thermal resistance

WZP Thermal resistance (thermal resistor) produced by SHANGHAI AUTOMATION INSTRUMENTATION CO. LTD.

 

is the most commonly used in the low temperature zone of a temperature detector. Thermal resistance The temperature measurement is based on the resistance of the metal conductor with the temperature increase and increase the characteristics of the temperature measurement. Its main feature is the measurement of high precision, stable performance. Which platinum thermal resistance measurement accuracy is the highest, it is not only widely used in industrial temperature measurement, and was made a standard benchmark. Thermal resistance is mostly made of pure metal material, the most widely used platinum and copper, in addition, has now begun to use nickel, manganese and rhodium and other materials to create thermal resistance. Metal thermal resistance commonly used more types of temperature sensing materials, the most commonly used is platinum wire. Industrial measurement with metal thermal resistance material in addition to platinum wire, there are copper, nickel, iron, iron - nickel and so on.

 

 

Thermal resistance

1, compression spring-type temperature sensor, anti-vibration performance is good;

2, high temperature accuracy;

3, high mechanical strength, high temperature pressure resistance is good;

4, imported film resistive components, reliable and stable performance.

working principle

The thermal resistance of the principle of temperature measurement is based on conductors or semiconductors

The value of the resistance varies with temperature to measure the temperature and temperature-related parameters. Thermal resistance is mostly made of pure metal material, the most widely used is platinum and copper, has now begun to use nickel, manganese and rhodium and other materials to create thermal resistance. Thermal resistance usually requires the resistance signal to be transmitted to the computer control device or other secondary instrument through the lead.

Main species

Ordinary type thermal resistance

From the thermal resistance of the principle of temperature measurement shows that the measured temperature changes are directly measured by the thermal resistance of the resistance changes, therefore, the thermal resistance of the lead wire and other wire resistance changes in the temperature measurement will have an impact.

Armored thermal resistance

Armored thermal resistance is composed of temperature sensing element (resistance), lead, insulation material, stainless steel casing combination of solid body, its diameter is generally φ2 - φ8mm, the minimum up to φmm. Compared with the ordinary thermal resistance, it has the following advantages:

1, small size, no air gap inside, thermal inertia, the measurement of small lag;

2, good mechanical properties, vibration resistance, impact;

3, can be bent, easy to install;

4, long service life

End face thermal resistance

End face thermal resistance temperature sensing element by the special treatment of the resistance wire around the system, close to the thermometer end. Compared with the general axial thermal resistance, it can more accurately and quickly reflect the measured end of the actual temperature, suitable for measuring the bearing pad and other parts of the end surface temperature.

Flameproof thermal resistance

Explosion-proof thermal resistance through a special structure of the junction box, the shell of the internal explosive gas mixture due to spark or arc and other effects of the explosion confined to the junction box, the production site will not lead the super-explosion. Flameproof thermal resistance can be used for temperature measurement in explosive areas with Bla - B3c.

Principle of temperature measurement

Thermal resistance of the principle of temperature measurement and thermocouple temperature principle is different,

Thermal resistance is based on the thermal effect of resistance for temperature measurement, that is, the resistance of the resistor with the temperature changes in the characteristics of change. Therefore, as long as the measured temperature resistance of the resistance of the resistance changes, you can measure the temperature. At present, there are two types of metal thermal resistance and semiconductor thermistor.

The resistance and temperature of the metal thermal resistance can generally be expressed by the following approximate relationship

Rt = Rt0 [1 + α (t-t0)]

Where Rt is the resistance at temperature t; Rt0 is the corresponding resistance value when temperature t0 (usually t0 = 0 ℃); α is the temperature coefficient.

The relationship between the resistance and temperature of the semiconductor thermistor is

Rt = AeB / t

Where Rt is the resistance at temperature t; A, B depends on the constants of the structure of the semiconductor material.

In contrast, the thermistor temperature coefficient is greater, the resistance at room temperature higher (usually in the thousands of euros or more), but the interchangeability is poor, non-linear serious temperature range of only -50 ~ 300 ℃ , A large number of home appliances and automotive temperature detection and control. Metal thermal resistance is generally applicable to the temperature range of -200 ~ 500 ℃, which is characterized by accurate measurement, good stability, reliable performance, in the process of control is extremely extensive.

Industry commonly used metal thermal resistance from the resistance with the temperature changes, most of the metal conductors have this property, but not all can be used as thermal resistance, thermal resistance as a metal material General requirements: as large as possible and stable Of the temperature coefficient, the resistivity to be large (in the same sensitivity to reduce the size of the sensor), in the use of the temperature range has a stable chemical and physical properties, good copy of the material, the resistance value with temperature changes have a value function (Preferably in a linear relationship).

Practical application

Currently the most widely used thermal resistance material is platinum and copper: platinum resistance is high precision,

Suitable for neutral and oxidizing medium, good stability, with a certain non-linear, the higher the temperature resistance change rate; copper resistance in the temperature range of resistance and temperature linear relationship between the temperature line, No corrosive medium, more than 150 easy to be oxidized. China has the most commonly used R0 = 10Ω, R0 = 100Ω and R0 = 1000Ω and several other, their sub-degree were Pt10, Pt100, Pt1000; copper resistance R0 = 50Ω and R0 = 100Ω two, their index No. Cu50 and Cu100. Among them Pt100 and Cu50 are the most widely used.

Wiring

The thermal resistance is a primary element that converts the temperature change into a change in resistance value,

It is usually necessary to pass the resistance signal through the lead to the computer control device or other instrument. Industrial thermal resistance installed in the production site, and the control room there is a certain distance between, so the thermal resistance of the lead on the measurement results will have a greater impact.

At present, there are three main ways to lead the thermal resistance:

Two-wire system: the two ends of the thermal resistance to connect a wire to lead the way the resistance signal is called two-wire system: This lead method is very simple, but because the connection wire must exist lead resistance r, r size and wire material and length Factors, so this lead only applies to the measurement of low precision occasions

Three-wire system: one end of the root of the thermal resistance to connect a lead, the other end of the way to connect two leads known as the three-wire system, this approach is usually used with the bridge supporting the cable can be better to eliminate the impact of resistance The most commonly used in process control.

Four-wire system: the two ends of the root of the thermal resistance of the two wires connected to the way known as the four-wire system, in which two leads to the thermal resistance to provide a constant current I, the R into the voltage signal U, and then through the other two leads Lead U to the secondary instrument. It can be seen that this lead way can completely eliminate the resistance of the lead, mainly for high-precision temperature detection.

Thermal resistance using three-wire connection method. The three-wire system is used to eliminate the measurement error caused by the connection wire resistance. This is because the circuit that measures the thermal resistance is generally an unbalanced bridge. Thermal resistance as a bridge arm resistance, the connection wire (from the thermal resistance to the control room) has become part of the arm resistance, this part of the resistance is unknown and with the ambient temperature changes, resulting in measurement error. Using a three-wire system, the wire connected to the power supply side of the bridge, the other two were connected to the thermal resistance of the bridge arm and its adjacent arm, thus eliminating the wire line resistance measurement error.

installation method

Installation requirements

The installation of thermal resistance should pay attention to the temperature measurement is accurate, safe and reliable and easy maintenance,

And does not affect the equipment operation and production operations. To meet the above requirements, in the choice of thermal resistance to the installation site and insert the depth of the following points to note:

1, in order to make the thermal resistance of the measuring end and the measured medium between the full heat exchange, should be a reasonable choice of measuring point position, try to avoid the valve, elbow and pipe and equipment near the dead angle installed thermal resistance.

2, with a protective tube thermal resistance with heat transfer and heat loss, in order to reduce the measurement error, thermocouple and thermal resistance should have sufficient insertion depth:

1) For measuring the thermal resistance of the center of the pipe, the measuring end should normally be inserted into the center of the pipe (vertical or tilted). If the measured fluid pipe diameter is 200 mm, then the thermal resistance insertion depth should be selected 100 mm;

2) For high temperature and high pressure and high temperature fluid temperature measurement (such as the main steam temperature), in order to reduce the protective sleeve on the resistance of the fluid and to prevent the protective cover in the fluid under the action of the fracture can take protective tube or insert the use of warm sets Thermal resistance. Shallow plug of the thermal resistance of the casing, the insertion of the main steam pipe depth should not be less than 75mm; thermal sleeve thermal resistance of the standard insertion depth of 100mm.

3) If you need to measure the temperature of the flue gas flue, although the flue diameter of 4m, thermal resistance can be inserted into the depth of 1m.

4) When measuring the original insertion depth of more than 1m, should be installed as far as possible vertical, or the installation of support and protective casing.

Installation Attention

1, the thermal resistance should be installed vertically as far as possible on the horizontal or vertical pipe,

Installation should be protective casing, to facilitate maintenance and replacement.

2, measuring the temperature inside the pipe, the component length should be in the pipeline center line (that is, the protection tube insertion depth should be half the diameter).

3, the temperature dynamic ring table installation, the hole size to be appropriate, the installation should be beautiful and generous.

4, high temperature zone using high temperature cable or high temperature compensation line.

5, according to the different temperature to choose a different measurement components. General measurement temperature is less than 400 ℃ when the choice of thermal resistance.

6, wiring should be reasonable and beautiful, indicating the needle to be correct.

The main difference

Thermocouple and thermal resistance are part of the temperature measurement of contact temperature,

Although the same effect is to measure the temperature of objects, but their principles and characteristics are not the same.

Thermocouple is the most widely used temperature measurement temperature, his main feature is the wide temperature range, the performance is relatively stable, while the structure is simple, dynamic response, and more remote transmission 4-20mA electrical signal, easy to control and centralized control The Thermocouple temperature measurement principle is based on thermoelectric effect. Connect two different conductors or semiconductors to a closed loop. When the temperatures at the two junctions are different, the thermoelectric potential will be generated in the loop. This phenomenon is called thermoelectric effect, also known as the Seebeck effect. The thermoelectric potential generated in the closed loop has two potentials; the temperature difference potential and the contact potential. Temperature difference potential refers to the same conductor at both ends due to different temperatures generated by the potential, different conductors have different electron density, so they produce the potential is not the same, and the contact potential as the name suggests refers to two different contact with the conductor, Because of their electronic density, there is a certain amount of electron diffusion, and when they reach a certain equilibrium, the magnitude of the contact potential depends on the material properties of the two different conductors and the temperature of their contact points. At present the international application of the thermocouple has a standard specification, the international requirements of the thermocouple is divided into eight different degrees, respectively, B, R, S, K, N, E, J and T, the minimum temperature can be measured Measured at 270 ℃, up to 1800 ℃, where B, R, S are platinum series of thermocouples, because platinum is precious metals, so they are called precious metal thermocouple and the rest of the few are called cheap metal thermoelectric I. Thermocouple structure has two kinds, ordinary type and armored type. Ordinary thermocouples are typically made of hot electrodes,

Insulation tube, protective casing and the junction box and other components, and armored thermocouple is the thermocouple wire, insulating materials and metal protective sleeve combination of the three after assembly, after stretching from a solid Combination. But the thermocouple's electric signal requires a special wire to carry, this wire we call the compensation wire. Different thermocouples need different Compensation wire, the main role is to connect with the thermocouple, so that the reference side of the thermocouple away from the power supply, so that the reference end temperature stability. Compensation wire is divided into two types of compensation and extension, extending the chemical composition of the wire and the compensated thermocouple the same, but in practice, the extension of the wire is not the same material and thermocouple metal, the general use and thermal Even with the same electronic density of the wire instead. The connection of the compensating conductor to the thermocouple is generally clear, and the positive pole of the thermocouple connects the red wire of the compensating conductor, while the negative pole connects the remaining color. Most of the material of the compensation wire is copper nickel alloy.

Thermal resistance is not only widely used in industrial temperature measurement, but also made into a standard benchmark. But because of his temperature range so that his application has been a certain limit, the thermal resistance of the temperature measurement principle is based on the conductor or semiconductor resistance with the temperature changes in the characteristics of change. Its advantages are many, but also remote signal, high sensitivity, stability, interchangeability and accuracy are better, but the need for power excitation, can not instantaneous measurement of temperature changes. Industrial thermal resistance generally use Pt100, Pt10, Cu50, Cu100, platinum thermal resistance of the temperature range is generally minus 200-800 ℃, copper thermal resistance of minus 40 to 140 ℃. Thermal resistance and thermocouple the same type of distinction, but he does not need to compensate for the wire, and cheaper than the thermocouple.

Platinum thermal resistance installed in many forms, a fixed thread installation, the activities of threaded installation, fixed flange installation, the activities of the flange installation, the activities of the pipe fittings installation, straight pipe fittings and so on.

Thermal resistance and thermocouple selection is the biggest difference between the choice of temperature range, thermal resistance is measured low temperature sensor,

Generally measured temperature of -200 ~ 800 ℃, and thermocouple is to measure the temperature of the temperature sensor, the general measurement temperature of 400 ~ 1800 ℃, in the choice of the measured temperature of about 200 ℃ should choose the thermal resistance measurement, if the measured temperature At 600 ℃ should choose K-type thermocouple, if the measured temperature at 1200 ~ 1600 ℃ should choose S-type or B-type thermocouple.