Armoured thermocouples are used as temperature measurement sensors and are usually used in conjunction with temperature transmitters, regulators and display instruments to form a process control system to directly measure or control fluids in the range of 0-1800 ℃ in various production processes. The temperature of steam and gaseous media and solid surfaces. Armored thermocouples have many advantages such as flexibility, high pressure resistance, fast thermal response time, sturdiness and durability. They are the same as industrial assembled thermocouples. They are used as temperature sensors and are usually matched with display instruments, recording instruments and electronic regulators. use.

Armoured thermocouples have many advantages such as flexibility, high pressure resistance, fast thermal response time and durability. They are the same as industrial assembled thermocouples. They are used as temperature sensors and are usually matched with display instruments, recording instruments and electronic regulators. At the same time, it can also be used as a temperature sensing element for assembled thermocouples. It can directly measure the temperature of liquid, steam, its gaseous medium and solid surface in the range of 0℃~800℃ in various production processes. Compared with fabricated thermocouples, armored thermocouples have the advantages of flexibility, high pressure resistance, short thermal response time and durability.

working principle

Armoured thermocouples are two different components of conductors that are welded at both ends to form a loop. The direct temperature measurement terminal is called the working terminal, and the terminal terminal is called the cold terminal, also called the reference terminal. When there is a temperature difference between the working end and the reference end, thermal current will be generated in the loop. Connect the display instrument, and the instrument will indicate the corresponding temperature value of the thermoelectromotive force generated by the thermocouple. The thermoelectromotive force of the armored thermocouple will increase as the temperature of the measuring terminal increases. The thermoelectromotive force is only related to the thermocouple conductor material and the temperature difference between the two ends, and has nothing to do with the length and diameter of the thermoelectrode. The structural principle of the armored thermocouple is that it is made of a conductor, a high-insulation magnesium oxide, and a 1Cr18Ni9Ti stainless steel protective tube with an outer jacket. Armored thermocouple products are mainly composed of junction box, terminal block and armored thermocouple, and are equipped with various installation fixtures.

Armored thermocouple is the most widely used temperature device in temperature measurement. Its main feature is wide temperature measurement range, relatively stable performance, simple structure, good dynamic response, and can 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 potentials; thermoelectric potential and contact potential. Thermoelectric potential refers to the potential generated by the two ends of the same conductor due to different temperatures. Different conductors have different electron densities, so they generate different potentials. The contact potential, as the name implies, refers to when two different conductors are in contact. Because their electron densities are different, a certain amount of electron diffusion occurs. When they reach a certain equilibrium, the potential formed by the contact potential depends on the material properties of the two different conductors and the temperature of their contact points.

There is a standard specification for thermocouples used internationally. The international regulations stipulate that thermocouples are divided into eight different divisions, namely B, R, S, K, N, E, J and T, and the lowest measurable temperature can be measured. Minus 270 degrees Celsius, up to 1800 degrees Celsius, of which B, R, S belong to the platinum series of heat
Armoured thermocouple
Armoured thermocouple
Galvanic couples, because platinum is a precious metal, they are also called precious metal thermocouples and the remaining ones are called cheap metal thermocouples. There are two types of thermocouples, common type and armored type. Ordinary thermocouples are generally composed of thermode, insulating tube, protective sleeve and junction box, while armored thermocouple is a combination of thermocouple wire, insulating material and metal protective sleeve. A solid combination formed by stretching. But the electrical signal of the thermocouple needs a special wire to transmit, this kind of wire is called compensation wire. Different thermocouples require different compensation wires, and their main function is to connect with the thermocouple to keep the reference terminal of the thermocouple away from the power supply, so that the temperature of the reference terminal is stable. Compensation wires are divided into two types: compensation type and extension type. The chemical composition of the extension wire is the same as that of the thermocouple being compensated. However, in practice, the extension wire is not made of the same material as the thermocouple. Even wires with the same electron density are replaced. The connection between the compensation wire and the thermocouple is generally very clear. The positive pole of the thermocouple is connected to the red wire of the compensation wire, and the negative pole is connected to the remaining color. Most of the general compensation wires are made of copper-nickel alloy.


Because the materials of armored 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 armored thermocouple materials and reduce costs, compensation wires are usually used to protect the armored thermocouple. The cold end (free end) of the couple extends into the control room where the temperature is relatively stable, and is connected 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 armored thermocouple 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 and cannot compensate. effect. Therefore, other correction methods are needed to compensate for the influence of cold junction temperature t0≠0℃ on temperature measurement. When using the sheathed 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 sheathed thermocouple connection end cannot exceed 100℃.

A thermocouple is welded together with two conductors of different components. When the temperature at both ends is different, a thermoelectric potential will be generated in the loop. Therefore, the electric potential
Armoured thermocouple
Armoured thermocouple
It is a kind of temperature sensing element that measures the temperature by measuring the electric potential. It is a kind of converter, which can convert the temperature signal into an electric signal and then display it by the display instrument.

The basic principle of thermocouple temperature measurement is the thermoelectric effect. Two metal conductors of different components are connected end to end into a closed loop. If the temperature of the two junctions is not equal, a thermoelectromotive force will be generated in the loop to form a thermal current. This is Thermoelectric effect. The thermocouple is made by welding one end of two different metal materials. The welded end is called the measurement end, and the unwelded end is called the reference end. The reference end is usually constant at a certain temperature (such as 00C) when the measurement end is heated. At the time, a thermoelectric potential is generated at the contact. If the temperature of the reference terminal is constant, the magnitude and direction of the thermoelectric potential are only related to the characteristics of the two metal materials and the temperature of the measuring terminal, but not to the fineness and length of the galvanic couple. When the temperature of the measuring terminal changes, the electric potential also changes, and there is a fixed functional relationship between the temperature and the thermoelectric potential, and the temperature can be measured by using this relationship.

Commonly used temperature instruments are armored thermocouples.

Armored thermocouple: measure the high temperature above 500℃, the temperature of the main steam of the thermal power plant, the temperature of the superheater tube wall, and the temperature of the high temperature flue gas. Features:
Armoured thermocouple
Armoured thermocouple
It can measure high temperature, stable performance, accurate and reliable, simple structure, easy to maintain, convenient for signal transmission and multi-point switching measurement. Main model: Index number: S or LB-3, the upper limit is 1300℃ (1600℃ for short time). B or LL-2 upper limit 1600℃ (short-time 1800℃) K or EU-2 upper limit 1200℃ (short-time 1300℃) T or CK upper limit-200~350℃ (short-time 400℃) E or EA-2 upper limit- 200~900℃ thermal resistance: high measurement accuracy, stable performance, high sensitivity, wide application range, remote inquiry, automatic temperature control and recording.

Platinum thermal resistance, the highest temperature measurement is 650℃, Pt50, Pt100, copper resistance: 50-150℃ Cu50, Cu100. Note: The error caused by the self-heating effect, the Pt working d current is less than 6mA, the effect of hysteresis, the large heat capacity, and the sufficient heat exchange, the measurement is accurate. Installation: When installing, it will form a countercurrent with the measured medium, at least 90°. Indexing number: S or LB-3, the upper limit is 1300°C (short-term 1600°C). B or LL-2 upper limit 1600℃ (short-time 1800℃) K or EU-2 upper limit 1200℃ (short-time 1300℃) T or CK upper limit-200~350℃ (short-time 400℃) E or EA-2 upper limit- 200~900℃: High measurement accuracy, stable performance, high sensitivity, wide application range, remote inquiry, and automatic temperature control and recording. Platinum, the highest temperature is 650℃, Pt50, Pt100, copper resistance:

Different materials and diameter armored thermocouple models, index numbers and recommended temperature electrode materials
Blowout-proof armored thermal resistance
Blowout-proof armored thermal resistance
The thermal response time of the sheathed thermocouple When there is a step change in temperature, the time required for the output value of the thermocouple to change to a certain percentage of the step change is called the thermal response time, which is represented by τ (take 50 % Is expressed by τ0.5). Thermal response time of armored thermocouple τ0.5 (seconds)

Note: The insulation resistance is expressed in MΩ・m, which is the product of the insulation resistance at room temperature and the length of the armored couple.

For example: 1000MΩ・m means the insulation resistance of a sample with a length of 1m is

The insulation resistance of a sample with a length of 1000MΩ and a length of 10m is 100MΩ.

For armored couples with a length less than 1m, it is calculated as 1m.

Socket-type junction boxes and armored thermocouples with compensation wires are not in this case.

Measuring range and accuracy

When the temperature changes in stages, the time required for the output of the thermocouple to change to 50% of the step change is called the thermoelectric response time.

The response time of the armored thermocouple should not be greater than the data in the following table: Unit: s

form

Exposure type: short thermal response time; suitable for measuring engine exhaust and other temperature measurement requiring fast response; low mechanical strength.
Armored thermocouple 191
Armored thermocouple 191
Shell connection type: short thermal response time; high nominal pressure (up to 34MPa); not suitable for occasions with electromagnetic interference.

Insulation type: The thermal response time is longer than the previous two, and the service life is longer; anti-electromagnetic interference; this type is often used in occasions where there is no special requirement for the thermal response time.

Separate type: Double branch, avoid signal interference, its characteristics are the same as the insulated type.

The armored thermocouple is mainly composed of junction box, terminal and armored thermocouple components, etc., and is equipped with various installation and fixing devices. Installation fixed type fixing device is for users to use when installing. Armoured thermocouples have five structural types: no fixed device, fixed ferrule type, movable ferrule type, fixed flange type, and movable flange type. The fixed ferrule type is only for one-time fixed use, and the movable ferrule type can be used for multiple fixed uses.
Measuring end (hot end) structure


The verification method and device of the metal melt fast armored thermocouple. The device is mainly composed of a flat heating coil that can accommodate two quartz tubes at the ends of the tested couple, two armored thermocouples with the same shape as the tested couple, and a corresponding temperature control display output device. The verification method is to first use two calibrated thermocouples to find out the point in the flat heating coil that makes the two calibrated couples have the same thermoelectric potential, and replace one of the calibrated couples with the tested couple. Under other conditions unchanged, wait After the reading of the tested couple is stable, compare it with the reading of the calibration couple to know whether the value of the tested couple is accurate. This method provides a laboratory verification method for rapid temperature measuring thermocouples, and can test and make a comprehensive evaluation of rapid temperature measuring thermocouples at multiple temperature points.

It is characterized in that: a. The device is composed of a flat heating coil, a pair of calibrated armored thermocouples and a temperature control display output device. b. The verification method is to first insert two calibrated thermocouples from both ends of the heating coil to make the hot ends of the armored thermocouple contact, and change the position of the calibrated thermocouple in the heating coil to make the thermoelectric potential of the two calibrated thermocouples the same , Replace one of the calibrated thermocouples with the tested fast thermocouple, read its steady state thermoelectric potential value and compare it with the calibrated thermocouple to know the accuracy of the tested fast armored thermocouple.

There are actually many techniques for using armored thermocouples.

1. It has bendable performance, except for the head, the thermal resistance can be bent in any direction, so it is suitable for the complicated structure and the temperature of the small equipment.

2. Armored thermocouple has good vibration and impact resistance. Therefore, its life is longer than ordinary thermal resistance.

3. Armored thermocouple has a long service life. Because the resistance body of the Kay installed thermal resistance is maintained by the twisted cover and metal sleeve of the Quanhua iron insulation material, the thermal resistance wire is not easily corroded by harmful media.

4. Explosion-proof thermal resistance through the junction box of special structure, the explosion of the explosive mixed gas inside the shell due to the influence of sparks or arcs, etc. is confined in the junction box.

5. The production site will not cause an explosion. Explosion-proof thermal resistance can be used for temperature measurement in places with explosion hazard in Bla-B3c level zone. Anti-corrosion thermal resistance 5) Anti-corrosion thermal resistance adopts PTFE anti-corrosion material.

6. As an integral protective sleeve or a two-section sleeve, the anti-corrosion treatment of this material can also be directly applied to the protective tube, divided into three forms: spraying, sintering and sleeve sealing.

7. It is suitable for measuring in strong alkali and corrosive medium, with a temperature resistance of 250℃, and the fixed installation situation can also use the same PTFE fixed thread.

8. According to the measurement temperature range and side measurement object, select the appropriate thermal resistance model, specification and maintenance tube information.


Differentiation method

The working principle is a closed loop composed of two different material conductors. Whenever there is a temperature ladder at both ends, a loop will be generated, and current will flow through this loop, and there will be electrical energy between the two ends. And this electrical energy is called thermoelectromotive force, which is the famous Seebeck effect (Seebeckeffect).

Two conductors of different materials are thermoelectrodes, and here, if the end with higher temperature is the normal working end, on the contrary, the end with lower temperature is the free end. Normally, the free end is in a constant temperature state. Down. According to the functional relationship between thermoelectromotive force and temperature, a thermocouple indexing table is made; the indexing table is obtained when the free end temperature is at 0℃, and different thermocouples have different indexing tables.

In the entire circuit of the thermocouple, if a third metal material is connected, and 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 connected to the third metal Influence in the loop. Therefore, when the thermocouple measures the temperature, the measuring instrument can be connected, and the temperature of the measured medium can be known after the thermoelectromotive force is measured. When the thermocouple measures temperature, the temperature of its cold end (the measuring end is the hot end, and the end connected to the measuring circuit through the lead is called the cold end) is required to maintain the same temperature, so that its thermoelectric potential is proportional to the measured temperature. If the (environment) temperature of the cold junction changes during measurement, it will seriously affect the accuracy of the measurement. Taking certain measures at the cold junction to compensate for the influence caused by the temperature change of the cold junction is called the cold junction compensation of the thermocouple. Dedicated compensation wire for connection with measuring instrument.

K-type thermocouple is currently the largest metal thermocouple used in the temperature measurement zone above 500℃, and its use is the sum of other metal thermocouples. Today we will understand how to divide the positive and negative poles of an armored thermocouple.

First, let's understand what the positive and negative poles are. The nominal chemical composition of the positive electrode (KP) is: Ni:Cr≈90:10, and the chemical composition of the negative electrode (KN) is: Ni:Si≈97. The negative electrode material is ferromagnetic and can be used to distinguish positive and negative electrodes.

K-type thermocouple has good linearity and large thermoelectromotive force, so it can be used in oxidizing and inert atmosphere. However, K-type thermocouples cannot be used directly at high temperatures in sulfur, reducing, or reducing and oxidizing alternate atmospheres and vacuum, nor are they recommended for use in weakly oxidizing atmospheres. Its positive electrode is nickel-chromium alloy (KP) containing 10% chromium, and the negative electrode is nickel-silicon alloy (KN) containing 3% silicon. Therefore, how to divide the positive and negative poles of the sheathed thermocouple? According to this characteristic, the positive and negative poles of the sheathed thermocouple can be easily distinguished with a magnet.

If the above method is used, the positive and negative poles of other thermocouples can be easily distinguished.

The ceramic pores of the armored thermocouple will fail if the components are blocked, and the method of electrification and dust removal is often used, but the effect is not ideal, and it cannot be used in an inflammable and explosive environment, which can reflect the measured more accurately and quickly The actual temperature of the end face is suitable for measuring the end face temperature of bearing bushes and other mechanical parts. It is not only widely used in industrial temperature measurement, but also made into a standard reference instrument. It has its 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. The thermoelectromotive force has a single value function relationship with the working end temperature. The index tables for the relationship between the temperature of various thermocouples and the thermoelectromotive force are all made when the temperature of the cold junction is zero. For this reason, a special compensation wire is used to extend the cold junction.

The main failure modes are wire breakage and thermocouple breakage caused by vibration, and thermocouple short circuit or disconnection caused by bending.

Application: Usually used with display instruments, recording instruments, electronic computers, etc. Directly measure the surface temperature of liquid, steam and gaseous media and solids in the range of 0-1300 degrees in various production processes.

Features

1 Less thermal response time, reducing dynamic errors;

2 Flexible installation

3 Large measuring range;

4 High mechanical strength, good pressure resistance;

International temperature scale
a. Temperature unit: Thermodynamic temperature is a basic physical quantity. Its unit Kelvin is defined as 1/273.16 of the thermodynamic temperature of the triple point of water. The difference with 273.15K (freezing point) is used to represent the temperature, so this is still retained method. By definition, the degree of Celsius is equal to Kelvin, and the temperature difference can also be expressed in degrees Celsius or Kelvin. The international temperature standard ITS-90 defines both the international Kelvin temperature (symbol T90) and the international Celsius temperature (symbol t90).

b. The general rules of the international temperature standard ITS-90: ITS-90 increases from 0.65K to the highest temperature actually measurable by the Planck radiation law using monochromatic radiation. ITS-90 is formulated in such a way that in the full range, any best estimate of T when the temperature is adopted, the measurement of T90 is much more convenient and more precise than the direct measurement of thermodynamic temperature, and has a high degree of complexity Present.

c. Definition of ITS-90:

The first temperature zone is between 0.65K and 5.00K, and T90 is defined by the relationship between the vapor pressure and temperature of 3He and 4He.

The second temperature zone is between 3.0K and the triple point of neon (24.5661K). T90 is defined by a helium gas thermometer.

The third temperature zone is from the triple point of flat hydrogen (13.8033K) to the freezing point of silver (961.78°C). T90 is defined by a platinum resistance thermometer, which uses a set of defined interpolation methods to index. For the temperature zone above the freezing point of silver (961.78°C), T90 is defined by Planck's law of radiation, and the reproduction instrument is an optical pyrometer.

Installation notes

1. The installation height of the pressure block must be the same, usually at 150~200mm above the level;

2. Three-sided spot welding is required for briquette welding. Do not spot welding the thermocouple insertion port. For side spot welding, be careful not to weld the stop screw;

3. The pipe wall temperature should be installed symmetrically up and down;

4. The armored thermocouple must be inserted in place, and the stop screw must be in place;

5. It is recommended to use a small slot box for armoured thermocouple leads;

6. Stainless steel wire must be used to replace the armored thermocouple;

7. The interface between the armored thermocouple and the furnace wall must be bandaged, and then pass through a self-made protective tube to lead out;

8. In order to prevent the settlement of the boiler, it is recommended to use a transition junction box, and then connect to the proximitor with a compensation wire;

9. The bridge frame or protective tube of the furnace wall and the proximitor should not be closed. It is necessary to prevent the high heat flow directly into the proximitor and damage the components inside (Tianjin Power Construction did not pay attention to it, causing the proximate damage);

10. The installation position of the proximitor should not be too far, otherwise the thermocouple length is not enough;

11. The wiring should be correct and reliable.