Industrial commonly used temperature measuring components are: thermocouple, thermal resistance, bimetal; Thermocouple and thermal resistance belong to the contact temperature measurement in temperature measurement. Although their functions are the same as measuring the temperature of the object, their principles and characteristics are not the same.
Thermocouple is the most widely used temperature device in temperature measurement, its main characteristics are wide measurement range, relatively stable performance, while the structure is simple, dynamic response is good, more can far transmit 4-20mA electrical signal, easy to automatic control and centralized control. The temperature measurement principle of thermocouple is based on thermoelectric effect. When two different conductors or semiconductors are connected into a closed loop, when the temperature at the two junctions is different, the loop will produce thermoelectric potential, this phenomenon is called thermoelectric effect, also known as Seebeck effect. Thermoelectric potential generated in a closed loop consists of two kinds of potential: thermoelectric potential and contact potential.
Thermoelectric potential means the same at both ends of the conductor potential because of the different temperature, different conductors with different electron density, so they produced by electric potential is not the same, the contact potential as its name implies is refers to the two different conductor in contact, because of their different electron density so produce certain electronic diffusion, when they reach a certain potential, which is formed by the balance after The size of the contact potential depends on the material properties of the two different conductors and the temperature of their contact point.
At present the application of thermocouple on the international has a standard specification, the regulations on international thermocouple is divided into eight different degree, respectively B, R, S, K, N, E, J and T, its measuring temperature of minus 270 degrees Celsius, the minimum detectable up to 1800 degrees c, B, R, S belong to platinum series thermocouple, because platinum are precious metals, So they're called precious metal thermocouples and the rest are called cheap metal thermocouples.
There are two types of thermocouple structure, ordinary type and armored type. The ordinary thermocouple is generally composed of thermal electrode, insulating tube, protective sleeve and junction box, etc., while the armored thermocouple is a solid combination of thermocouple wire, insulating material and metal protective sleeve, which is assembled by stretching and processing.
The electrical signal of a thermocouple needs a special kind of conductor to carry on, this kind of conductor is called compensating conductor. Different thermocouples need different compensation conductors, whose main role is to connect with the thermocouple, so that the reference end of the thermocouple is far from the power supply, so that the temperature of the reference end is stable. Compensation wire is divided into two types of compensation and extension, extension of the chemical composition of the conductor and the thermocouple is the same, but in practice, the extension of the conductor is not with the same material and the thermocouple metal, generally used and thermocouple has the same electron density of the conductor instead. The connection between the compensation conductor and the thermocouple is generally clear. The positive pole of the thermocouple is connected to the red line of the compensation conductor, while the negative pole is connected to the rest of the color. The material of general compensation conductor is mostly copper nickel alloy.
Although thermal resistance is widely used in industry, its application is limited due to its range of temperature measurement. The temperature measurement principle of thermal resistance is based on the characteristic that the resistance value of conductor or semiconductor changes with the temperature. Its advantages are also many, can also be far electrical signals, high sensitivity, strong stability, interchangeability and accuracy are better, but the need for power supply excitation, can not be instantaneous measurement of temperature changes.
Industrial heat resistance generally adopts Pt10, Pt100, Pt1000, Cu50, Cu100, platinum thermal resistance temperature measurement generally below 200-800 degrees Celsius, the range of thermal resistance of copper of 40 to 140 degrees Celsius below zero. Thermal resistors are of the same type as thermocouples, but they do not require compensating conductors and are cheaper than thermocouples.
Bimetallic thermometer
The bimetallic thermometer is a kind of field measuring instrument for low and medium temperature. It can directly measure the temperature of liquid vapor and gas medium in the range of -80℃ to +500℃ in various production processes. The main component of the industrial bimetallic thermometer is a multilayer metal sheet composed of two or more metal sheets stacked together, using the principle that two different metals expand to different degrees when the temperature changes. It is based on a bimetallic sheet wound into a circular bend. When one end is heated and expanded, it drives the pointer to rotate, and the working instrument will show the temperature value of the thermoelectric potential.
