The current flow through one side of a current loop will be affected by EMI created by, say a portable radio. Doing this will eliminate error due to EMI. The current is then measured by a PLC input card that has approximately 250 Ω resistance. This is accomplished by measuring the thermocouple or RTD signal at the sensor and converting that measurement to a 4…20 mA current. One of the best ways to minimize error due to EMI, is to minimize the length of the sensor wiring, thereby minimizing the length of the “antenna”. change based on the location/orientation of sensor wiring.change based on time of day or weather patterns.change when a mobile radio is transmitting.change when a part of a process is turned off or on.immediately change when a nearby motor or heater starts.Some symptoms of noise-affected temperature signals are that temperature measurements: Therefore, the sense wires also act like antennas, picking up a wide range of noise present in the industrial environment. Even though the RTD element is excited by a tiny constant current source, (typically 0.3 mA), virtually no current flows on the sense wires used on 3- and 4-wire RTDs. The same thing can happen when measuring RTDs. Electrical noise from 50/60 Hz power mains, burst noise from lightning, static electricity, radio frequency interference from portable radios, commutator noise from DC motors and many other sources of electrical noise can be “received” by a thermocouple – and the longer the wires are, the more opportunity there is to “receive” electrical noise. The combination of a low mV signal and no current flow makes thermocouples behave like an antenna. Therefore, any device that measures a thermocouple must have a very high input resistance, (usually 1 million Ω or more). Most thermocouples create a voltage signal of less than 50 mV, and a thermocouple has practically no ability to create a current flow. By converting thermocouple and RTD signals to industry standard 4…20 mA current, however, errors due to EMI can be effectively eliminated. This loss of accuracy is often caused by electromagnetic interference, (EMI). In cases where these sensors are connect directly to the PLC, the result of the measurement accuracy is sometimes worse than expected. Thermocouple and RTD sensors are commonly used to measure temperature in industrial processes.
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