As one of the seven basic values of the International Unit System, temperature is a parameter that needs to be monitored in an industrial field. There are many ways to measure temperature value. Temperature sensing can be done either through direct contact with the heating source, or remotely. There are currently four types of temperature sensors commonly used in the industry, such as thermocouples, thermal resistance RTDs, thermistors and integrated circuit temperature sensors.
Thermocouple is a contact measurement and is the most widely used temperature device in temperature measurement. The main features are wide temperature 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 principle of thermocouple temperature measurement is based on thermoelectric potential, which connects two different conductors or semiconductors into a closed circuit. When the temperatures at the two junctions are different, a thermoelectric potential will be generated in the circuit. The thermoelectric potential is composed of thermoelectric potential and contact potential. composition. Thermoelectric potential refers to the electric potential produced by the two ends of the same conductor due to different temperatures. Different conductors have different electron densities, so they generate different electric potentials. Contact potential refers to when two different conductors are in contact, because their electron densities are different, a certain amount of electron diffusion is produced. When they reach a certain balance, the potential is formed. The magnitude of the contact potential depends on the materials of the two different conductors. Nature and the temperature of their contact point. The temperature measurement range of thermocouple is -270°C～1800°C.
2. Thermal Resistance RTD
Thermal resistance is also a contact measurement in temperature measurement. It is based on the thermal effect of resistance for temperature measurement, that is, the resistance of the resistor changes with temperature. Therefore, as long as the resistance of the thermal resistance is measured, the temperature can be measured. The thermal resistance commonly used in industry generally adopts PT100 and PT1000. The thermal resistance is suitable for occasions with low temperature but high linearity requirements. The measurement temperature is -200°C～800°C. In thermal resistance measurement, instantaneous temperature changes cannot be measured, and an excitation power supply is required. Industrial thermal resistance is installed at the production site, and there is a certain distance between it and the control room, so the lead of the thermal resistance has a greater impact on the measurement results. There are currently three main ways to lead the thermal resistance:
(1) Two-wire system: Connect a wire at both ends of the thermal resistance to lead out the resistance signal. This lead method is very simple, but there must be lead resistance “r” in the connecting wire, and the value of “r” depends on the material and length of the wire. Factors are related, so this lead method is only suitable for occasions with low measurement accuracy.
(2) Three-wire system: The method of connecting one lead at one end of the root of the thermal resistance and connecting two leads at the other end is called three-wire system. This method is usually used in conjunction with a bridge, which can better eliminate the influence of lead resistance. This is the most commonly used in industrial process control.
(3) Four-wire system: The method of connecting two wires at each end of the thermal resistance is called a four-wire system, in which two leads provide a constant current “I” for the thermal resistance, convert “R” into a voltage signal “U”, and then pass another two leads lead “U” to the secondary meter. This type of lead wire can completely eliminate the influence of lead wire resistance and is mainly used for high-precision temperature detection.
Thermistor is a kind of thermal resistance, but thermistors are semiconductor resistors. This is because the temperature coefficient of semiconductor thermal resistance is 10-100 times larger than that of metal, and it can detect temperature changes of 10-6°C. Thermistor The temperature measurement range is -55°C～315°C. According to the temperature characteristics, thermistors can be divided into two categories. The resistance that increases with the temperature rises is the positive temperature coefficient thermistor, and vice versa is the negative temperature coefficient thermistor.
(1) The positive temperature coefficient thermistor is made of barium titanate (BaTio3) as the basic material, mixed with an appropriate amount of rare earth elements, and sintered at high temperature using ceramic technology. The positive temperature coefficient thermistor has a very slow change in resistance with temperature before it reaches the Curie point. It has the functions of constant temperature, temperature adjustment and automatic temperature control. It only generates heat, does not emit red, has no open flame, and is not easy to burn. The voltage can be AC or DC 3～440V and has a long service life. It is very suitable for overheating detection of electric devices such as motors.
(2) Negative temperature coefficient thermistors are made of metal oxides such as manganese oxide, cobalt oxide, nickel oxide, copper oxide and aluminum oxide as the main raw materials, and are manufactured by ceramic technology. There are many types of negative temperature coefficient thermistors. They are used to distinguish low temperature (-60°C～300°C), medium temperature (300°C～600°C), and high temperature (>600°C). They have high sensitivity, good stability, fast response, long life, low price and other advantages. It is widely used in automatic temperature control circuits that require fixed-point temperature measurement, such as temperature control systems for refrigerators, air conditioners, and greenhouses.
4. Integrated Circuit Temperature Sensor
The integrated temperature sensor is made of silicon semiconductor integration process. It integrates the temperature sensing element, expansion circuit, compensation circuit, etc. on a small chip. It has good linearity, fast response, moderate precision, high sensitivity, the advantages of small size, convenient use, etc., can effectively compensate for the traditional temperature sensor's slow response time, large thermal inertia, and poor high-temperature linearity.
The integrated temperature sensor is divided into digital temperature sensor, logic output temperature sensor, and analog temperature sensor according to the output signal. Most of the digital temperature sensors are equipped with a BUS interface, which has address and data transmission functions, which overcomes the shortcomings of analog signals being easily interfered and large errors during long-distance transmission. The logic output temperature sensor will set the temperature range when in use. Once the temperature exceeds the specified range, it will send an alarm signal to start or shut down other control equipment. Analog temperature sensors are divided into current output type and voltage output type. The main feature of analog integrated temperature sensor is single function, only measuring temperature, small temperature measurement error, low price, fast response speed, no non-linear calibration, peripheral circuit simple.