If you work in the field of automation or process control, you may have come across a 4 wire RTD (Resistance Temperature Detector) at some point. An RTD is a temperature sensor that measures temperature by detecting changes in resistance. It is one of the most popular temperature sensors in the industry today. In this article, we will discuss the 4 wire RTD wiring diagram and how it works.
What is a 4 Wire RTD?
A 4 wire RTD is a type of temperature sensor that uses four wires to measure temperature. It consists of a sensing element made of a pure metal or ceramic. The sensing element is connected to four wires, two for the current source and two for the measurement of the voltage across the sensing element. The two wires for the current source, also called the excitation wires, provide a constant voltage to the sensing element. The other two wires, also known as the sense wires, measure the voltage across the sensing element.
Wiring Diagram for a 4 Wire RTD
The wiring diagram for a 4 wire RTD is as follows:
| Wire Color | Wire Function |
|---|---|
| Red | Excitation Positive |
| Black | Excitation Negative |
| Yellow | Measurement Positive |
| Green | Measurement Negative |
The two excitation wires (red and black) are connected to a constant current source. The two measurement wires (yellow and green) are connected to a voltmeter, which measures the voltage drop across the sensing element. The resistance of the sensing element changes as the temperature changes, which causes a change in voltage at the measurement wires.
Advantages of a 4 Wire RTD
There are several advantages of using a 4 wire RTD:
Accuracy
A 4 wire RTD is more accurate than a 2 wire RTD because it eliminates the effects of lead resistance. Lead resistance is the resistance of the wires that connect the RTD to the measuring device. In a 2 wire RTD, lead resistance can cause errors in the measurement. A 4 wire RTD eliminates these errors by using separate wires for the excitation and measurement.
Stability
A 4 wire RTD is more stable than a 2 wire RTD because it eliminates the effects of lead resistance and thermal gradients. Thermal gradients are differences in temperature between the sensing element and the wires that connect it to the measuring device. These differences can cause errors in the measurement. A 4 wire RTD eliminates these errors by using separate wires for the excitation and measurement.
No Self Heating
A 4 wire RTD does not produce self-heating because the excitation current flows through separate wires. In a 2 wire RTD, the excitation current flows through the sensing element, which can cause self-heating and errors in the measurement.
FAQs
What is the maximum distance between the RTD and the measuring device?
The maximum distance between the RTD and the measuring device depends on several factors such as the wire gauge, the excitation voltage, and the resistance of the sensing element. However, in general, the maximum distance is around 1000 feet.
What is the temperature range of a 4 wire RTD?
The temperature range of a 4 wire RTD depends on the material used for the sensing element. Common materials include platinum, nickel, and copper. Platinum is the most commonly used material and has a temperature range of -200°C to 850°C.
Can a 4 wire RTD be used in hazardous areas?
Yes, a 4 wire RTD can be used in hazardous areas. However, it must be properly installed and certified for use in the specific hazardous area.
What is the response time of a 4 wire RTD?
The response time of a 4 wire RTD depends on several factors such as the size and shape of the sensing element, the temperature range, and the measurement device. However, in general, the response time is around 5-10 seconds.
What is the output signal of a 4 wire RTD?
The output signal of a 4 wire RTD is a change in resistance. The resistance change is proportional to the temperature change.
Conclusion
A 4 wire RTD is a type of temperature sensor that uses four wires to measure temperature. It is more accurate, stable, and reliable than a 2 wire RTD. The wiring diagram for a 4 wire RTD is simple and easy to understand. By using a 4 wire RTD, you can ensure accurate and reliable temperature measurements.
