Which module to use for Pt100 / Ni100 measurements?

FAQ #101032

 

Introduction
The resistance of Pt 100 and Ni 100 elements is low compared to the resistance of the line connecting the sensor element to the PCD. When using 2 wire mode (e.g. with PCDx.W350) the resistance of the lines add to the resistance of the element, which means that the measured temperatures are always too high. Worse than that the line resistance has as well a temperature coefficient and there is no way to compensate for that in 2 wire mode.

Best choice of module
The answer to this issue is the 4 wire mode as supported by the PCDx.W745. A pair of lines is used to supply the element, another pair of lines to measure the voltage drop over the element. Thanks to this the resistance of the line and its temperature coefficient do no longer matter. Another advantage of the PCDx.W745 is the 16 bit resolution, which is advantageous for temperature measurements.

Remarks
The effect described in the first paragraph is as well present with Pt1000 / Ni1000 elements, but 10 times weaker. For many applications it is therefore adequate to use 2 wire mode, compensate the offset with a subtraction and forget about the temperature effect of the line when using Pt1000 / Ni1000. If you have a choice avoid elements with low resistance like Pt 100 / Ni 100 / Balco 500.