Temperature error of 2-wire, 3-wire and 4-wire RTD temperature sensors

RTD temperature sensor or ohm input
The transmitter will accept various RTD temperature sensor configurations, including 2-wire, 3-wire, and 4-wire. If the transmitter is remotely mounted from a 3-wire or 4-wire RTD, it will operate within specifications without recalibration, with lead resistance of up to 60 ohms per lead (equivalent to 6000 feet of 20 AWG lead).
In this case, the lead between RTD temperature sensor and 4-20ma transmitter should be shielded. If only two leads are used, both RTD leads are in series with the sensor element, so if the lead length exceeds three feet of 20 AWG wire (approximately 0.05 ° C/foot), a serious error will occur. For longer runs, connect the third or fourth lead as described above.

Sensor Lead Wire Resistance Effect – RTD Temperature Sensor Input
When 4-wire RTD temperature sensor is used, the influence of lead resistance is eliminated, which has no influence on accuracy. However, the 3-wire sensor cannot completely eliminate the lead resistance error because it cannot compensate for the imbalance of resistance between the leads.
Using the same type of wire on all three wires will make the installation of the 3-wire RTD temperature sensor as accurate as possible. The 2-wire sensor produces the greatest error because it directly adds the lead resistance to the sensor resistance. For 2-wire and 3-wire RTD temperature sensors, changes in ambient temperature can cause additional lead resistance errors.
The tables and examples shown below help to quantify these errors.

RTD temperature error
Consider RTD temperature sensors with the following parameters. Calculate the resistance error or final temperature reading error in transmitters with 2-wire, 3-wire, and 4-wire configurations.

Given data:

Total cable length: 150 m
Unbalance of conductor at 20 ° C: 1.5 Ω
Resistance/length (18 AWG Cu): 0.025 Ω/m ° C
Cu ( α Temperature coefficient of Cu): 0.039 Ω/Ω ° C
Pt( α Temperature coefficient of Pt): 0.00385 Ω/Ω ° C
Ambient temperature change (∆ Tamb): 25 ° C
RTD resistance at 0 ° C (Ro): 100 Ω (for Pt 100 RTD)

Pt100 4  Line RTD：
No lead resistance effect

Pt100 3 Line RTD：

Pt100 2 Line RTD：