Analysis of thermocouple and thermal resistor test parameters!

Thermocouple is one of the most commonly used temperature detection elements in industry. Its advantages are:
① High measurement accuracy. Because the thermocouple is in direct contact with the object to be measured, it is not affected by the intermediate medium.

② Wide measurement range. Commonly used thermocouples can measure continuously from -50~+1600℃, and some special thermocouples can measure as low as -269℃ (such as gold, iron, nickel and chromium) and as high as +2800℃ (such as tungsten-rhenium).
③ Simple structure and easy to use. Thermocouples are usually composed of two different metal wires, and are not limited by size and opening. There is a protective sleeve outside, which is very convenient to use.
1. Basic principle of thermocouple temperature measurement

Weld 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 conductors A and B, an electromotive force is generated between the two, and thus a current of a certain size is formed in the loop. This phenomenon is called the thermoelectric effect. Thermocouples work by using this effect.
2. Types and structures of thermocouples
(1) Types of thermocouples
Common thermocouples can be divided into two categories: standard thermocouples and non-standard thermocouples. The standard thermocouple refers to a thermocouple whose thermoelectric potential and temperature relationship, allowable error, and unified standard scale are specified by national standards. It has a matching display instrument for selection. Non-standard thermocouples are not as good as standardized thermocouples in terms of use range or order of magnitude, and generally do not have a unified scale. They are mainly used for measurements in certain special occasions. Standardized thermocouples Since January 1, 1988, my country has produced all thermocouples and thermal resistors in accordance with IEC international standards, and designated seven standardized thermocouples, S, B, E, K, R, J, and T, as my country's unified design thermocouples.
(2) Thermocouple structure In order to ensure that the thermocouple works reliably and stably, the structural requirements are as follows:
① The welding of the two thermocouples must be firm;
② The two thermocouples should be well insulated from each other to prevent short circuit;
③ The connection between the compensation wire and the free end of the thermocouple should be convenient and reliable;
④ The protective sleeve should ensure that the thermocouple is fully isolated from harmful media.
3. Temperature compensation of the cold end of the thermocouple

Since the materials of thermocouples are generally expensive (especially when precious metals are used), and the distance between the temperature measurement point and the instrument is very far, in order to save thermocouple materials and reduce costs, compensation wires are usually used to extend the cold end (free end) of the thermocouple to the control room where the temperature is relatively stable and connect it to the instrument terminal. It must be pointed out that the role of the thermocouple compensation wire is only to extend the thermocouple and move the cold end of the thermocouple to the instrument terminal in the control room. It itself cannot eliminate the influence of the cold end temperature change on the temperature measurement and does not play a compensation role. Therefore, other correction methods are required to compensate for the influence of the cold end temperature t0≠0℃ on the temperature measurement.
When using thermocouple compensation wires, it is necessary to pay attention to the matching of models, the polarity cannot be connected incorrectly, and the temperature of the connection end of the compensation wire and the thermocouple cannot exceed 100℃.
2. Application principle of thermal resistors
Thermal resistors are the most commonly used temperature detectors in the medium and low temperature range.
Its main features are high measurement accuracy and stable performance. Among them, platinum thermal resistors have the highest measurement accuracy. They are not only widely used in industrial temperature measurement, but also made into standard reference instruments.
1. Thermocouple temperature measurement principle and materials

Thermal resistor temperature measurement is based on the characteristic that the resistance value of metal conductors increases with the increase of temperature to measure temperature. Thermal resistors are mostly made of pure metal materials. Currently, platinum and copper are the most widely used. In addition, materials such as dian, nickel, manganese and rhodium have begun to be used to manufacture thermal resistors.
2. Thermocouple structure

(1) Proficient thermal resistor The structure and characteristics of the commonly used thermal resistor temperature sensing element (resistor body) in industry are shown in Table 2-1-11. From the temperature measurement principle of thermal resistors, it can be known that the change of the measured temperature is directly measured by the change of the resistance value of the thermal resistor. Therefore, the change of the resistance of various wires such as the lead wire of the thermal resistor will affect the temperature measurement. In order to eliminate the influence of the lead wire resistance, a three-wire system or a four-wire system is generally used. For details, please refer to the first section of Chapter 3 of this article.

(2) Armored thermal resistors Armored thermal resistors are a solid body composed of a temperature sensing element (resistance body), lead wires, insulating materials, and stainless steel sleeves, as shown in Figure 2-1-7. Its outer diameter is generally φ2~φ8mm, and the minimum can reach φmm.

Compared with ordinary thermal resistors, it has the following advantages: ① Small size, no air gap inside, small measurement lag in thermal inertia; ② Good mechanical properties, vibration resistance, and impact resistance; ③ Can be bent, easy to install ④ Long service life.

(3) End face thermal resistors The end face thermal resistor temperature sensing element is wound with specially treated resistance wire and is close to the end face of the thermometer. Its structure is shown in Figure 2-1-8. Compared with general axial thermal resistors, it can more accurately and quickly reflect the actual temperature of the measured end face, and is suitable for measuring the end face temperature of bearings and other parts.

(4) Flameproof thermal resistors Flameproof thermal resistors use a special structure of the junction box to confine the explosion of the explosive mixed gas inside its shell due to the influence of sparks or arcs to the junction box, so that the production site will not cause an explosion. Flameproof thermal resistors can be used for temperature measurement in explosion-hazardous places in the Bla~B3c level area.
3. Composition of thermal resistor temperature measurement system
The thermal resistor temperature measurement system generally consists of thermal resistors, connecting wires and display instruments. The following two points must be noted:
① The graduation number of the thermal resistor and the display instrument must be consistent
② In order to eliminate the influence of the resistance change of the connecting wire, the three-wire connection method must be adopted. For details, please refer to Chapter 3 of this article.

(2) Armored thermal resistor The armored thermal resistor is a solid body composed of a temperature sensing element (resistor), lead wire, insulating material, and stainless steel casing, as shown in Figure 2-1-7. Its outer diameter is generally φ2~φ8mm, and the minimum can reach φmm. Compared with ordinary thermal resistors, it has the following advantages: ① Small size, no air gap inside, small measurement lag in thermal inertia; ② Good mechanical properties, vibration resistance, and impact resistance; ③ Can be bent and easy to install; ④ Long service life.

(3) End face thermal resistor The end face thermal resistor temperature sensing element is wound with specially treated resistance wire and is close to the end face of the thermometer. Its structure is shown in Figure 2-1-8. Compared with general axial thermal resistors, it can more accurately and quickly reflect the actual temperature of the measured end face, and is suitable for measuring the end face temperature of bearings and other parts.

(4) Flameproof thermal resistors Flameproof thermal resistors use a specially structured junction box to isolate the explosive mixed gas inside the shell from the fire caused by sparks or arcs. Repairing the resistor will inevitably change the length of the resistor wire, which will affect the resistance value. Therefore, it is better to replace the resistor with a new one. If welding is used for repair, it must be calibrated and qualified before use.