Technical Blog

# RTDs; 2 Wire, 3 Wire and 4 Wire – What’s the Difference? RTDs are a type of temperature sensor; a “Resistance Temperature Detector”. They are available with different wire configurations; 2 wire, 3 wire and 4 wire.

The wire configuration will be suited to different aspects of the application at hand.

RTDs measure temperatures within an electric circuit, the temperature is determined based on the amount of resistance within the circuit.

## 2 Wire RTDs

2 wire RTDs contains a single wire connecting each end of the RTD element. It is the most simple wire configuration. The resistance calculated will include all elements of the circuit which means there is a higher degree for error. As there are only 2 wires, they are unable to compensate for any additional resistance caused by other elements within the circuit.

Systems can be calibrated to eliminate the error, however, this is still not always accurate or the best solution.

Therefore, a 2 wire RTD is often used in applications with short wires, high resistance sensors or where high accuracy is not of the utmost importance. In this instance the measured resistance is Rt=R1+R2+Rb, otherwise known as RTOTAL as it is the total amount of resistance within the circuit. The most simple measurement with a degree of error included.

## 3 Wire RTDs

The 3 wire construction of an RTD is the most common type where on side of the element has one wire connected and the other side has two wire connections. This allows for the extra resistance created in the circuit to be compensated for giving a more accurate reading. The resistance is calculated by first calculating the resistance between wires 1 and 2 and then subtracting the resistance between wires 2 and 3 which results in an accurate measurement for the resistance element (Rb). This method assumes that all wires are measuring the same resistance; they therefore have to be identical.

A 3 wire RTD is the most common type as they are accurate for most types of application. However, there are still inaccuracies if the wires have different resistances. Therefore for a fully accurate solution a 4 wire RTD is recommended.

## 4 Wire RTDs

It goes without saying that the 4 wire RTD is the most complex solution – this is also reflected in the price of this type of RTD. They are primarily used in laboratory applications where high accuracy is essential.

This circuit is used for longer wire lengths between the measuring element and the measuring electronics as it is able compensate for errors caused by the wire resistance.

In contrast to the 3-wire circuit, where it is assumed that all wires have the same resistance. The 4 wire RTD circuit assumes that each wire has a unique measurement of resistance. This compensates for the entire wire resistance to give the most accurate reading of all RTDs.

When using a 4-wire RTD, two of the wires are connected to each end of the measuring element (usually wires 1 and 4).  A distinct small measuring current is constantly applied in the measuring element, so that it can be constantly checked as a comparison variable. The voltage drop that occurs at the measuring resistor can measured on wires 2 and 3. Using the "fault current" flowing through the very high-resistance input, the voltage drop at the wire resistances can be neglected and the measuring input can then detect the real voltage drop at the measuring resistor almost 1 to 1. Ohm's law can be used to determine the exact measuring resistance by dividing the measuring voltage by the constant current. Umeas / I = Rmeas

## RTDs from Variohm

We have a range of RTDs which are available to view on our website. For more information on RTDs take a look at the following blog posts:

How does an RTD work?

Difference between PT100 and PT1000