RTD or Ohm Inputs
The transmitters will accept a variety of RTD configurations, including 2-wire, 3-wire, 4-wire. If the transmitter is mounted remotely from a 3-wire or 4-wire RTD, it will operate within specifications, without recalibration, for lead wire resistances of up to 60 ohms per lead (equivalent to 6,000 feet of 20 AWG wire).
In this case, the leads between the RTD and 4-20ma transmitter should be shielded. If using only two leads, both RTD leads are in series with the sensor element, so significant errors can occur if the lead lengths exceed three feet of 20 AWG wire (approximately 0.05 °C/ft). For longer runs, attach a third or fourth lead as described above.
Sensor Lead Wire Resistance Effect – RTD Input
When using a 4-wire RTD, the effect of lead resistance is eliminated and has no impact on accuracy. However, a 3-wire sensor will not fully cancel lead resistance error because it cannot compensate for imbalances in resistance between the lead wires.
Using the same type of wire on all three lead wires will make a 3-wire RTD installation as accurate as possible. A 2-wire sensor will produce the largest error because it directly adds the lead wire resistance to the sensor resistance. For 2- and 3-wire RTDs, an additional lead wire resistance error is induced with ambient temperature variations.
The table and the examples shown below help quantify these errors.
Temperature Error of RTD
Consider a RTD with the following parameters mentioned below. Calculate Resistance error or 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 (∆Tamb): 25 °C
RTD Resistance at 0 °C (Ro): 100 Ω (for Pt 100 RTD)
No lead wire resistance effect
Reference : Rosemount Temp Manual