Thermocouples are used extensively for temperature sensing in various industrial and domestic applications. A 3 wire thermocouple is a type of thermocouple that has three wires instead of two, which enhances the accuracy of temperature sensing. In this article, we will discuss the wiring diagram for a 3 wire thermocouple, its working principle, advantages, and some frequently asked questions.
Working Principle of a 3 Wire Thermocouple
A thermocouple consists of two dissimilar metals joined at one end, which produces a voltage that is proportional to the temperature difference between the two junctions. The working principle of a 3 wire thermocouple is similar to that of a 2 wire thermocouple, but it has an additional wire that makes it more accurate.
The third wire in a 3 wire thermocouple is called the compensation wire or the reference wire. It is connected to the same metal as the negative wire at the measurement end of the thermocouple. This prevents any errors due to resistance differences between the positive and negative wires.
The compensation wire acts as a reference point to measure the resistance of the positive and negative wires, which helps in eliminating any errors that may be caused by resistance variations. The voltage drop across the reference wire is zero because it is made of the same metal as the negative wire. Therefore, the reference wire does not contribute to the thermocouple output.
Wiring Diagram for a 3 Wire Thermocouple
The wiring diagram for a 3 wire thermocouple is a bit more complex than that of a 2 wire thermocouple. The positive and negative wires are connected to the instrument that measures the voltage difference, while the third or compensation wire is connected to the same metal as the negative wire.
The wiring diagram for a 3 wire thermocouple is shown in the table below:
| Wire Color | Description |
|---|---|
| Red | Positive wire (measuring end) |
| Blue | Negative wire (measuring end) |
| Green | Compensation wire (reference end) |
The wiring diagram shows that the red wire is connected to the positive terminal of the instrument, while the blue wire is connected to the negative terminal. The green wire is connected to the negative wire at the reference end of the thermocouple.
Advantages of a 3 Wire Thermocouple
A 3 wire thermocouple has several advantages over a 2 wire thermocouple:
- Higher accuracy: The additional compensation wire eliminates any errors due to resistance variations in the positive and negative wires.
- Longer cable runs: A 3 wire thermocouple can be used for longer cable runs without losing accuracy.
- Cost-effective: A 3 wire thermocouple is more cost-effective than a 4 wire thermocouple, which has two compensation wires.
FAQ
Q. What is the difference between a 2 wire and a 3 wire thermocouple?
A. The main difference between a 2 wire and a 3 wire thermocouple is the presence of an additional compensation wire in the latter. The compensation wire eliminates any errors caused by resistance variations in the positive and negative wires, resulting in higher accuracy.
Q. Can a 3 wire thermocouple be used for longer cable runs?
A. Yes, a 3 wire thermocouple can be used for longer cable runs without losing accuracy. The compensation wire ensures that any resistance differences between the positive and negative wires are eliminated, thus maintaining the accuracy of temperature sensing.
Q. Is a 3 wire thermocouple more expensive than a 2 wire thermocouple?
A. Yes, a 3 wire thermocouple is slightly more expensive than a 2 wire thermocouple due to the additional compensation wire. However, it is more cost-effective than a 4 wire thermocouple, which has two compensation wires.
Q. Can a 3 wire thermocouple be used with any instrument?
A. Yes, a 3 wire thermocouple can be used with any instrument that is capable of measuring the voltage difference between the positive and negative wires. However, it is important to ensure that the compensation wire is connected to the same metal as the negative wire, as shown in the wiring diagram above.
Q. What is the maximum cable length for a 3 wire thermocouple?
A. The maximum cable length for a 3 wire thermocouple depends on the type of thermocouple and the accuracy required. However, in general, a 3 wire thermocouple can be used for cable runs up to 100 feet without losing accuracy.
Conclusion
A 3 wire thermocouple is a type of thermocouple that has three wires instead of two, which enhances the accuracy of temperature sensing. It has an additional compensation wire that eliminates any errors due to resistance variations in the positive and negative wires. The wiring diagram for a 3 wire thermocouple is slightly more complex than that of a 2 wire thermocouple, but it is still easy to understand. With its higher accuracy and longer cable runs, a 3 wire thermocouple is an ideal choice for various industrial and domestic applications.
