As you may or may not know already, an RTD is a device containing an electrical resistance source that changes resistance value based on its temperature. That change of resistance could be used and computed to identify the process’s temperature or of a material.
RTDs are typically bought with 2, 3, or 4 lead wires per element:
- 4-wire construction is utilized in labs where close accuracy is needed. In a four-wire RTD, the lead wires’ actual resistance can be identified and removed from the sensor measurement.
Take note that the 4-wire circuit is considered a true 4-wire bridge, working by using wires 1 and 4 to energize the wires and circuits 2 and 3 to read. That true bridge technique will compensate for any differences within lead wire resistances.
- 3-wire construction is the most typically utilized in industrial uses, where the third wire offers a technique for detaching the usual lead wire resistance from the sensor measurement. Substantial savings could be done using a 3-wire cable rather than a 4-wire cable, especially when extended distances exist between the measurement or control instrument and the sensor.
The 3-wire circuit also functions by computing the resistance between #1 and #2 and subtracting the resistance between #2 and #3. That leaves only the resistance of the RTD bulb. That technique assumes that wires 1, 2, and 3 are all similar resistance.
- The 2-wire connection is finally the least precise of the three types of RTDs. That’s because there’s no way of removing the lead wire resistance from the sensor measurement. Further, the two-wire RTDs are mostly utilized where close accuracy is not needed or with short lead wires.
Now that you have a basic understanding of the difference between RTDs let’s focus more on 4-wire RTD.
Introducing 4-Wire RTDs
A four-wire RTD offers a higher level of temperature accuracy as compared to the 3- and 2-wire RTD configurations. That’s because the 4-wire configuration gets rid of the possible differences in-circuit resistance that leads to a difference in lead wire length, materials, and diameters.
Take note that a 4-wire RTD design was initially used in research centers and labs because of the higher cost of the sensor as well as the special instrumentation to accept a four-wire input.
Moreover, the amplified utilization of these RTD designs is occurring throughout the industry. The cost has lowered, and accessibility for the instrumentation and sensors has risen. The value for enhanced temperature preciseness has improved product efficiency, quality, and new product development.
RTD connectors along with 4-wire capabilities have also impacted the continuous adoption of that configuration. The circular technology like the M12 design and other circular connectors and small connectors used in automotive, aerospace, electronics, and automation are being significantly used in other industries.
Did you know that platinum 1000 ohm (PT1000) and platinum 100 ohm (PT100) resistance temperature devices are the most typically used design with this 4-wire RTD configuration? It’s worth mentioning as well that the 4-wire RTD’s temperature operating range is the same as the 3-wire and 2-wire configurations. That means it has -196 to 850°C or -320°F to 1,562°F depending on the design chosen.
Should You Use 4-Wire RTD?
One of the best things about using a 4-wire RTD is that this configuration removes the issues due to resistance imbalance between the leads and extension wire length. That being said, environmental conditions and corrosion are less of a problem.
A four-wire RTD can also utilize a tinier gauge wire as there is no problem with lead resistance. For such reasons, you will find numerous applications that could benefit from using a four-wire configuration. Those applications need high accuracy, where the receiving device and sensor are some distance apart and in corrosive settings.
Also, a four-wire RTD configuration stops lead wires and eliminates the impacts of incompatible resistances like contact points. A typical version is the continuous current circuit. It drives an exact measuring current through L4 and L1, and L3 and L2 compute the voltage drop across the RTD element.
It should have a higher impedance to stop current flow in the potential leads. You see, a four-wire configuration might be practical and helpful over long distances, unlike a 3-wire circuit. Nonetheless, you need to consider using a transmitter, especially if you’re using it in an electrically noisy condition.
How Can You Test a 4-Wire RTD?
Testing a 4-wire RTD is a very simple task that must be done frequently, as it’s a vital element in most applications it’s used for, particularly when it’s used for measuring the cooling load consumption.
Many four-wire circuits are made of platinum materials with a linear temperature versus resistance relationships. Would you like to know how you can test your 4-wire RTD? Just follow the necessary steps below.
But first, to test your RTD, make sure you identify its type first. Are you using Pt100, Pt500, or Pt1000? Take note that the differences between these are in the temperature measuring ranges and accuracy.
Knowing the RTD type, you can download its resistance table. That table will give you the resistance output in ohms you should receive from the sensor at various temperatures.
Let’s say you measure the resistance at room temperature. That output should be 109.35 ohms.
For your 4-wire RTD, the outlets of the same color are shorted. Hence, you should measure it between two points of different colors. Otherwise, you will receive a 0 reading if you test it between two points of the same color.
- Put your digital multimeter (DMM) on the ohms unit.
- Take your hook leads and check across the node. You like to read close to zero or just around one.
- Take one of your leads and measure across the element itself. Center a room temperature, and you should be reading around anywhere from 100 to 120 ohms.
- 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 4- wire RTD is working just fine.
We hope you now learned how to test your 4-wire circuit with a digital multimeter. Share this post to your colleagues, so they are aware too!