1. The wires and internal wiring between the signal source and the instrument cause interference in the circuit through magnetic coupling. There is a strong AC magnetic field in the surrounding space of high-power transformers, AC motors, and power lines, and the closed loop will generate electromotive force in this changing magnetic field. This kind of induced potential is connected in series with the useful signal. When the sensor is far away from the digital display, this kind of series mode interference is particularly prominent.
2. The interference source forms interference in the loop through capacitive coupling, which is the result of the interaction of the two electric fields. Through electrostatic coupling, a common voltage to the ground can be induced at the two input terminals, which appears in the form of common mode interference. Since common mode interference does not overlap with the signal, it does not directly affect the instrument. However, it can form a leakage current to the ground through the measurement system, and the leakage current can directly act on the meter through the coupling of resistance and cause interference. Electromagnetic induction and electrostatic induction are mostly power frequency interference voltages, but frequency converters, motors with commutators, etc. will produce harmonic interference.
3. In some temperature measurement occasions, the thermocouple electrode is directly welded to the metal part that is heated by electricity. Because the metal part has a potential difference at each point parallel to the current direction, the interference voltage introduced at this time is also very large. Under high temperature conditions, the insulation resistance of refractory materials decreases sharply, and the insulation performance of the thermocouple protection tube also decreases. Then the power supply voltage leaks to the thermocouple wire through refractory bricks, thermocouple bushings, etc., between the thermocouple electrode and the ground. Interference voltage is generated between them.
4. There are often potential differences between different points in the earth, especially near high-power electrical equipment. When the insulation performance of these equipment decreases, the potential difference is greater. When the field instrument is in use, sometimes carelessly will cause the loop to have more than two grounding points, and the potential difference of different grounding points will be introduced into the digital display meter to form common mode interference.
5. When the bridge power supply of the meter is grounded, in addition to the unbalanced signal voltage output by the bridge, there is also a common voltage between the signal line and the ground. This common voltage is not the signal voltage to be measured, but a manifestation of common mode interference.
Dec 29, 2022
