RTD temperature sensor error Two-wire, three-wire, four-wire
RTD or ohm input
Transmitters accept a variety of RTD settings, including 2-wire, 3-wire, and 4-wire. If the transmitter consists of 3 wires or 4 wires, it works in specifications for wire resistance of up to 60 ohms per wire (equivalent to 6000 feet 20 AWG wires). In this case, the paths between the RTD transmitter and 4-20m must be protected.
If two wires are used, both types of RTDs are in series with the sensor element, so if the length of the wire exceeds 1, significant errors will occur.
Effect of wire resistance (sensor – RTD input)
When using 4-wire RTD, the effect of wire resistance is lost and has no effect on accuracy. However, a 3-wire sensor does not completely eliminate the wire resistance error because it cannot compensate for the resistance imbalance between the wires. Using the same type of wire in all three wires, the rtd installation makes the three wires as accurate as possible.
A 2-wire sensor can produce the largest error. Because it directly adds the resistance of the wire to the resistance of the sensor.
For 2- and 3-wire RTDs, an additional resistance wire error occurs with changes in ambient temperature. The following table and examples help quantify these errors.
Consider RTD with the parameters listed below. Calculate the resistance error or the final temperature reading error in the transmitter with 2-wire, 3-wire and 4-wire configurations
Total cable length: 150 m-
Imbalance of the lead wires at 20 ° C: 1.5 Ω-
Resistance / length (18 AWG Cu): 0.025 Ω / m ° C-
Temperature coefficient of Cu (αCu): 0.039 Ω / Ω ° C-
Temperature coefficient of Pt (αPt): 0.00385 Ω / Ω ° C-
Change in Ambient Temperature (ambTamb): 25 ° C-
RTD Resistance at 0 ° C (Ro): 100 Ω (for Pt 100 RTD) –
Pt100 4-wire RTD
No wire resistance
Pt100 3-wire RTD
Pt100 2-wire RTD
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