Relationship between thermal resistance connection and measurement accuracy

It is often said that details determine success or failure! That's right, take the measurement of thermal resistors as an example. During the installation of the thermal resistor, if the thermal resistor is not carefully connected, the accuracy of the measurement will be affected.

Let's take PT resistors as an example to explain the problem of connection method and measurement accuracy.

The accuracy of PT thermal resistors is basically divided into two levels, A and B. This classification is only the accuracy of itself and has nothing to do with the number of wires connected. In addition to the measurement error from the PT resistor itself, there is also the influence of the lead resistance on the measurement, so PT resistors are divided into 2, 3, and 4. Let's analyze these three lines:

Theoretically, 2-wire PT resistors cannot offset the lead error. Although you can measure the lead resistance and subtract it with a program in actual use, due to the discreteness of the length and material of the resistor lead, this error cannot be eliminated.

The 3-wire PT resistor offsets the error by measuring the total voltage drop of the resistor minus the voltage drop of one lead arm. As long as the material and cross-section of the three connected resistor leads are exactly the same, the error can be completely eliminated in theory. The reason is the same as above. Due to the existence of discreteness, the accuracy can only be limited.

The 4-wire PT resistor uses a completely different measurement method. Its 2-wire is used as the power supply arm and the other two wires are used as the measurement arm. Because there is basically no current in the measurement arm (high impedance input), the resistance of the measurement arm will not affect the measurement result at all. Therefore, its measurement is the most accurate, and the measurement accuracy has nothing to do with the material and cross-section of the connected resistor lead. Summary:

The shorter the 2-wire, the more accurate the measurement

The more the material and cross-section of the 3-wire lead are the same, the more accurate the measurement

There is no requirement for 4-wire, it is inherently accurate

Through the above analysis, I believe that everyone has a certain understanding of the relationship between the connection of thermal resistors and the accuracy of measurement. Therefore, in our daily practice, we must pay close attention to this issue to minimize measurement errors.