How to Check a 3 Wire RTD with a Multimeter

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Are you familiar with high-performance applications? If yes, you may have heard of RTDs. Resistance temperature detectors or RTDs measure temperature in most industrial applications. In a programmable logic controller (PLC) or a distributed control system (DCS), one data-acquisition module might track the temperature of most remotely located RTDs. 

The ideal accuracy will be acquired for high-performance purposes if each RTD has its ADC and excitation circuit. However, the data-acquisition module will be big, costly, and hungry for power. Multiplexing results in a smaller, cost-efficient, lower power module, but a few accuracies could be compromised and lost. 

Take note that RTDs are accessible in 2-wire, 3-wire, and 4-wire configurations. 4-wire devices are considered the most accurate, while the 2-wire devices are the least costly. Further, 3-wire RTDs could be excited by two similar current sources to stop lead resistance. 

Today’s post will discuss what a 3-wire RTDs is, how it works, and ways to help you check it with a multimeter. If you’re ready, let’s dive in! 

What are the Different RTD Wires?  

While we briefly discussed the differences between the different RTDs above, this section will dig deeper into each of them. 

  • 2 Wire RTD

Two-wire RTDs are known to be less accurate of the three circuit configurations. That’s because there is no way of calculating or eliminating the lead wire resistance between the detector and the reading. That creates uncertainty in the reading itself.

Thus, the sensors are frequently utilized only with short lead wires, where accuracy isn’t of the utmost importance. Take note that 2 wire sensors are also less expensive for a smaller number of critical measurements.  

  • 3 Wire RTD

A 3 wire RTD is considered the most typically utilized RTD sensor. Considering all three lead wires are the same, the third wire computes the average lead wire resistance over the circuit and detaches it from the sensor measurement.

That makes a 3-wire RTD much preferred as it gives more accurate measurement, unlike the 2-wire counterparts, but less accurate than 4-wire configurations. Nonetheless, keep in mind that substantial readings could be done using 3 wire configurations in circuits with long lead wires and where there are long distances between the reading and the detector. 

It’s worth mentioning as well that 3 wire RTD is used where high accuracy over a massive temperature range is needed. 

  • 4 Wire RTD

4-wire RTDs are used where close accuracy is important, normally used for lab purposes. In this RTD resistance, each lead wire is calculated and removed, leading to the exact detector’s resistance.

Moreover, the four wire circuit functions by utilizing the first two lead wires to fuel the circuit while the fourth and third wires read the resistance in every lead wire compensation for any differences within lead wire resistance. 

How Does 3 Wire RTD Configuration Work?

The three-wire configuration offers a compensation loop, which can be utilized to minimize the lead wire resistance from the element loop’s resistance measurement. That leads to a value for the element resistance only. 

Obtaining an exact measurement with this approach is established on the resistance of every lead is the same and equal. Unluckily, that is not always the case. Steps should be taken in the entire design and application of a 3-wire sensor to ensure accuracy.

It’s a standard practice to use a 3-wire circuit to lessen the impacts of the line resistances and their instability with temperature. It is composed of running an extra wire to one contact of the RTD. That then leads to two measuring circuits, where one is utilized as a reference.

You see, the three-wire RTD makes it possible to compensate for the line resistance both in its temperature variation and its amount. Nonetheless, it is a prerequisite that all three conductors should have equal properties and are exposed to the same temperatures. 

That typically applies to a sufficient degree. That’s the reason why the 3-wire RTD is the most widely utilized method these days, as it doesn’t need line balancing.

How to Test Your 3 Wire RTD with a Digital Multimeter

Are you now ready to test and check your RTD for functionality? This section will offer you the steps you need to follow. To do that, make sure you have your digital multimeter in place. 

  1. Grab your digital multimeter and set it to Ohms. 
  1. Now check your 3-wire RTD. It will help if you check across the node and the element as well. Just some advice though, not all RTDs have the same color wiring. Make sure you check with the MFG first for the proper wiring arrangement and color. 
  1. Take your hook leads and check across the node. You like to read close to zero or just around one. 
  1. Take one of your leads and measure across the element itself. Center a room temperature, and you should be reading around anywhere from 115 to 209 ohms. 
  1. Let’s move on to your 3-Wire RTD 830. This RTD has a connector where the red dot means the positive side. Take your jump lead and measure it across the node. Take one of your leads and jump over to the element. Again, you should receive a reading somewhere around 109. That means your RTD is working just fine. 

Final Thoughts

3-wire RTDs have a three-wire that offers a measurement of the resistance of the lead wire and subtracts that resistance from the real value. That correction compensates for the effect of the resistance of the long lead wires on the temperature measurement and leads to enhanced accuracy.

You see, 3-wire RTDs are so cheap and efficient that they have become the industry standard. They are widely utilized in industrial processes and applications, particularly in refineries, petrochemical, and chemical plants where temperature control and monitoring are of utmost importance.

To sum up, there’s no doubt that 3 wire RTDs deliver excellent repeatability and accuracy at an affordable cost and have been used by all big temperature transmitter companies as the standard type of sensor for many applications.

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