What are the differences between 2 wire, 3 wire, and 4 wire RTD Wiring for Pt100 Sensors?

An RTD sensor (Resistance Temperature Detector) is a temperature sensor that is ideally suited to a wide variety of industrial and laboratory applications. These sensors, usually Pt100, use the relationship between resistance and temperature to provide a reliable and predictable resistance value which can then be measured and converted to display in ºF or ºC by appropriate instrumentation. They are highly accurate with high stability and repeatbility. The resistance values of a Pt100 RTD are provided in our resistance vs temperature tables.

RTDs are available in a wide range of styles, configurations and terminations and are available as either a 2-wire, 3-wire or 4-wire device. Each wiring configuration has its own characteristics and advantages and they are usually colour coded as below:

2-wire RTD Pt100 Sensor = 1 x red lead wire and 1 white lead wire

3-wire RTD Pt100 Sensor = 2 x red lead wires and 1 white lead wire

4-wire RTD Pt100 Sensor = 2 x red lead wires and 2 white lead wires

2-wire configuration

The simple two wire rtd connection shown in Figure 3.1 is used only where high accuracy is not essential - the resistance of the connecting wires is always included with that of the sensor, leading to errors in the signal (resistance of element + lead resistance, usually copper). In fact, a standard restriction with this installation is a maximum of 1 - 2 ohms resistance per conductor - which is typically about 300 feet of cable. This applies equally to balanced bridge and fixed bridge systems. The values of the lead resistance can only be determined in a separate measurement (without the RTD sensor) and therefore a continuous correction during the temperature measurement is not possible.

3-wire configuration

A better wiring configuration is shown in Figure 3.2. In this rtd circuit diagram, the two leads of the sensor are on adjoining legs. Although there is lead resistance in each leg of the bridge, the lead resistance is cancelled out from the measurement. It is assumed that the two lead resistances are equal, therefore demanding high quality connection cables.This allows an increase to 10 ohms - usually allowing cable runs of around 1500 feet or more, if necessary.

Also, with this wiring configuration, if fixed bridge measurement is being made, compensation is clearly only good at the bridge balance point. Beyond this, errors will grow as the imbalance increases. This, however, can be minimized by using larger values of resistance in the opposite bridge circuits to reduce bridge current changes.

The pt100 3 wire configuration is very popular for general industrial applications and is widely used in terminal heads when used with 4 to 20mA current transmitters and where dual element duplex sensors are used.

4-wire configuration

The best wiring arrangement is the four wire configuration as depicted in figure 3.4. This provides for full cancellation of spurious effects with the bridge type measuring technique. Cable resistance of up to 15 ohms can be handled with this arrangement, accommodating cable runs of around 3,000 feet. Incidentally, the same limitation as for three wire connections applies if the fixed-bridge, direct-reading approach is being used.

The resistance thermometer can also be energised from a constant current source, and the potential difference developed across it measured directly by some kind of potentiometer. An immediate advantage is that here, incidentals like conductor resistance and selector switch contact resistance are irrelevant. The essentials for this voltage-based method are simply a stabilised and accurately known current supply for the RTD sensor (giving a direct relationship of voltage to resistance and thus to temperature) and a high impedance voltmeter (DVM, or whatever) to measure the voltage developed with negligible current flow.

With this approach, absolute temperature can be derived as long as the current is known. Even where it is not known, if it is stable, differential resistance (and thus temperature) is provided. Also, a number of RTD's can be connected in series using the same current source. Voltage signals from each can then be scanned by high impedance measuring instrumentation.

Again, a four wire configuration is appropriate, although clearly somewhat different to that used with bridge systems. Using the configuration in Figure 3.7 the resistance of the leads has a negligible effect on measurement accuracy.