English is currently available.
Control & Instrumentation Engineering DISCUSSION
I need to measure a tank temperature of about 80 °C with a Pt100 that sits 50 m from the control panel. The probe is available in 2-wire, 3-wire and 4-wire versions, and the input module supports all three.
How much error does the cable actually add in the 2-wire case, how does the third wire cancel it, and is 4-wire worth the extra cores for a plant measurement?
A Pt100 is 100 Ω at 0 °C and rises by about 0.385 Ω per °C, so it reads about 130.9 Ω at 80 °C. In a 2-wire connection the instrument cannot tell sensor resistance from cable resistance. 50 m of 0.5 mm² copper is about 1.75 Ω per conductor, 3.5 Ω for the pair. 3.5 / 0.385 means the reading is about 9 °C high.
You can subtract a fixed offset, but the resistance of copper itself changes by about 0.4 percent per °C, so the error moves with the ambient temperature along the cable run.
The 3-wire connection adds a second lead to one end of the sensor. The instrument uses it to measure the resistance of the leads and subtracts that, on the assumption that all three conductors have the same resistance. With cores of the same cable, same length and same temperature, that holds well and the residual error is small.
It fails when the leads are not equal: a corroded terminal, a different core size on one leg, or an extra junction in one wire appears directly as temperature error, 0.1 Ω of mismatch being about 0.26 °C.
The 4-wire connection removes the lead resistance from the measurement entirely. Two wires carry the excitation current, the other two sense the voltage right at the element, and almost no current flows in the sense pair, so there is no voltage drop in it. Lead resistance, and unequal lead resistance, no longer matter.
Use it where you want the accuracy of the sensor itself, such as laboratory work, energy metering and reference probes. For a general plant measurement over 50 m, 3-wire is the usual compromise.
Another option for a long run is to put a head-mounted temperature transmitter at the probe and send 4-20 mA to the panel. The Pt100 leads are then only a few centimetres long, and the 50 m of cable carries a current signal that is insensitive to its resistance.
Whichever wiring you use, keep the excitation current low. 1 mA through 131 Ω dissipates about 0.13 mW, which is harmless, but several milliamps in a small element in still air causes measurable self-heating. Also check the sensor tolerance class, since a Class B element is already allowed ±0.3 °C at 0 °C and more at higher temperature.
Back to Control & Instrumentation Engineering community
