Problems with thermistor measurement
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The thermal resistance meters on site are of the two-wire type, while the modules in the control room are three-wire types. Now, it is desired to change the configuration from two-wire to three-wire at the meters (that is, two wires are to be led out from the negative terminal of the thermal resistance element). The transmission distance is 1600 meters; will this result in a large measurement error? If the transmission distance of a three-wire heating resistor cable is too long, does the resistance of the circuit have a significant impact on the measurements? For example, 1600 meters in this case.
This post was last edited by chensf123 on 2018-11-25 at 22:24. The purpose of the three-wire connection is to eliminate errors caused by changes in the loop resistance of the transmission wires, but 1600 meters is indeed too long a distance.
Support the view from above. Even when changing it to a 4–20 mADC signal, the attenuation is very high; there is a concern that the field transmitter won’t be able to handle it.
Variable digital signals, fiber optic transmission; P
1600 meters is too far; it is recommended that you use a temperature transmitter instead
It turns out to be a secondary thermistor, which isn’t accurate to begin with. As for connecting it to the module, as long as there is a short circuit at the module end, that’s sufficient. It’s meaningless to simply use the original three wires; this does not eliminate the effect of wire resistance. Either switch to a proper third-class thermistor, or change the transmitter after all.
1600 meters is a bit far. . . . . .
It’s too far, resulting in large resistance errors and instability. If a 4-20MA current signal from a transmitter is used, a wire with a nominal cross-sectional area of 1.5 square millimeters can transmit the signal over a distance of 2626 meters, while a wire with a nominal cross-sectional area of 0.8 square millimeters allows transmission over a distance of 1400 meters. In practical applications, the actual length of the wire should be set slightly lower than the calculated value, as the nominal cross-sectional area of wires is almost always on the high side. And from such a distance, it’s worth checking whether wired or fiber optic transmission can be used
Generally, when the signal distance exceeds one kilometer, a repeater is needed to amplify the signal; otherwise, the accuracy of the measurements will be significantly affected
So which option is the most suitable in the end?
This post was last edited by 1111111 on 2018-11-26 at 13:48. Answer 1: By using two wires led from the negative terminal of the thermistor to implement a three-wire system, the measurement error is minimal and can be ignored in industrial measurements. Answer 2: For a three-wire thermistor, the transmission distance is 1600 meters. As for the error, it isn’t very large; it’s an erratic error, roughly within the range of 0–3°C. . . And with remote transmission, there are issues that cannot be avoided or ignored – when using a three-wire setup for remote transmission, it’s necessary to determine whether the signals transmitted over the wires can be delivered reliably, whether they will be affected by interference, and whether the temperature values can be obtained stably from the temperature sensor at the remote end! Once you have to put in a lot of effort to deal with interference issues, it’s better to use local heating and transmission via the 2-wire 4–20mA protocol – this results in minimal measurement errors, reliable signal transmission, and saves time and effort while also being simpler to handle. . . Why? Read the post: “In-depth explanation of the three-wire thermal resistor temperature measurement circuit, https://bbs.hcbbs.com/thread-1977228-1-1.html