Problems with measuring four-wire thermal resistors with a multimeter. .
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It is common to directly cut off the power at the site in order to measure the resistance values of the common terminal and the other two terminals of a three-wire thermistor. The actual resistance value can be determined by subtracting the resistance measured between the other two terminals using a multimeter from the resistance measured with a PT100 sensor; alternatively, this value can be divided by 0.385 per degree to obtain the actual temperature of the measuring terminal. However, this method takes into account the resistance of the circuit loop of the thermistor (and is suitable for three-wire thermistors). How is measurement carried out for four-wire systems at the site? Four-wire measurement converts the voltage across the thermal resistance to determine the resistance value. When using a multimeter on-site for measurement, is it necessary to subtract the resistance between the wires? (It seems that subtraction is actually required.) There is an ABB transmitter on site, which operates in a three-wire configuration. Therefore, the PT100 thermoresistor is adapted for use in this three-wire system: its positive terminal is connected to the transmitter, while the other two terminals are connected separately to the transmitter. Will there be any measurement errors as a result of converting this four-wire system to a three-wire one? And why was the resistance value measured on-site at around 165 ohms. The table shows around 170 degrees, but the transmitter displays 230 degrees? Based on the measurement results from another identical heat resistance and transmitter set on the same heat exchanger, as well as process considerations, the value is generally around 230 degrees. . Why is there such a large error (difference in resistance)? Is it a problem with my measurement method or what? The transmitter is probably fine. . (Data read from hart)
There’s too much information, hahaha. I encountered one instance where a four-wire setup was converted to a three-wire setup, with an error of 30 degrees; I was quite surprised by that. Additionally, for 165 ohms, if it’s calculated based on 385, it shouldn’t be 170 degrees either. It is recommended to change the wiring in the module to match that of the sensor, rather than making changes using jumpers
The only difference between a three-wire thermal resistor and a four-wire thermal resistor is that the four-wire version has two wires leading from each end, while the three-wire version has only one wire leading from one end; by removing one of the wires in a four-wire thermal resistor, it becomes a three-wire thermal resistor. To measure the resistance value of a thermal resistor on-site, it is necessary to remove the thermal resistor from its connection terminals; measurement cannot be carried out while it is still part of the circuit, as this would include the resistances present in the circuit itself. The wire coming from the thermoresistor is quite short, so its resistance is very low and can generally be ignored. If the measured resistance value does not match what is displayed, first check whether the measurement method is correct (for example, whether it was taken while the circuit was connected) and whether the multimeter is accurate. After ruling out these factors, it can be determined that there is a problem with the display; it is recommended to calibrate the transmitter or the display instrument.
It’s fine to connect 4 wires to 3 – just leave one wire unconnected. The resistance value measured by the original poster does not match what the transmitter shows; perhaps as mentioned by someone else, you measured the resistance value without disconnecting the wires. :D
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