What is a third -line RTD temperature sensor?

The compromise between the two -line and the fourth -line RTD connection is the three -line connection, as shown below:

In the three -line RTD temperature sensor circuit, the voltage meter "A" measures the voltage of the RTD temperature sensor and plus the voltage drop on the bottom load wire. The voltage gauge "B" only measures the voltage drop on the top load wire. Assuming that the two load wires will have the same resistance (very close), subtract the instructions of the voltage meter "B" from the instructions given by the voltage gauge "A", and obtain the voltage drop at both ends of the RTD temperature sensor: V RTD = V Table (A) − V Table (B)

Similarly, the RTD resistance is calculated based on Ohm's law based on the RTD temperature sensor pressure and the known current source value, just like in the 4 -line circuit.

If the resistance of the two load wires is exactly the same (which includes any connected resistors in these current load paths, such as the wiring box), the calculated RTD electric temperature sensor will be the same as the real RTD temperature sensor voltage, and it will not not be voltage and not There will be a cable blocker error. However, if one of these streaming wires is more than the other resistance, the calculated RTD temperature sensor voltage will be different from the actual RTD temperature sensor voltage, which will cause measurement errors.

Therefore, we see that the three -line RTD temperature sensor circuit saves our wiring cost compared to the four -wire circuit, but the cost is potential measurement errors. The beauty of the four -wire design is that the wire resistance is completely unrelated: no matter how large the resistance of each wire, or even if the wire resistor is different from each wire, the RTD voltage (and RTD resistance) can be truly determined. other. In contrast, the error elimination characteristics of the third -line circuit depends on the assumptions that the two load lines have exactly the same resistance, which may actually be true or not.

It should be understood that the real three -line RTD instrument does not use the direct indicator voltage table shown in these simplified examples. The actual RTD temperature sensor uses analog or digital "adjustment" circuit to measure the voltage and perform the necessary calculations to compensate the wire resistance. The voltage table displayed in the four -line and three -line chart is only used to illustrate the basic concepts, not the design of the actual instrument.