Thermocouple is a sensor that measures temperature

Thermocouple is one of the most commonly used temperature detection components in the industry. 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

Show. 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, thus

A large and small current is formed in the loop. This phenomenon is called the electric effect of thermal zirconia probe. Thermocouples use this effect to work

Made.

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 standard thermocouple called refers to the country

The standard specifies the relationship between its thermoelectric potential and temperature, the allowable error, and the thermocouple with a unified standard graduation table. It

A matching display instrument is available for selection. Non-standardized thermocouples are not as good as the standard in the scope or order of magnitude

For thermocouples, there is generally no unified indexing table. Precision digital pressure gauges are 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, the structure requirements are as follows:

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

② The two hot electrodes 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 protection tube should be able to ensure that the hot electrode is fully isolated from harmful media.

A thermocouple is a temperature sensor. It is a temperature sensor like a thermal resistance, but the main difference between it and thermal resistance is:

1. The nature of the signal. The thermal resistance itself is a resistance. The temperature change causes the thermal resistance to produce a positive or negative resistance change; and a thermocouple produces a change in the induced voltage, which changes with the change of temperature.

2. The temperature ranges detected by the two sensors are different. The thermal resistance generally detects a temperature range of 0-150 degrees, and the highest measurement range can reach about 600 degrees (of course it can detect negative temperatures).

The thermocouple can detect the temperature range of 0-1000 degrees (or even higher). Therefore, the former Rosemount 3051 transmitter is low temperature detection, and the latter is high temperature detection.

3. From the point of view of material, thermal resistance is a kind of metal material with temperature-sensitive change of metal material, and thermocouple is a bimetal material, which is two different metals. Due to temperature changes, the two different metal wires A potential difference is generated at the end.

4. The input modules of thermal resistance and thermocouple corresponding to PLC are also different. This sentence is no problem, but generally PLC is directly connected to 4-20ma signals, while thermal resistance and thermocouple generally have transmitters. Before connecting to the PLC. If you connect to DCS, you don't need to use a transmitter! The thermal resistance is the RTD signal, and the thermocouple is the TC signal!

5. PLC also has thermal resistance module and thermocouple module, which can directly input resistance and electrical couple signals.

6. Thermocouples are available in J, T, N, K, S and other models. Rosemount temperature transmitters are more expensive than resistors, and some are cheaper than resistors. However, including compensation wires, the overall cost of thermocouples is higher.

Thermal resistance is a resistance signal, and thermocouple is a voltage signal.

Thermal resistance temperature measurement principle and material

The principle and material of thermal resistance temperature measurement 1. The principle and material of thermal resistance temperature measurement are currently the most widely used platinum and copper. The thermal resistance temperature measurement is based on the characteristic that the resistance value of a metal conductor increases with the increase in temperature for temperature measurement. Most of the resistors are made of pure metal materials. In addition, materials such as dian, nickel, manganese and rhodium have now been used to make thermal resistance. 2. The structure of the thermal resistance. The change in the measured temperature is directly measured by the change in the resistance of the thermal resistance. Therefore,

1Proficient thermal resistance: The structure and characteristics of thermal resistance temperature sensing elements (resistors) commonly used in industry. Known from the principle of temperature measurement of thermal resistance. The change of the resistance of various wires such as the lead wire of the thermal resistance body will affect the temperature measurement. In order to eliminate the influence of lead resistance, a three-wire system or a four-wire system is generally adopted. The outer diameter is generally φ2~φ8mm and the smallest can reach φmm. Compared with ordinary thermal resistance,

2 Armored thermal resistance: Armored thermal resistance is a solid body composed of temperature sensing element (resistor body) leads, insulation materials, and stainless steel casing. It has the following advantages: no air gap inside, small size. In terms of thermal inertia, the measurement lag is small; impact resistance ②good mechanical performance and vibration resistance. Convenient device ③ can be bent. ④Long service life. Close to the end of the thermometer. Compared with general axial thermal resistance,

3 End surface thermal resistance: The end surface thermal resistance temperature sensing element is wound by a specially treated resistance wire. 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 parts. The explosion of explosive mixed gas inside the enclosure due to the influence of sparks or arcs is confined to the junction box. 4 Flameproof thermal resistance: Flameproof thermal resistance passes through a special structure junction box. The production site will not cause an explosion. Explosion-proof thermal resistance can be used for temperature measurement in Bla~B3c zone with explosion hazard.

Introduction to the knowledge of thermocouple probes

Knowledge of the electric couple/probe: The thermocouple probe is one of the most commonly used temperature detection components in the industry. Its advantages are: ① High measurement accuracy. Because the thermocouple/probe is in direct contact with the measured object, it is not affected by the intermediate medium. ②Wide measuring range. Commonly used thermocouples/probes can be measured continuously from -50 to +1600℃, some special thermocouples/probes can measure as low as -269℃ thermocouples/probes thermocouples/probes (such as gold-iron-nickel-chromium), up to +2800℃ (such as tungsten-rhenium). ③Simple structure and convenient use. The thermocouple/probe is usually composed of two different metal wires, and is not limited by the size and beginning, and there is a protective sleeve outside, which is very convenient to use.

　　What is a thermocouple

　Thermocouple is a kind of temperature sensing element, which belongs to the contact temperature measurement in temperature measurement. It can convert the temperature signal into a thermoelectric potential signal, and the measured temperature can be measured with the cooperation of electrical measuring instruments.

The basic principle of thermocouple/probe temperature measurement

Two conductors or semiconductors A and B of different materials are welded together to form a closed loop. 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, thus in the loop The formation of a large current in the pyroelectric effect is called the thermoelectric effect. This is the so-called Seebeck effect. Thermocouples/probes use this effect to work.

Conductors A and B are called hot electrodes. The end with higher temperature (T> is called the working end (usually welded together); the end with lower temperature (To> is called the free end (usually at a constant temperature)).

According to the relationship between thermoelectric potential and temperature. It can be made into thermocouple index table. The index table is obtained under the condition of the free end temperature To=00C. Different thermocouples have different index tables.

When the third metal material is connected in the thermocouple circuit, as long as the temperature of the two junctions of the material is the same, the thermoelectric potential generated by the thermocouple will remain unchanged, that is, it will not be affected by the third metal in the circuit. Therefore, when the thermocouple measures the temperature, it can be connected to the measuring instrument. After the thermoelectric potential is measured, the temperature of the measured medium can be known. In theory, any two kinds of conductors can be made into thermocouples, but in fact, not all materials can make thermocouples, so the following points must be met for the thermoelectrode material: (1) After the thermocouple material is subjected to temperature It can produce higher thermoelectric potential, and the relationship between thermoelectric potential and temperature is preferably a linear or approximately linear single-valued function relationship; (2) It can measure higher temperatures and apply in a wider temperature range. After long-term use, the physical and chemical properties and thermoelectric properties remain stable; (3) The temperature coefficient of resistance of the material is required to be small, the resistivity is high, the electrical conductivity is good, and the heat capacity is small; (4) The reproducibility is good, which is convenient for mass production and Interchange, easy to formulate a unified indexing table; (5) Good mechanical performance, uniform material; (6) Rich resources and low price.

Types and structure formation of thermocouples/probes (1) Types of thermocouple probes Commonly used thermocouple probes can be divided into two categories: 1, standard thermocouple/probe 2, and non-standard thermocouple probe.

The so-called standard thermocouple probe refers to a thermocouple probe that has a unified standard graduation table that specifies the relationship between its thermoelectric potential and temperature, the allowable error, and its matching display instrument is available for selection. Non-standardized thermocouples/probes are inferior to standardized thermocouples/probes in the range or order of magnitude, and there is generally no unified indexing table, which is mainly used for measurement in some special occasions.

Standardized thermocouple probes. Since January 1, 1988, thermocouples/probes and thermal resistances have all been produced in accordance with IEC international standards, and seven standardized thermocouples/probes have been designated S, B, E, K, R, J, and T. The thermocouple/probe is uniformly designed for our country.

(2) The structure of the thermocouple/probe In order to ensure the reliable and stable operation of the thermocouple/probe, its structure requirements are as follows: ①The welding of the two thermoelectrodes that make up the thermocouple probe must be firm; ②The two thermoelectrodes must be connected to each other. It should be well insulated to prevent short circuit; ③The connection between the compensation wire and the free end of the thermocouple/probe 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

Since the materials of thermocouple probes 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 cool the thermocouple/probe. The end (free end) extends into the control room where the temperature is relatively stable, and is connected to the instrument terminal. It must be pointed out that the function of the thermocouple/probe compensation wire is only to extend the thermoelectrode and move the cold end of the thermocouple/probe to the instrument terminal of the control room. It cannot eliminate the influence of the temperature change of the cold end on the temperature measurement. Play compensation. Therefore, other correction methods are needed to compensate for the influence of cold junction temperature t0≠0℃ on temperature measurement.

When using thermocouple/probe compensating wire, you must pay attention to the matching of the model, the polarity can not be wrong, the temperature of the compensating wire and the thermocouple/probe connection end should not exceed 100℃