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A large collection of common thermocouples

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Temperature is an important parameter in industrial automation. Temperature instruments are generally divided into contact measuring instruments and non-contact measuring instruments. Modern contact temperature instruments are mainly composed of thermal resistance and thermocouple. There are many classification methods for thermal resistance and thermocouple, but they are generally classified by material. Today, let's take a look at common thermocouple types.

Thermocouple

As shown in the figure below, a thermocouple is a sensor made of two different metal or semiconductor materials. So all thermocouples are made of two materials.



Standard thermocouple

There are two kinds of materials for thermocouples. One is thermocouple materials which have been produced and used in large quantities, have stable process, meet professional standards or national standards, and have the same scale at the same time. This kind of thermocouple is called standard thermocouple;



Among them, the International Electrotechnical Commission has formulated a unified standard for thermocouple materials with good performance. Currently, there are 8 kinds of standardized thermocouples, namely:

Platinum Rhodium 10 platinum thermocouple: the graduation number is s, which belongs to precious metal thermocouple. The positive electrode is platinum rhodium alloy containing 10% rhodium and 90% platinum, bright white and hard; the negative electrode is pure platinum, bright white and soft.

The short-term working temperature can reach 1600 ℃, and the long-term working temperature is 0 ~ 1300 ℃;

Advantages: good corrosion resistance, good chemical stability, good resistance to corrosion and oxidation;

Because of its high accuracy, it is generally used in high temperature measurement with high accuracy requirements;

It has good reproducibility and can be used as a standard.

Disadvantages: the thermoelectric EMF is small, so the sensitivity is low;

It is more fragile in reductive gas environment;

The error of compensating wire is large;

The price is relatively high and the economic cost is high.

Platinum rhodium 13 platinum thermocouple: the graduation number is r, which belongs to precious metal thermocouple. The positive electrode is platinum rhodium alloy containing 13% rhodium and 87% platinum, bright white and hard; the negative electrode is pure platinum, bright white and soft.

Compared with s graduation, R graduation has almost the same performance except that the thermal electromotive force is about 15% larger.

Platinum rhodium 30 platinum rhodium 6 thermocouple: the graduation number is B, which belongs to precious metal thermocouple. The positive electrode is platinum rhodium alloy containing 30% rhodium and 70% platinum, which is bright white and hard; the negative electrode is platinum rhodium alloy containing 6% rhodium and 94% platinum, which is slightly soft.

The short-term working temperature can reach 1800 ℃, and the long-term measuring temperature range is 0-1600 ℃. There is no need to compensate the conductor when the free end is within 0-50 ℃.

Advantages: it is an ideal high temperature thermocouple, which can measure the high temperature of 1800 ℃ in a short time;

Under normal temperature, the EMF is very small, so it is not necessary to compensate the conductor;

It has good oxidation resistance and corrosion resistance;

The heat resistance and mechanical strength are better than those of R type.

Disadvantages: in the medium and low temperature environment, the thermoelectric EMF is very small, and the temperature measurement below 600 ℃ is very inaccurate;

The linearity of thermoelectric EMF is not good;

It belongs to precious metal and its price is high.


Nickel chromium nickel silicon thermocouple: the graduation number is k, the positive electrode is nickel chromium (nickel 90% chromium 10%), not magnetic; the negative electrode is nickel silicon (nickel 97% silicon 3%), slightly magnetic (so it can be judged by magnet).

The short-term working temperature can reach 1300 ℃, and the long-term working temperature is 0 ~ 1200 ℃.

Advantages: the thermoelectric electromotive force of this kind of thermocouple has good linearity, and the error is generally within 6 ~ 8 ℃;

The oxidation resistance is good below 900 ℃;

Among the metal thermocouples, it is the most stable one among the non precious metals. It is cheap and widely used.

Disadvantages: it is not suitable for reducing gas environment, especially carbon monoxide, sulfur dioxide, hydrogen sulfide and other gases. In this case, only the temperature below 500 ℃ can be measured;

In the low temperature environment, the nonlinear error is large.

Nickel chromium silicon nickel silicon thermocouple: the graduation number is n, the positive electrode is nickel chromium silicon (nickel 84% chromium 14% silicon 2%), not magnetic; the negative electrode is nickel silicon (nickel 95% silicon 5%), slightly magnetic (so it can be judged by magnet).

The short-term working temperature can reach 1300 ℃, and the long-term working temperature is - 200 ~ 1200 ℃.

The advantages of EMF are good linearity;

The oxidation resistance is good below 1200 ℃;

The heat resistance temperature is higher than that of K type,

cheapness.

Disadvantages: not suitable for reducing gas environment;

Compared with the precious metal thermocouple, the thermoelectric EMF changes greatly over time.



Nickel chromium constantan thermocouple: the graduation number is e, the positive electrode is nichrome (nickel 90% chromium 10%), dark green; the negative electrode is constantan (copper 55% nickel 45%), bright yellow;

The short-term working temperature can reach 800 ℃, and the long-term working temperature is - 200 ~ 760 ℃.

Advantages: it is a kind of thermocouple with the largest thermoelectric potential, with high measurement accuracy. Among the existing thermocouples, the sensitivity is the best and the price is low.

Disadvantages: easy to oxidize, not suitable for reducing gas environment.

Copper constantan thermocouple: the graduation number is t, the positive electrode is pure copper, red; the negative electrode is constantan (55% copper, 45% nickel), bright yellow;

The short-term working temperature can reach 400 ℃, and the long-term working temperature is - 200 ~ 350 ℃.

Advantages: good performance at low temperature, high sensitivity, it is widely used in low temperature thermocouple, good stability, good reproducibility, high precision, can be used in reduced gas environment, and the price is low.

Disadvantages: low temperature limit;

The positive electrode is easy to oxidize;

The error of heat conduction is large.

Iron constantan thermocouple: the division number is j, the positive pole is pure iron, and the magnetic affinity can be judged by magnet; the negative pole is constantan (55% copper, 45% nickel), not magnetic;

The short-term working temperature can reach 1200 ℃, and the long-term working temperature is - 200 ~ 750 ℃.

Advantages: it can be used in reductive gas environment;

The thermoelectric EMF is 20% higher than that of K thermocouple;

Good linearity, low price, suitable for medium temperature area.

Disadvantages: the positive electrode is easy to rust, can not dare to use in high temperature or sulfur medium, and poor reproducibility.

It should be pointed out that because S-type, K-type and B-type thermocouples are suitable for use in strong oxidizing and weak reducing atmospheres, while J-type and T-type thermocouples are suitable for weak oxidizing and reducing atmospheres. If protective tubes with better air tightness are used, the requirements for atmosphere are not very strict.



Non standard thermocouple

The production process of the other kind of thermocouple is not mature enough. It is not as good as the standard thermocouple in the application range and quantity. There is no unified index table and no matching display instrument. These thermocouples are mostly used to meet some special measurement requirements, such as ultra-high temperature, extremely low temperature, high vacuum or nuclear radiation environment. These thermocouples are called nonstandard thermocouples.

The common non-standard thermocouples are as follows:

It is recommended that the temperature range of rhenium-tungsten thermocouple is 0-25 ℃.

Advantages: it is the best among high temperature thermocouples. It has good plasticity and stability at low temperature.

Disadvantages: poor reproducibility, need to separate graduation.

Application: vacuum, reduction or neutral environment.

W-w-re-26 thermocouple: the recommended temperature range of this thermocouple is 0 ~ 3000 ℃.

Advantages: the upper limit of temperature measurement is the highest, and the electromotive force rate is large.

Disadvantages: poor reproducibility, need to separate graduation;

Tungsten is brittle when exposed to air.

Application: vacuum, reduction or neutral environment.

Pt Rh 20 Pt Rh 5 thermocouple: the recommended temperature range of this thermocouple is 500 ~ 1700 ℃.

Advantages: the upper limit of temperature measurement is higher than that of S-type, with good stability and no need of cold end compensation.

Disadvantages: it is easy to be polluted by metal vapor, and the electromotive force rate is small.

Application: in vacuum, oxidation or neutral environment.



Platinum rhodium 40 - platinum rhodium 20 thermocouple: the recommended temperature range of this thermocouple is 1000 ~ 1850 ℃.

Advantages: the upper limit of temperature measurement is high, the stability and reproducibility are good, and the influence of cold end can be ignored.

Disadvantages: it is easy to be polluted by metal vapor, and the electromotive force rate is small.

It is suitable for neutral or oxidizing environment.

Iridium rhodium iridium thermocouple: the recommended temperature range of this thermocouple is 1000 ~ 2200 ℃.

Advantages: the only thermocouple can be used in oxidation atmosphere above 1850 ℃.

Disadvantages: general stability and reproducibility, short life, easy to brittle, easy to iron stain, low EMF rate.

Application: in vacuum, oxidation or neutral environment.

Tungsten molybdenum thermocouple: the recommended temperature range of this thermocouple is 1200 ~ 2400 ℃.

Advantages: low price, can be used in reducing atmosphere.

Disadvantages: low electromotive force rate, the polarity of thermoelectric EMF turns at 1200 ℃, and is easily polluted by carbon, oxygen and silicon.

Application: vacuum, reduction or neutral environment.

Boron carbide graphite thermocouple: the recommended upper limit of temperature measurement is 2200 ℃.

Advantages: high EMF rate, stable physical and chemical properties, low price and simple structure.

Application: in carbon or neutral environment.

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