Questions regarding the use of thermistor signals in control and regulation
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I’ve heard that in a temperature control loop, it’s best to convert the PT100 signal into a 4–20mA signal before adjusting the control valve. Is this statement correct? What are the regulatory bases?
The thermocathode at the site is not actually a signal in the physical sense; it is only when a certain intensity of current flows through the circuit of the thermal resistor that a voltage drop occurs, and it is this voltage drop that is detected. You can imagine that due to the resistance of the thermal element and the wires, the current applied will not be high, the resulting voltage drop will not be significant, and it is susceptible to various other influences. Therefore, by installing a temperature transmitter on-site to convert the signal into a current signal, it becomes easier to transmit the signal over long distances, and the system becomes more stable and reliable.
The advantage of this approach is that if the distance from the field site to the control center is too great, connecting the electrical signals to the DCS using wires requires high-quality wires; moreover, it’s possible to use a temperature sensor to convert the temperature value into an electrical current on-site, thereby improving accuracy
There are no specific rules; it depends on the production requirements – whatever is more convenient is used. The thermal resistance value can also be entered directly into the card, and RTD cards can be utilized as well for temperature measurement