A frequently asked question is how to connect measurement modules. You often have pressure sensors that provide two lines. Here, one line is typically connected to a 24 V DC power supply, while the other supplies the signal (4...20 mA). However, there are also pressure sensors with three lines, in which case the influence on the measurement data of the line resistance is largely compensated, provided the line resistances are the same.
What is a 2-wire circuit?
As the name suggests, a 2-wire circuit offers two lines that are connected from the measuring modules to the sensor. The output signal of a 2-wire circuit means that there is less wiring work, which means that wiring errors are less likely to occur. Furthermore, the EMC protection is significantly better, since interference can be easily filtered.
One disadvantage is that the lead resistance is fully included in the measurement result. If the pressure is constant, this can be compensated for by a power resistor. This means that the lead resistance value, as a fixed value, is deducted.
What is a 3-wire circuit?
The 3-wire circuit is wired like the 2-wire circuit, but with the difference that it is directly grounded at the pressure sensor. The pressure sensor can thus be operated with a higher intrinsic resistance, which means that fewer measuring errors occur. Ultimately, it is not the absolute current that flows when measuring that matters. The ratio of the internal resistance of the source to the intrinsic resistance of the pressure sensor is important.