Thermocouples are a common temperature sensing device used in many industrial applications. J type thermocouples in particular are low cost, versatile, and durable. Connecting them properly is important for accurate temperature measurements.
A thermocouple consists of two dissimilar metal wires joined together at one end. This junction between the two metals produces a small voltage that varies with temperature. By measuring this voltage, the temperature at the junction can be determined.
J type thermocouples use iron and constantan as the two metals. They are inexpensive, accurate, and measure a wide temperature range of -40 to 750°C (-40 to 1382°F). Some key properties of J type thermocouples:
- Inexpensive and readily available
- Rugged – can withstand high temperatures and harsh conditions
- Wide temperature range
- Fairly linear output
- Common in industrial applications
To use a J type thermocouple, the two metal wires must be properly connected to create the sensing junction, and the voltage output measured with a thermocouple meter, thermocouple module, or temperature controller.
Creating the Sensing Junction
The sensing junction is where the two dissimilar metals join together. This can be created by:
- Twisting or welding the two wires together
- Allowing the wires to overlap inside a closed tip probe
- Inserting the wires into a connector block or terminal
Twisting the wires together forms the simplest junction. The wires can be twisted tightly together and optionally welded, soldered, or crimped for a solid connection.
Twisted Wires Example
Overlapping the wires in a sealed probe protects the junction while still allowing temperature sensing at the end of the probe tip.
Special thermocouple connectors are also available to easily connect the two wires. These contain metal blocks into which the two wires are inserted, creating a solid junction.
Extension Wires and Connectors
Once the sensing junction is created, the thermocouple wires must be connected to a measuring instrument. This often requires extending the leads to reach the equipment.
Important considerations when extending thermocouple leads:
- Use properly rated thermocouple extension wire. Match the type of thermocouple (J, K, T, etc.) and wire gauge.
- Avoid introducing extra junctions which can create errors. Use continuous extension wire without splices.
- Use connectors rated for the expected temperature ranges. Use twisted pair cable for EMI/RFI noise reduction.
Thermocouple Extension Wire
| Wire Color | Polarity |
|---|---|
| White | Negative (-) |
| Red | Positive (+) |
The red wire is positive, while the white wire is negative. Connecting with reversed polarity will result in reversed, incorrect temperature measurements.
Thermocouple connectors are color coded for proper mating with extension wires or instrumentation.
Connecting to Instruments
The thermocouple can be connected to various devices to read or record the temperature:
Multimeter
An inexpensive way to measure thermocouple temperatures. However, most multimeters only have a limited cold junction compensation accuracy of +/- 2°C.
Temperature Controller
Controllers allow temperature monitoring, control, and recording. Provides cold junction compensation and sometimes linearization for enhanced accuracy.
Data Logger
Data loggers record thermocouple measurements over time. Small self-contained units are available for portable data logging.
PLC Analog Input Module
For industrial automation systems, PLC analog input modules can connect directly to thermocouples.
DAS (Data Acquisition System)
DAQ systems interface multiple thermocouples with a computer for high speed monitoring and recording.
Isothermal Blocks
When connecting multiple thermocouples, isothermal terminal blocks help reduce temperature measurement errors. These contain metal connection terminals in a heated block to maintain a uniform temperature.
This minimizes thermal EMFs between the copper terminals and different wire types, providing better accuracy than simple screw terminals.
Cold Junction Compensation
Thermocouples measure the temperature difference between the sensing junction and connecting terminals. This connecting point is called the cold junction.
To determine the true temperature at the sensing junction, the temperature at the cold junction must also be measured. The cold junction temperature is used to compensate for the thermal EMFs created here. This is known as cold junction compensation (CJC).
Some ways to implement cold junction compensation:
- Measure cold junction temperature with a thermistor or RTD
- Use an isothermal terminal block
- Electronically simulate a reference temperature in the thermocouple circuitry
- Use thermocouple wire for all connections (creating multiple thermocouple junctions)
Proper cold junction compensation is critical for accurate thermocouple temperature measurements.
Key Steps to Connect a J Type Thermocouple
To summarize, connecting a J type thermocouple involves:
- Creating the sensing junction – twist, weld, overlap, or terminate the iron and constantan wires
- Extending the leads with proper extension wire and connectors if needed
- Connecting to a temperature measurement or control device
- Implementing cold junction temperature sensing at the terminal connection
- Ensuring proper polarity – white is negative, red is positive
Following these basic steps will allow robust and accurate temperature measurement with a J type thermocouple.
Conclusion
J type thermocouples are versatile sensors for industrial temperature monitoring and control. While low cost, they can provide very accurate measurements when properly connected with cold junction compensation. Paying attention to polarity, proper wire extension, and termination is key. Following basic wiring best practices will ensure success in interfacing J type thermocouples with your temperature measurement system.
