RTD Connections, 2 wire, 3 wire, 4 wire

TEMPERATURE SENSORS -
RTDs

RTD Connections: 2 Wire, 3 Wire, 4 Wire?

Because an RTD is a resistance type sensor, resistance introduced by connecting copper extension wires between the RTD and control instrument will add to readings. Furthermore, this additional resistance is not constant but increases with ambient temperature. To estimate leadwire error in 2-wire circuits, multiply the total length of the extension leads times the resistance per foot in the table below. Then divide by the sensitivity of the RTD, given in the next two pages, to obtain an error figure in °C. For example, assume you have connected 100 feet of AWG 22 wires to a 100 Ω platinum RTD (PD element).

Lead resistance is:

Approximate error is:

You can reduce leadwire error by:

Using larger gauge extension wires.
Specifying an RTD with greater sensitivity; 1000 Ω instead of 100 Ω, for example.
Employing a 3 or 4-wire resistance canceling circuit as shown at right. Common leads, connected to the same end of the sensing element, are the same color.
Using a 2-wire current transmitter. Its linearized signal is immune to electrical noise as well as resistance and can maintain accuracy over runs of several thousand feet.

2-wire circuit

Shown above is a 2-wire RTD connected to a typical Wheatstone bridge circuit. ES is the supply voltage; EO is the output voltage; R₁, R₂, and R₃ are fixed resistors; and R_T is the RTD. In this uncompensated circuit, lead resistances L₁ and L₂ add directly to R_T.

3-wire circuit

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 L₁ and L₃. Minco matches RTD leads within 5%.

4-wire circuit

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

If necessary you can connect a 2-wire RTD to a 3-wire circuit or 4-wire circuit, as shown to the above. As long as the junctions are near the RTD, as in a connection head, errors are negligible.

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TEMPERATURE SENSORS -
RTDs