WZPM-201 Thermal Resistance
Product Name: WZPM-201 Thermal Resistance WZPM-201 Thermal Resistance
Number of products: 153443-226
Product model: WZPM-201
Produced by: Thermocouple Thermocouple, thermal resistance, bimetal thermometer, polytetrafluoroethylene, tetrafluorogasket--Shanghai Feilong Instrument & Electrical Co., Ltd. Ordering instructions: Temperature probe and compensation line can be customized according to customer needs
Product details The working principle of thermal resistance Thermal resistance is the most commonly used temperature detector in low and medium temperature areas. Its main features are high measurement accuracy and stable performance. Among them, the measurement accuracy of platinum thermal resistance is the highest. It is not only widely used in industrial temperature measurement, but also made into a standard reference instrument.
Different from the temperature measurement principle of the thermocouple, the thermal resistance is based on the thermal effect of the resistance for temperature measurement, that is, the characteristic of the resistance of the resistor body changing with temperature. Therefore, as long as the resistance change of the temperature-sensing thermal resistance is measured, the temperature can be measured. There are mainly two types of metal thermistors and semiconductor thermistors.
The resistance value and temperature of metal thermal resistance can generally be expressed by the following approximate relationship, that is, Rt = Rt0 [1 + α (t-t0)]
In the formula, Rt is the resistance value at temperature t; Rt0 is the corresponding resistance value at temperature t0 (usually t0 = 0 ° C); 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 value at temperature t; A and B depend on the constant of the structure of the semiconductor material.
In comparison, the thermistor has a larger temperature coefficient and a higher resistance value at normal temperature (usually above several thousand ohms), but it has poor interchangeability and serious non-linearity, and the temperature measurement range is only -50 to 300 ° C. It is widely used for temperature detection and control of home appliances and automobiles. Metal thermal resistance is generally suitable for temperature measurement in the range of -200 ~ 500 ℃. Its characteristics are accurate measurement, good stability, reliable performance, and it is widely used in process control.
Thermal resistance material Thermal resistance temperature measurement is based on the characteristic that the resistance value of a metal conductor increases with increasing temperature. Most of the thermal resistances are made of pure metal materials, and platinum and copper are currently the most widely used. In addition, materials such as nickel, manganese and rhodium have been used to make thermal resistances.
Thermal resistance types (1) Precision thermal resistance: The structure and characteristics of thermal resistance temperature sensing elements (resistors) commonly used in industry. From the temperature measurement principle of the thermal resistance, it can be known that the change of the measured temperature is directly measured by the change of the resistance value of the thermal resistance. Therefore, changes in 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 the lead resistance, a three-wire system or a four-wire system is generally used.
(2) Armored thermal resistance: Armored thermal resistance is a solid body composed of a temperature sensing element (resistor), leads, insulation material, and stainless steel sleeve. Its outer diameter is generally φ2 ~ φ8mm, and the minimum can reach φmm. Compared with ordinary thermal resistance, it has the following advantages:
①Small volume, no air gap inside, small measurement lag in thermal inertia;
② Good mechanical properties, vibration resistance and impact resistance;
③ Can be bent for easy installation;
④ Long service life.
(3) End face thermal resistance: The end face thermal resistance temperature sensing element is wound by a specially treated resistance wire and is closely attached to the end face of the thermometer. Compared with the general axial thermal resistance, it can more accurately and quickly reflect the actual temperature of the measured end face, and is suitable for measuring the end face temperature of the bearing pad and other parts.
(4) Explosion-proof thermal resistance: Explosion-proof thermal resistance uses a special structure of the junction box to limit the explosion of the explosive mixed gas inside its casing to the junction box due to the effects of sparks or arcs. Super explosion. Explosion-proof thermal resistance can be used for temperature measurement in explosion-hazardous areas in the Bla ~ B3c class area.
From the perspective of changes in resistance with temperature, most metal conductors in industry have this property, but not all can be used as temperature measuring thermal resistance. Metal materials as thermal resistance generally require that it be as large and stable as possible. Temperature coefficient, resistivity should be large (reduce the size of the sensor under the same sensitivity), have stable chemical and physical properties in the used temperature range, good reproducibility of the material, and the value of resistance should have a relationship function with temperature (Preferably linear).
At present, the most widely used thermal resistance materials are platinum and copper: platinum has high resistance, is suitable for neutral and oxidizing media, has good stability, has a certain non-linearity, the higher the temperature, the smaller the resistance change rate; copper resistance is under test In the temperature range, the resistance value has a linear relationship with the temperature, and the number of temperature lines is large. It is suitable for non-corrosive media, and it is easy to be oxidized beyond 150. The most commonly used in China are R0 = 10Ω, R0 = 100Ω, and R0 = 1000Ω. Their division numbers are Pt10, Pt100, and Pt1000. Copper resistors have two types: R0 = 50Ω and R0 = 100Ω. The numbers are Cu50 and Cu100. Among them, Pt100 and Cu50 are the most widely used.
Signal connection method of thermal resistance Thermal resistance is a primary element that converts temperature changes into resistance changes. Usually, resistance signals need to be transmitted to computer control devices or other primary instruments through leads. Industrial thermal resistance is installed at the production site, and there is a certain distance from the control room, so the lead of the thermal resistance will have a greater impact on the measurement results.
At present, there are three main ways of thermal resistance. ○ Two-wire system: The method of connecting a wire at each end of the thermal resistor to lead the resistance signal is called a two-wire system: This method of wiring is very simple, but because the connecting wire must have a resistance The size of r and r is related to the material and length of the wire. Therefore, this lead method is only suitable for occasions with low measurement accuracy. 2 Three-wire system: One lead is connected to one end of the root of the thermal resistance, and the other end is connected to two. The lead method is called a three-wire system. This method is usually used in conjunction with an electric bridge to better eliminate the effect of lead resistance. It is the most commonly used lead resistance in industrial process control.
○ 3 Four-wire system: The method of connecting two wires at the two ends of the root of a thermal resistor is called a four-wire system, in which two leads provide a constant current I for the thermal resistor, convert R into a voltage signal U, and then pass the other two The lead leads U to the secondary meter. It can be seen that this lead method can completely eliminate the resistance effect of the lead and is mainly used for high-precision temperature detection.
Thermal resistance adopts three-wire connection method. The three-wire system is used to eliminate measurement errors caused by the resistance of the connecting wires. This is because the circuit for measuring thermal resistance is generally an unbalanced bridge. The thermal resistance is a bridge arm resistance of the bridge, and its connecting wire (from the thermal resistance to the central control room) also becomes a part of the bridge arm resistance. This part of the resistance is unknown and changes with the ambient temperature, causing measurement errors. The three-wire system is used to connect one lead to the power supply terminal of the bridge, and the other two to the bridge arm where the thermal resistance is located and the bridge arm adjacent to it. Industry generally adopts the three-wire connection method. Thermocouples produce millivolt signals, which is not a problem.
Composition of thermal resistance temperature measurement system
(1) Thermal resistance temperature measurement system generally consists of thermal resistance, connecting wires and display instruments. You must pay attention to the following two points:
① The graduation number of the thermal resistance and the display instrument must be the same
② In order to eliminate the influence of the resistance change of the connecting wire, a three-wire connection method must be used. For details, see Chapter 3 of this article.
(2) Armored thermal resistance Armored thermal resistance is a solid body composed of a combination of temperature sensing elements (resistors), leads, insulation materials, and stainless steel sleeves. Its outer diameter is generally φ2 ~ φ8mm, and the minimum can reach φmm. . Compared with ordinary thermal resistance, it has the following advantages:
①Small volume, no air gap inside, small measurement lag in thermal inertia;
② Good mechanical properties, vibration resistance and impact resistance;
③ Bendable for easy installation ④ Long service life.
(3) End-face thermal resistance The end-face thermal resistance temperature-sensing element is wound by a specially treated resistance wire and is closely attached to the end surface of the thermometer. Compared with the general axial thermal resistance, it can more accurately and quickly reflect the actual temperature of the measured end face, and is suitable for measuring the end face temperature of the bearing pad and other parts.
(4) Explosion-proof thermal resistance Explosion-proof thermal resistance uses a junction box with a special structure to repair the explosive mixed gas inside its casing due to the interruption of the spark resistor or the arc resistor, which must change the length of the resistance wire and affect the resistance value For this reason, it is better to replace the new resistor body. If welding repair is used, it can only be used after it has been verified after welding.
The difference between thermocouples and thermal resistors Both thermocouples and thermal resistors are contact temperature sensors in temperature measurement. Although their functions are the same to measure the temperature of an object, their principles and characteristics are not the same.
First of all, introduce the thermocouple. Thermocouple is the most widely used temperature device in temperature measurement. Its main characteristics are wide kiss range, stable performance, simple structure, good dynamic response, and the ability to transmit 4-20mA electricity. Signal for easy 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, a thermoelectric potential will be generated in the loop. This phenomenon is called the thermoelectric effect, also known as the Seebeck effect. The thermoelectric potential generated in a closed loop is composed of two potentials; the temperature difference potential and the contact potential. The temperature difference 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 its name implies, is when two different conductors are in contact. Because of their different electron densities, there is a certain amount of electron diffusion. When they reach a certain equilibrium, the potential formed. The size of the contact potential depends on the material properties of the two different conductors and the temperature of their contact points. At present, the thermocouples used in the world have a standard specification. The international regulations stipulate that the thermocouple is divided into eight different graduations, which are B, R, S, K, N, E, J, and T. The lowest possible measurement temperature is Measured at minus 270 degrees Celsius, up to 1800 degrees Celsius, of which B, R, and S belong to platinum series thermocouples. Since platinum is a precious metal, they are also called precious metal thermocouples and the remaining ones are called cheap metal thermoelectric I. There are two types of thermocouples, ordinary and armored. Ordinary thermocouples are generally composed of thermal electrodes, insulating tubes, protective sleeves, and junction boxes. Armored thermocouples are assembled by combining thermocouple wire, insulating material and metal protective sleeve. It is a solid combination made by drawing. However, the electrical signal of the thermocouple needs a special kind of wire to transmit. This kind of wire is called a compensation wire. Different thermocouples require different compensation wires. Their main function is to connect the thermocouple to keep the reference end of the thermocouple away from the power supply, so that the temperature of the reference end is stable. Compensation wires are divided into compensation type and extension type. The chemical composition of extension wires is the same as that of the thermocouple being compensated. However, in practice, extension wires are not made of the same material as thermocouples. Replace the wires with the same electron density. The connection between the compensation wire and the thermocouple is generally clear. The positive electrode of the thermocouple is connected to the red wire of the compensation wire, while the negative electrode is connected to the remaining color. Most of the materials of general compensation wires are copper-nickel alloys.
Secondly, we introduce the thermal resistance. Although the thermal resistance is also widely used in industry, but because of its temperature measurement range, his application is limited. The temperature measurement principle of the thermal resistance is based on the resistance value of the conductor or semiconductor. A characteristic that changes with temperature. It also has many advantages. It can also transmit electrical signals remotely, with high sensitivity, strong stability, interchangeability and accuracy. Industrial thermal resistance generally uses Pt100, Pt10, Cu50, Cu100. The temperature range of platinum thermal resistance is generally minus 200-800 degrees Celsius, and the copper thermal resistance is minus 40 to 140 degrees Celsius. The thermal resistance is the same type as the thermocouple, but it does not need a compensation wire and is cheaper than a hot couple.