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Temperature Error of 2-Wire, 3-Wire & 4-Wire RTD

Temperature Error of 2-Wire, 3-Wire & 4-Wire RTD

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.

RTD Sensor Error

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.

Given Data:

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)

Pt100 4-wire RTD:

No lead wire resistance effect

Pt100 3-wire RTD:

Three Wire RTD Lead Wire Resistance Error

Pt100 2-wire RTD:

Two Wire RTD Lead Wire Resistance Error

Reference : Rosemount Temp Manual

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4 comments

Venkatesh March 2, 2017 at 3:22 am

Very very useful data thanq for giving me such a data

Reply
Ali March 2, 2017 at 3:10 pm

Thankyou so much Sir.You are doing a great job. Your efforts are helping thousands of people like me. God bless!

Reply
Shailesh January 1, 2019 at 4:38 pm

Very very useful

Reply
Thamprasad January 6, 2019 at 6:45 pm

I was having a question. It will be of grate help if someone could help me out.

How are you going to compensate temperature in a RTD??

Reply

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