Questions encountered in temperature measurement
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I would like to ask everyone for some advice; I had some doubts while checking a PT100 temperature sensor a few days ago. The process indicated that the display on the control panel at that measurement point was low. An inspection was carried out; the instrument technicians did not want to go to the site for measurements, so they measured the resistance values at the terminals of the control panel. The results showed that the display on the control panel was indeed low (only the resistance values between points A and B were measured). Subsequent measurements taken on-site gave results that matched those displayed on the control panel. They say it’s because there is resistance in the wiring during measurement at the control panel; the actual temperature should be measured on-site for accuracy. But we have other temperature points that enter the system after being converted by temperature-sensitive safety gates located at the cabinet. When we measure these temperature points, the values obtained by measuring the resistance at points A and B in the control room match those displayed on the control screen. The question is this: since the resistance at points A and B is measured in the control room as well, and wire resistance is present in all cases, why does it need to be measured on-site for signals that go directly into the system, while the measurements taken through the safety gates show the correct values on the control screen? Does that mean the values displayed for the temperature points at our safety barriers are currently higher than the actual temperatures?
To connect to the system, there is a thermistor card and an analog card; the transmission of thermistor signals has distance requirements. 1. Convert the Pt100 resistance signal into a standard DC4–20mA signal; this method can also eliminate interference that occurs during signal transmission, allowing the temperature displayed by the computer to return to normal, as the DC4–20mA signal has excellent resistance to interference! 2. The output of a thermal resistor is actually the resistance value; the material of the wires affects this value through the resistance of the wires themselves and its variation with temperature. To address this issue, a three-wire or four-wire system is used, with the three-wire system being the most common. In essence, wires of the same material, cross-section, and length are employed to perform differential processing on the resistance value of the thermal resistor, thereby eliminating the influence of the wires. So it is basically unrelated to the wire material; instead, it requires that the cross-sectional area and length of the wire materials be the same.
The two-wire system yields higher values, while the three-wire system doesn’t present any issues. In the case of temperature sensors, if the sensor is located on-site, the short distance between the wires in a PT100 system means there are no problems; the signal remains fine even after connecting the temperature sensor. The three-wire system also has no issues
It’s actually a problem of remote temperature compensation. In the temperature measurement systems we design nowadays, the practice of using safety barriers for remote compensation is rarely employed; instead, temperature transmitters are used for remote transmission to avoid such unnecessary errors.
Two methods: use a three-wire system, or install the temperature sensor on-site.