1. Overview

Thermocouple is a common temperature measuring element in the temperature measuring instrument. It directly measures the temperature, converts the temperature signal into thermoelectric EMF signal, and converts it into the temperature of the measured medium through the electrical instrument (secondary instrument). The shapes of various thermocouples are often very different due to their needs, but their basic structures are generally the same, usually composed of the main parts such as the thermal electrode, insulating sleeve protection tube and junction box, which are usually used together with the display instrument, recording instrument and electronic regulator.

In the process of industrial production, temperature is one of the important parameters to be measured and controlled. In temperature measurement, thermocouple is widely used. It has many advantages, such as simple structure, convenient manufacture, wide measurement range, high precision, small inertia and easy remote transmission of output signal. In addition, thermocouple is a kind of active sensor, which does not need external power supply, so it is very convenient to use, so it is often used to measure the temperature of gas or liquid in furnace and pipeline and the surface temperature of solid.

2, classification

(1) classification by fixture type

As the main temperature measurement means, thermocouples are widely used, so there are many requirements for the fixed device and technical performance. Therefore, the fixed device of thermocouples can be divided into six types: no fixed device type, screw type, fixed flange type, movable flange type, movable flange angle ruler type, cone-shaped protective tube type.

(2) classification by assembly and structure

According to the performance and structure of thermocouple, it can be divided into detachable thermocouple, flameproof thermocouple, armored thermocouple and pressure spring fixed thermocouple.

3. Types of common thermocouples

(1) K type thermocouple nickel chromium (nickel silicon (nickel aluminum) thermocouple)

K-type thermocouple is a base metal thermocouple with strong oxidation resistance. It can measure the medium temperature of 0-1300 ℃, which is suitable for continuous use in oxidizing and inert gases. Its short-term use temperature is 1200 ℃, long-term use temperature is 1000 ℃, and its relationship between thermal potential and temperature is approximately linear, so it is a thermocouple with large amount at present. However, it is not suitable to be used in the atmosphere of vacuum, sulfur, carbon and alternate oxidation-reduction; when the oxygen partial pressure is low, the chromium in the nickel chromium electrode will be preferentially oxidized, which makes the thermal potential change greatly, but the metal gas has little effect on it, so the metal protective tube is mostly used.

Disadvantages of K-type thermocouple:

① the high temperature stability of thermoelectric potential is worse than that of n-type thermocouple and precious metal thermocouple, which is often damaged by oxidation at higher temperature (for example, over 1000 ℃);

② the short-term thermal cycle stability is not good in the range of 250-500 ℃, that is to say, at the same temperature point, in the process of heating and cooling, the thermoelectric potential indication is different, and the difference can reach 2-3 ℃;

(3) the negative electrode will undergo magnetic transformation in the range of 150-200 ℃, resulting in the deviation of the graduation value from the graduation table in the range of room temperature to 230 ℃, especially the time independent thermoelectric interference when used in the magnetic field;

(4) in the environment of high flux medium system irradiation for a long time, the elements such as manganese (MN) and cobalt (CO) in the negative electrode have been transformed, which leads to poor stability and great changes in thermoelectric potential.

(2) S-type thermocouple (Platinum Rhodium 10 platinum thermocouple)

The positive component of the thermocouple is a platinum rhodium alloy containing 10% rhodium, and the negative electrode is pure platinum.

It is characterized by:

① it has stable thermoelectric property, strong oxidation resistance, suitable for continuous use in oxidizing atmosphere, long-term use temperature up to 1300 ℃, and over 1400 ℃, even in the air, the pure platinum wire will recrystallize, making the grain coarse and fracture;

② high precision, high accuracy level in all thermocouples, usually used as standard or measuring higher temperature;

③ it has a wide range of use, good uniformity and interchangeability;

The main disadvantages are: the differential thermoelectric potential is small, so the sensitivity is low; the price is expensive, and the mechanical strength is low, so it is not suitable for use in reducing atmosphere or under the condition of metal vapor.

(3) e type thermocouple (nichrome copper nickel [constantan] thermocouple)

E-type thermocouple is a new product, the positive electrode is nickel chromium alloy, the negative electrode is copper nickel alloy (Constantan). Its great characteristic is that in the commonly used thermocouple, its thermal potential is very large, that is, its sensitivity is very high; although its application scope is not as wide as K-type couple, it is often selected under the conditions of requiring high sensitivity, low thermal conductivity and large resistance tolerance; the limiting conditions in use are the same as K-type, but it is not very sensitive to corrosion with high humidity atmosphere.

(4) n-type thermocouple (Ni Cr Si Ni Si thermocouple)

The main characteristics of the thermocouple are as follows: under 1300 ℃, it has strong temperature regulation and oxidation resistance, good long-term stability and short-term thermal cycle recurrence, good nuclear radiation resistance and low temperature resistance. In addition, in the range of 400-1300 ℃, the linearity of n-type thermocouple is better than that of K-type thermocouple. But in the range of low temperature (- 200-400 ℃), the nonlinear error is large, and the material is hard Processing.

(5) J type thermocouple (iron constantan thermocouple)

J-type thermocouple: the positive pole of the thermocouple is pure iron and the negative pole is constantan (copper nickel alloy), which is characterized by low price. It is suitable for reduction or inert atmosphere of vacuum oxidation. The temperature range is from - 200 to 800 ℃, but the commonly used temperature is only below 500 ℃, because the oxidation rate of iron hot electrode is accelerated after exceeding this temperature. If the wire with coarse wire diameter is used, it can still be used in high temperature The thermocouple can resist hydrogen (H2) and carbon monoxide (CO) gas corrosion, but can not be used in high temperature (such as 500 ℃) sulfur (s) atmosphere.

(6) T-type thermocouple (copper copper nickel thermocouple)

T-type thermocouple: the positive pole of the thermocouple is pure copper, and the negative pole is copper nickel alloy (also known as Constantan). Its main features are: in base metal thermocouple, its accuracy is very high, and the uniformity of the thermoelectric pole is good; its use temperature is - 200 ~ 350 ℃, because the copper hot electrode is easy to oxidize, and the oxide film is easy to fall off, so when it is used in the oxidizing atmosphere, it can not exceed 300 ℃, In the range of - 200 ~ 300 ℃, their sensitivity is relatively high. Copper constantan thermocouples are also characterized by low price, which is one of the most commonly used type products.

(7) r type thermocouple (platinum rhodium 13 platinum thermocouple)

The positive electrode of the thermocouple is a platinum rhodium alloy containing 13% and the negative electrode is pure platinum. Compared with the S-type thermocouple, its potential rate is about 15%, and other properties are almost the same. This thermocouple is widely used as a high-temperature thermocouple in the Japanese industry, but less in China.