The difference between thermal resistor and thermocouple

Thermal resistors are the most commonly used temperature detectors in the medium and low temperature zones. Its main features are high measurement accuracy and stable performance. Among them, platinum thermal resistors have the highest measurement accuracy. They are not only widely used in industrial temperature measurement, but also made into standard reference instruments.

Unlike the temperature measurement principle of thermocouples, thermal resistors measure temperature based on the thermal effect of resistors, that is, the resistance of the resistor changes with the change of temperature. Therefore, as long as the resistance change of the temperature-sensitive thermal resistor is measured, the temperature can be measured. At present, there are mainly two types of metal thermal resistors and semiconductor thermistors. The resistance value and temperature of metal thermal resistors 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℃); α is the temperature coefficient. The relationship between the resistance value and temperature of semiconductor thermistors is Rt=AeB/t

In the formula, Rt is the resistance value at temperature t; A and B are constants depending on the structure of the semiconductor material. In comparison, thermistors have a larger temperature coefficient and a higher resistance value at room temperature (usually above several thousand ohms), but their interchangeability is poor, their nonlinearity is serious, and their temperature measurement range is only about -50~300℃. They are widely used in temperature detection and control of household appliances and automobiles. Metal thermal resistors are generally suitable for temperature measurement in the range of -200~500℃. They are characterized by accurate measurement, good stability, and reliable performance. They are widely used in process control.

Thermal resistor materials Thermal resistor temperature measurement is based on the characteristic that the resistance value of metal conductors increases with the increase of temperature to measure temperature. Thermal resistors are mostly made of pure metal materials. Currently, platinum and copper are the most widely used. In addition, nickel, manganese and rhodium are now being used to manufacture thermal resistors.

Types of thermal resistors

(1) Armored thermal resistors: Armored thermal resistors are solid bodies composed of temperature sensing elements (resistance bodies), leads, insulating materials, and stainless steel casings. Their outer diameter is generally φ2~φ8mm, and the minimum can reach φmm. Compared with ordinary thermal resistors, it has the following advantages: ① small size, no air gap inside, small measurement lag in thermal inertia; ② good mechanical properties, vibration resistance, and impact resistance; ③ can be bent and easy to install; ④ long service life.

(2) Precision thermal resistor: the structure and characteristics of the thermal resistor temperature sensing element (resistor) commonly used in industry. From the temperature measurement principle of thermal resistors, it can be seen that the change of the measured temperature is directly measured by the change of the resistance value of the thermal resistor. Therefore, the change of various wire resistances such as the lead wire of the thermal resistor will affect the temperature measurement. In order to eliminate the influence of the lead wire resistance, a three-wire or four-wire system is generally used.

(3) End face thermal resistor: The end face thermal resistor temperature sensing element is wound with specially treated resistance wire and is close to the end face of the thermometer. Compared with general axial thermal resistors, 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 bearings and other parts.

(4) Flameproof thermal resistor: The explosion of the explosive mixed gas inside the shell of the flameproof thermal resistor is confined to the junction box through a special structure, so that the explosion caused by the influence of sparks or arcs will not be caused at the production site. Flameproof thermal resistors can be used for temperature measurement in explosion-hazardous places in the Bla~B3c level area.

Commonly used metal thermal resistors in industry From the perspective of the change of resistance with temperature, most metal conductors have this property, but not all of them can be used as temperature measuring thermal resistors. The metal materials used as thermal resistors generally require: as large and stable temperature coefficient as possible, large resistivity (reducing the size of the sensor at the same sensitivity), stable chemical and physical properties within the temperature range of use, good material reproducibility, and the resistance value should have an intermediate value function relationship with temperature change (linear relationship).

Signal connection method of thermal resistor The thermal resistor is a primary element that converts temperature change into resistance value change. It is usually necessary to transmit the resistance signal to the computer control device or other primary instrument through the lead wire.

At present, the most widely used thermal resistor materials are platinum and copper: platinum resistors have high precision, are suitable for neutral and oxidizing media, have good stability, have certain nonlinearity, and the higher the temperature, the smaller the resistance change rate; copper resistors have a linear relationship between resistance value and temperature within the temperature measurement range, have a large temperature line number, are suitable for non-corrosive media, and are easily oxidized when the temperature exceeds 150.

Industrial thermal resistors are installed at the production site, and there is a certain distance between them and the control room, so the leads of the thermal resistors will have a greater impact on the measurement results.

At present, there are three main ways to lead the thermal resistor. ○1 Two-wire system: Connecting a wire at each end of the thermal resistor to lead out the resistance signal is called two-wire system: This lead method is very simple, but since the connecting wire must have a lead resistance r, the size of r is related to the material and length of the wire, so this lead method is only suitable for occasions with low measurement accuracy. ○2 Three-wire system: Connecting a lead at one end of the root of the thermal resistor and two leads at the other end is called a three-wire system. This method is usually used in conjunction with a bridge, which can better eliminate the influence of lead resistance. It is the most commonly used lead resistance in industrial process control. ○3 Four-wire system: Connecting two wires at each end of the root of the 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 lead U to the secondary instrument through the other two leads. It can be seen that this lead method can * eliminate the influence of lead resistance, and is mainly used for high-precision temperature detection.

The thermal resistor adopts a three-wire connection method. The three-wire system is used to eliminate the measurement error caused by the resistance of the connecting wire. This is because the circuit for measuring thermistors is generally an unbalanced bridge. Thermistors are a bridge arm resistance, and their connecting wires (from thermistors to the central control room) also become part of the bridge arm resistance. This part of the resistance is unknown and changes with the ambient temperature, causing measurement errors. Using a three-wire system, one wire is connected to the power supply end of the bridge, and the other two are connected to the bridge arm where thermistor is located and the bridge arm adjacent to it, thus eliminating the measurement error caused by the line resistance of the wire.

In industry, the three-wire connection method is generally used. Thermocouples generate millivolt signals and do not have this problem. Composition of the thermal resistor temperature measurement system

(1) The thermal resistor temperature measurement system is generally composed of a thermal resistor, connecting wires and a display instrument. The following two points must be noted:
① The thermal resistor and the display instrument must have the same graduation number
② In order to eliminate the influence of the change in the resistance of the connecting wire, a three-wire connection method must be used. For details, please refer to Chapter 3 of this article.

(2) Armored thermal resistor The armored thermal resistor is a solid body composed of a temperature sensing element (resistor), lead wire, insulating material, and stainless steel casing. Its outer diameter is generally φ2~φ8mm, and the minimum can reach φmm. Compared with ordinary thermal resistors, it has the following advantages:
① Small size, 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 resistor The end face thermal resistor temperature sensing element is wound with specially treated resistance wire and is tightly attached to the end face of the thermometer. Compared with general axial thermal resistors, 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 bearings and other parts.

(4) Flameproof thermal resistors Flameproof thermal resistors use a special structure of the junction box to isolate the explosive mixed gas inside the shell from the sparks or arcs. The circuit repair of the resistor body will inevitably change the length of the resistor wire and affect the resistance value. Therefore, it is better to replace the resistor body with a new one. If welding is used for repair, it must be calibrated after welding before it can be used.

The difference between thermocouples and thermal resistors
Both thermocouples and thermal resistors belong to contact temperature measurement in temperature measurement. Although they have the same function of measuring the temperature of an object, their principles and characteristics are different.

First, let's introduce thermocouples. Thermocouples are the most widely used temperature devices in temperature measurement. Their main features are wide measurement range, relatively stable performance, simple structure, good dynamic response, and the ability to transmit 4-20mA electrical signals remotely, which is convenient for automatic control and centralized control. The temperature measurement principle of thermocouples is based on the thermoelectric effect. When two different conductors or semiconductors are connected into a closed loop, when the temperatures at the two junctions are 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 consists of two potentials: temperature difference 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 the potentials they generate are also different. As the name suggests, the contact potential refers to the potential formed when two different conductors are in contact because their electron densities are different, so a certain amount of electron diffusion is generated. When they reach a certain balance, the size of the contact potential depends on the material properties of the two different conductors and the temperature of their contact points. The thermocouples currently used in the market have a standard specification, which stipulates that thermocouples are divided into eight different divisions, namely B, R, S, K, N, E, J and T. The lowest temperature they can measure is minus 270 degrees Celsius and the highest is 1800 degrees Celsius. Among them, B, R, and S belong to the platinum series of thermocouples. Since platinum is a precious metal, they are also called precious metal thermocouples, and the remaining ones are called cheap metal thermocouples.

There are two structures of thermocouples, ordinary type and armored type. Ordinary thermocouples are generally composed of thermocouples, insulating tubes, protective sleeves and junction boxes, while armored thermocouples are a solid combination of thermocouple wires, insulating materials and metal protective sleeves assembled and stretched. However, the electrical signal of the thermocouple requires a special wire to transmit, which we call a compensation wire. Different thermocouples require different compensation wires, whose main function is to connect with the thermocouple, so that the reference end of the thermocouple is far 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 the extension wire is the same as that of the compensated thermocouple, but in practice, the extension wire is not made of the same metal as the thermocouple. It is generally replaced by a wire with the same electron density as the thermocouple. 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.

Secondly, let us introduce the thermal resistor. Although the thermal resistor is widely used in industry, its application is limited to a certain extent due to its temperature measurement range. The temperature measurement principle of the thermal resistor is based on the characteristic that the resistance value of the conductor or semiconductor changes with the temperature. It has many advantages, such as remote transmission of electrical signals, high sensitivity, strong stability, good interchangeability and accuracy, but it requires power excitation and cannot measure temperature changes instantly. Industrial thermal resistors generally use Pt100, Pt10, Cu50, Cu100. The temperature measurement range of platinum thermal resistors is generally minus 200-800 degrees Celsius, and that of copper thermal resistors is minus 40 to 140 degrees Celsius. Thermal resistors are classified into the same types as thermocouples, but they do not require compensation wires and are cheaper than thermal couples.