Why is there a discrepancy between the on-site shorting of terminals B and C for PT100 and the shorting of terminals B and C inside the DCS cabinet?
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Why is there a discrepancy between the local shorting of terminals B and C for PT100 and the shorting of terminals B and C inside the DCS cabinet?
PT100 is a thermistor; the temperature it measures is linearly related to its resistance, so the resistance of the wiring also affects the measurement. To reduce the impact of line resistance on measurements, it is necessary to tell the system what the line resistance is And the resistance between BC is used to represent the line resistance. Shorting inside the cabinet is equivalent to telling the system that my circuit resistance is very low, essentially zero. But in fact it’s not, so this leads to deviations. The line resistance in chemical plants is generally around 3-6 ohms (depending on the length of the line).
It’s right to have deviations; that’s why DCS provides compensation for all temperature points, namely to compensate for this resistance value
The deviation is caused by the resistance of the circuit
The deviation is caused by the line resistance. The thermal resistor is one of the bridge arms of a bridge; the purpose of using a three-wire system is to introduce the same three wires into different bridge arms, thereby eliminating the influence of wire resistance on the measurement. By creating a short circuit inside the DCS cabinet, the wire becomes part of the resistance value of the thermistor, which in turn increases the measured value and leads to measurement errors ； In the case of on-site shorting, the length of the circuit is very short, so the circuit resistance is extremely low and can be ignored; this is why the wires of thermal resistors are always shorted on site.
Thank you, everyone! Got it! The B wiring of the PT100 sensor used to monitor the temperature of the upper bearing shell of a circulation pump was damaged, preventing the pump from stopping. After making a short circuit inside the DCS cabinet, it was found that the temperature was 10 degrees lower than the normal levels recorded in the past; however, when a short circuit was made in the local wiring box, the temperature was close to the normal values from before. This led to confusion.
In other words, is your problem caused by a break in the circuit at point B?
It’s the component inside the testing point that is damaged; the AB connection is open, while AC shows a resistance value – it’s not the external wires. It can’t be handled temporarily.
I basically agree with the explanation given by “Yann (703362)”.