Who am I? Where am I? I – The Mystery of a Thermal Resistor
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This post was last edited by zhenlizhijian on 2018-8-8 at 12:38. Who am I? Where am I? 1. The mystery of a thermistor. These days, the weather in the north is unbearably hot, like being in a sauna; the weather alone is already annoying enough, and yet the usual steady rhythm of daily work gets disrupted by a single thermistor. In one of the company’s workshops, there are temperature sensors installed in the oxidation furnaces; all of these sensors are of the K-type thermocouple type. One day, the temperature readout from one of these sensors dropped from the normal level of 220 degrees Celsius to 155 degrees Celsius. Upon inspection, it was found that the thermocouple itself was functioning properly, but the temperature sensor was defective – it was the sensor itself that was causing the issue. The temperature sensor was connected directly to the cold end of the thermocouple. After replacing the sensor, the temperature reading returned to around 220 degrees Celsius. It seemed like the problem was resolved there, but during the daily workshop meeting in the afternoon, the supervisor learned that the ambient temperature around the sensor was 58 degrees Celsius. He considered this a potential risk, as such conditions were not suitable for the sensor. Even though we informed him that the sensor could operate within an ambient temperature range of -40 to 85 degrees Celsius, that didn’t help. He insisted on replacing the sensor with one that could function in a room-temperature environment. However, since we didn’t have any suitable compensation wires available, we abandoned this idea and decided to replace the sensors with thermal resistors instead. The thermal resistors were then placed inside the DCS cabinet. As expected, after making these changes, the temperature readout using the thermal resistors was 183 degrees Celsius, which was about 40 degrees Celsius lower than the previous reading using the thermocouples. When we measured the resistance value of the thermal resistors, it corresponded to a temperature of around 175 degrees Celsius. In other words, there was still a difference of over 40 degrees Celsius. We then replaced several more thermal resistors, as well as several thermocouples, but all of the thermocouples continued to function properly, while all of the thermal resistors gave incorrect readings. The incorrect readings were around 180 degrees Celsius, while the thermocouples showed around 220 degrees Celsius – a difference of about 40 degrees Celsius. When we compared the readings of the thermal resistors and thermocouples in the laboratory, the difference was only about 1 degree Celsius, so there seemed to be no problem. But once they were installed back in place, the difference again was around 40 degrees Celsius. There was no solution to this issue. . . . . . . . . . . . . The entire team discussed it collectively but was unable to figure out what the problem was; they hit a dead end. The only difference between the laboratory and the field is the cold-end temperature; the cold-end temperature in the field is around 58 degrees Celsius. Thermocouples perform automatic compensation for temperature changes, while thermal resistors do not require cold-end compensation. Where exactly is the problem? I can’t figure out anything about life...... I’m seeking advice from my brothers and sisters here; what are your thoughts and opinions? O(∩_∩)O Thank you! The content is a bit long and messy; to summarize: at the same temperature measurement point, using the same temperature sleeve and inserting armored thermoresistors and armored thermocouples of the same length, the temperature readings obtained by the sensors differ, with the thermoresistors showing values about 40 degrees Celsius lower than those of the thermocouples. «Cold-junction compensation for the thermocouples has been taken into account and included in the calculations.» This phenomenon was observed with numerous different thermoresistors and thermocouples. The cold-end temperature at the site is around 58 degrees Celsius. This is true whether the temperature change occurs on-site or in the DCS cabinet.
This post was last edited by xxkhc on 2018-8-8 at 14:32. From what is stated, is it the original two wires with temperature sensitivity that are used to connect the thermal resistor to the DCS cabinet? If so, then the wiring method for the thermal resistor is two-wire system. Also, the wire originally used for temperature sensing was connected to the 24V power supply; is the common ground from before still in place? Otherwise, errors will arise as a result.
Check the wires from the thermal resistor to the control room (to see if ABC is correct), the temperature setting, and the DCS configuration; it’s likely caused by a minor issue.
If the thermal resistor also uses a two-wire system and there is a certain distance from the field site to the control center, then it is normal for the temperature error at the control center to be in the tens of degrees. Therefore, it must be changed to a three-wire system to eliminate lead errors.
As a personal suggestion, after installing the thermistor, wait for the temperature to stabilize, then check the wiring terminals of the thermistor’s junction box to see what the temperature is. If everything is normal, it seems that either the temperature sensor you’re using is not suitable for this thermistor, or the temperature setting is incorrect. Regarding cable issues, if you use a two-core cable, the resistance should increase, which means the temperature would be higher than actual; it’s not the phenomenon you described. So it’s likely a problem related to temperature changes
In reply to the guys above, when I say there’s a difference of several dozen degrees, I mean that the temperature obtained by directly measuring the resistance value is about 40 degrees different from the temperature calculated by using the MV value measured by the thermocouple along with cold-junction compensation. Of course, this difference also exists when a temperature sensor is used. . . There’s nothing to do
Measure the insulation resistance of the thermistor on-site separately.
In practical production, since the working end (measurement end) of the thermocouple is very close to the cold end (reference end), and the cold end is exposed to the working environment, it is easily affected by fluctuations in the temperature of that surrounding environment. As a result, it is difficult to maintain a constant temperature at the cold end, leading to inaccurate measurements. In practical applications, special compensation wires are used to extend the cold end of the thermocouple to a location with a lower temperature and greater stability.
1. The guys upstairs are asking you what wire count the thermal resistance uses, and you haven’t answered either. A 2-wire system is not acceptable; a 3-wire or 4-wire heating resistance system can be considered. 2. Is there a difference in the insertion depth required for thermal resistors and thermocouples? ? ? 5 CM short – it’s a significant difference.
This post was last edited by zhenlizhijian on 2018-8-9 at 11:51. This has nothing to do with the wiring scheme; there might be too many words written above, making it a bit messy. 1. Temperature difference refers to the temperature obtained by measuring the resistance value on-site and then converting it using a scale – in other words, it is the resistance signal directly measured by the resistor as a sensor, with the measurement point being at the site itself. 2. For thermocouples, 550; for thermal resistors, those of 600, 800, and 1000 have all been tried. . . The thermal resistors are all three-wire --------------- Now a colleague has asked whether they shouldn’t be inserted all the way in You can try inserting it all the way in and then pulling it back a little. Try measuring again. .