With the development of industrial automation, nowadays sensors are used in many industries. The thermocouple that we often use is a kind of sensor. The thermocouple is a sensor that measures temperature based on the pyroelectric effect and is one of the commonly used temperature measuring components. But we sometimes encounter some interference when using thermocouples for measurement. Then let's take a detailed look at the effective measures to prevent the thermocouple from being interfered.
1. Isolation method The isolation method is to install the thermocouple in the air so that the thermocouple does not contact the refractory bricks on the furnace wall, and the thermocouple and the support are also isolated by insulation. This method can well prevent high temperature leakage interference.
2. Shielding method The shielding method is to shield the compensation wire of the thermocouple through an iron pipe or other metal shield. This can prevent electromagnetic interference and high-voltage electric field interference. When using this method, the iron pipe and shield should be well grounded, and the compensation wire should be twisted.
3. Grounding method This method is to ground the measurement loop and introduce interference to the ground to ensure the accuracy of the meter. This method has two ground forms: the first is the grounding of the thermocouple reference terminal, and the second is the grounding of the thermocouple measurement terminal. When the reference terminal grounding method is used, one end of the thermocouple (or compensation wire) output terminal is grounded through a large enough capacitor (the larger the capacitor when conditions permit, the better). The measuring terminal grounding method is to ground the measuring terminal of the thermocouple, that is, lead a metal wire from the measuring terminal of the thermocouple to ground. This method has a good preventive effect on high temperature leakage interference. When selecting the metal wire, the metal wire that is resistant to high temperature and harmless to the thermocouple electrode should be used.
When we use thermocouples, we should be prepared to prevent interference. Only in this way can the measurement of our thermocouple be more accurate, thus making our work more convenient and effective.
Avoid strong magnetic fields first
Two compensation wires plus shield
Three power cables and signal lines are routed separately to keep distance
Causes of system interference
Realization of monitoring and control in the industrial production process requires the use of various automation instruments, control systems and actuators. The signal transmission between them includes small signals ranging from faint to millivolt and microampere levels, as well as tens of volts. Even large signals of thousands of volts and hundreds of amperes; there are low-frequency DC signals, high-frequency pulse signals, etc. After forming a system, it is often found that the signal transmission between instruments and equipment interferes with each other, causing system instability or even misoperation. In this situation, in addition to the performance reasons of each instrument and device itself, such as anti-electromagnetic interference, another very important factor is that there is a potential difference between the signal reference point between the instrument and the device, which forms a "ground loop". "Causes distortion during signal transmission. Therefore, to ensure the stable and reliable operation of the system, the "ground loop" problem is a problem that must be solved in the process of system signal processing.
Solution to "ground loop"
According to theoretical and practical analysis, there are three solutions:
The first solution: Use signal isolation methods in each process loop to disconnect the process loop without affecting the normal transmission of the process signal, thereby completely solving the ground loop problem.
The second solution: all field devices are not grounded, so that all process loops have only one grounding point and cannot form a loop. This method seems simple, but it is often difficult to achieve in practical applications because some devices require grounding. To ensure measurement accuracy or to ensure the safety of life, some equipment may form new grounding points due to long-term corrosion and wear or weather effects.
The third scheme: Make the electric potential of the two grounding points the same, but because the resistance of the grounding point is affected by many factors such as geological conditions and climate change, this scheme cannot be completely achieved in practice.