A thermocouple is a sensor used to measure temperature. It consists of two dissimilar metal wires joined together at one end. When heat is applied to the junction of the two metals, a small voltage is generated that can be correlated back to the temperature.
Type K thermocouples are one of the most common types of thermocouples and are made from chromel (nickel-chromium alloy) and alumel (nickel-aluminum alloy). They are inexpensive, accurate, and can measure a wide temperature range from -200°C to +1200°C.
Type K thermocouples are commonly used in industrial applications such as food processing, plastics manufacturing, and power generation. They can also be found in HVAC systems and scientific labs.
This article provides a step-by-step guide on how to properly wire a type K thermocouple, from selecting the right gauge wire and connectors to wiring multiple thermocouples.
Selecting Wire
The first step in wiring a type K thermocouple is selecting the right thermocouple wire.
Several factors need to be considered when choosing wire:
- Temperature range – Make sure the wire is rated for the entire temperature range the thermocouple will experience.
- Gauge – Thicker wires are stronger and more durable. 24-16 AWG is common.
- Insulation – Look for wire with fiberglass, PTFE, or woven glass insulation for high temperatures.
- Quality – Use wire specifically designed for thermocouples, not generic hookup wire.
- Length – Shorter wire lengths are better to minimize resistance. Under 50 feet is ideal.
For most general purpose type K thermocouple applications, 20-24 AWG fiberglass insulated thermocouple extension grade wire is recommended.
Popular wire gauges include:
| AWG | Diameter (mm) |
|---|---|
| 24 | 0.51 |
| 22 | 0.64 |
| 20 | 0.81 |
The insulation material and diameter directly affect the maximum temperature rating. Refer to the wire specification sheet for exact values.
Thermocouple Connectors
The next component needed is a thermocouple connector. Connectors allow the thermocouple wire to interface with instrumentation or a data acquisition device.
There are a few types of connectors to choose from:
- Miniature – Smaller style used for connecting to circuit boards or smaller instruments.
- Subminiature – Intermediate size good for general applications.
- Standard – Larger heavy duty connectors.
The connector must match the thermocouple wire gauge you selected. Common options include:
| Connector Type | Matches Wire Gauge |
|---|---|
| Miniature | 30-24 AWG |
| Subminiature | 24-20 AWG |
| Standard | 20-16 AWG |
Choose nickel plated connectors for type K thermocouples. Make sure any pins or sockets are properly marked positive and negative.
Thermocouple Probe
In addition to the thermocouple wire and connectors, a probe is required to form the thermocouple junction and measure the temperature.
Probes are typically constructed from stainless steel or ceramic and come in different styles:
- General purpose
- Surface contact
- Pipe clamp
- Insulated
- High temperature
Select a probe suitable for the application environment and temperature range. The probe can be grounded or ungrounded. Grounded probes provide better noise immunity.
Wiring a Single Thermocouple
Follow these steps to wire a single type K thermocouple circuit:
- Cut two equal lengths of thermocouple extension grade wire, one chromel (yellow) and one alumel (red).
- Strip about 1/4″ of insulation from both ends of the wires using wire strippers.
- Twist the two wires together at one end and attach the thermocouple probe using a crimp connector or solder joint. This forms the hot junction.
- Connect the other ends of the wires to a thermocouple connector, matching polarity – yellow to positive, red to negative.
- Plug the thermocouple connector into the instrumentation input channel.
Wiring Multiple Thermocouples
To connect multiple type K thermocouples to a single instrument input channel:
- Wire each thermocouple as described in the single thermocouple steps.
- Use a thermocouple terminal block to connect the individual thermocouples. Choose a block with the number of channels needed.
- Connect the yellow wires from each thermocouple to the positive terminals on the block.
- Connect all the red wires from the thermocouples to the negative terminals.
- Run a pair of extension grade wires from the terminal block back to the instrument. Connect the yellow wire to the positive input and the red to negative.
This allows multiple thermocouples to be wired in parallel and measured by a single channel.
Thermocouple Extension Wire
For long distance thermocouple circuits, extension grade wire is used between the thermocouple and instrument:
- Polarity must be maintained – Positive (yellow) wire to positive terminal, negative (red) to negative terminal.
- Only use extension grade thermocouple wire, not generic wire.
- Minimize splices – Each adds connection errors.
- Use terminal blocks when transitioning between wire gauges.
The maximum recommended distance between a type K thermocouple and instrument is 50 feet. For longer distances, consider using thermocouple transmitters.
Thermocouple Instrumentation
The thermocouple must be connected to some type of instrumentation to read the output voltage and display the temperature.
Common instruments include:
- Digital thermometer
- Temperature controller
- Data logger
- PLC temperature input module
Most instruments designed for thermocouples use a reference junction and cold junction compensation to correct for ambient temperature effects. Make sure the instrument is set for the proper type K thermocouple.
Troubleshooting
Some common issues when wiring thermocouples and their solutions:
| Problem | Solution |
|---|---|
| No reading or low reading | Check wiring polarity and connections |
| Inaccurate or unstable readings | Use proper extension grade wire. Check for short circuits. |
| Measurement drift | Inspect thermocouple junction. Re-solder or replace probe. |
| External noise interference | Use shielded/grounded probes. Improve cable routing. |
| Reading doesn’t match temperature | Ensure proper type K selected in instrument settings. |
Carefully follow wiring polarity, only use extension grade thermocouple supplies, and inspect all connections to avoid issues.
Conclusion
Thermocouples are versatile temperature sensors but proper wiring techniques are critical for accurate measurements.
When working with type K thermocouples:
- Choose the appropriate gauge thermocouple wire with fiberglass or woven glass insulation.
- Use connectors and a probe suited for the temperature range and environment.
- Maintain proper polarity by matching wire colors when making connections.
- Use extension grade wire for long thermocouple circuits.
- Select instrumentation designed for type K thermocouples.
Following the wiring steps outlined in this article will help avoid common errors and ensure your type K thermocouple circuit is set up correctly. Taking care during installation will allow accurate and reliable temperature monitoring for your application.
