to the end face of the thermometer. Compared with general axial thermal resistance, it can reflect the actual temperature of the measured end surface more accurately and quickly, and is suitable for measuring the end surface temperature of bearing bushes and other mechanical parts.

4) Flameproof thermal resistance

Explosion-proof thermal resistance through a junction box with a special structure confines the explosion of the explosive mixed gas inside the shell due to the influence of sparks or arcs in the junction box, and the production site will not cause over-explosion. Explosion-proof thermal resistance can be used for temperature measurement in places with explosion hazard in Bla-B3c level zone.

Thermocouple is the most widely used temperature device in temperature measurement. Its main characteristics are wide measurement range, relatively stable performance, simple structure, good dynamic response, and the ability to transmit 4-20mA electrical signals remotely, which is convenient for automatic control and centralized control. The principle of thermocouple temperature measurement is based on the thermoelectric effect. The temperature measurement principle of thermal resistance is based on the characteristic that the resistance value of a conductor or semiconductor changes with temperature.

Thermocouple is the most widely used temperature device in temperature measurement. Its main characteristics are wide measurement range, relatively stable performance, simple structure, good dynamic response, and the ability to transmit 4-20mA electrical signals remotely, which is convenient for automatic control and centralized control. The principle of thermocouple temperature measurement is based on the thermoelectric effect. Two different conductors or semiconductors are connected into a closed loop. When the temperature at the two junctions is different, thermoelectric potential will be generated in the loop. This phenomenon is called pyroelectric effect, also known as Seebeck effect. The thermoelectric potential generated in the closed loop is composed of two kinds of electric potential: thermoelectric potential and contact potential.

Although thermal resistance is also widely used in industry, its application is limited due to its temperature measurement range. The temperature measurement principle of thermal resistance is based on the resistance value of conductor or semiconductor changing with temperature. characteristic. It also has many advantages. It can also transmit electrical signals remotely. It has high sensitivity, strong stability, interchangeability and accuracy. However, it requires power supply and cannot instantaneously measure temperature changes.

Industrial thermal resistance generally uses Pt100, Pt10, Cu50, Cu100. The temperature measurement range of platinum thermal resistance is generally minus 200-800 degrees Celsius, and copper thermal resistance is minus 40 to 140 degrees Celsius. Thermal resistance does not require compensation wires and is cheaper than hot spot couples.