4 Wire RTD

Why Use a 4 Wire RTD?

The 4-wire configuration alleviates the problems due to extension wire length and resistance imbalance between the leads. Because of this, corrosion and environmental conditions are less of a concern.

Since there is no issue with lead resistance, 4-wire RTD’s can use a smaller gauge wire. For these reasons there are many applications that can benefit from the use of a 4-wire RTD. These are applications that require high accuracy, where the sensor and receiving device are some distance apart,and in corrosive conditions.

4-wire RTD circuits not only cancel lead wires but remove the effects of mismatched resistances such as contact points. A common version is the constant current circuit shown here. Is drives a precise measuring current through L1 and L4; L2 and L3 measure the voltage drop across the RTD element. Eo must have high impedance to prevent current flow in the potential leads. 4-wire circuits may be usable over a longer distance than 3-wire, but you should consider using a transmitter in electrically noisy environments.

Lead wire colors are defined in the IEC 60751-2008 standard where all wire colors are shown as in the following figure.

Precision RTD calibration with ITS-90, IPTS-68, Calandar Van Dusan or Polynominal Chart

Industrial RTD Calibration with Data and DEviation from IEC-751

Meters, Transmitters & Switch Calibration

Thermistor & Bi-Metallic Calibration

In this circuit there are three leads coming from the RTD instead of two. L1 and L3 carry the measuring current while L2 acts only as a potential lead. No current flows through it while the bridge is in balance.Since L1 and L3 are in separate arms of the bridge,resistance is canceled. This circuit assumes high impedance at Eo and close matching of resistance between wires L2 and L3.