Thermocouples (TC) and Resistance Temperature Detectors (RTD) are widely used for temperature measurement in process industries across the globe. However, in many cases, the sensor is located far from the measuring point. In these instances, cables are needed to transfer the signal. The challenge is to select the correct extension cables to maintain measurement accuracy in long-distance transmissions.
Ashcroft temperature solutions are known for their accuracy, reliability and seamless integration into processes. That's why customers often ask us for guidance on how to ensure accurate readings, reduce installation errors and avoid costly downtime due to inaccurate readings.
Read this article to learn how thermocouples and RTD extension wires work, why they matter and how to select the correct extension cables for your specific application. You will also learn when transmitters may be a good investment for long-distance signal integrity.
How Thermocouple Extension Wires Work
Temperature sensors like the Ashcroft® S80 Thermocouple are made of two different metal alloy wires. Together, these wires generate a millivoltage using the Seebeck effect—a process that occurs when two dissimilar metal wires form a junction and experience a temperature difference between the "hot" and "cold" ends. That tiny voltage is then converted into a temperature value.
Figure 1. Ashcroft® S80 Thermocouple
The hot junction is where the two thermocouple wires are joined together and placed at the temperature measurement point. The cold junction is where the other ends of the wires connect to the measuring device.
Figure 2. Hot and Cold Junctions on a Thermocouple
However, because the cold junction also introduces a thermoelectric voltage, the thermocouple must compensate for it by measuring the ambient temperature at the connection point and applying a correction. This process, called cold junction compensation (CJC), ensures accurate readings.
Why Choosing Correct Thermocouple Cable Extensions Matters
In many industrial applications, temperature sensors such as thermocouples and resistance temperature detectors (RTDs) are installed far from the monitoring or control equipment. Extending these sensors over long distances introduces a key challenge of maintaining accuracy and signal integrity between the sensor and the readout device.
If thermocouple wires are extended using the wrong material (i.e., different metals), unintended thermoelectric junctions are introduced. This creates false signals, leading to inaccurate readings. That's why it's critical to use extension cables or compensation wires made from the same alloy—or at least ones with equivalent characteristics.
Proper shielding is also essential in industrial environments to protect the weak millivolt signal from EMI. Color-coded plugs and sockets help avoid compatibility errors between thermocouple types.
Figure 3. Thermocouple IEC 60584-3 Color Codes
Figure 4. Thermocouple ASTM MC96.1 Color Codes
How RTD Extension Wires Work
On the other hand, temperature sensors like the Ashcroft® S81 Resistance Temperature Detector (RTD) measure temperature through changes in the electrical resistance of a platinum element. Unlike thermocouples, RTD extension wires are typically made of copper or nickel-plated copper, chosen for their low resistance. Using improper or high-resistance wire can produce inaccurate readings.
To reduce lead wire resistance, three-wire and four-wire configurations are preferred. The four-wire method offers the highest accuracy by effectively eliminating lead wire error.
Figure 5. Ashcroft® S81 RTD Probe with 3-wire Configuration
When To Use a Transmitter for Signal Transfer
Both thermocouple and RTD temperature sensors can benefit from using analog transmitters. These devices convert the temperature signal from the sensor into a standardized output, such as 4-20 mA, or use a digital protocol to transfer the signals, minimizing the impact of wire resistance and EMI.
Using a transmitter, you can prevent signal loss or interference over long distances to ensure that the signal remains accurate.
Transmitters can be mounted directly on the sensor, remotely on walls or pipes, in control cabinets, or even integrated into programmable logic controllers (PLCs). No matter the setup, they help preserve signal integrity back to your control system.
Figure 6: Ashcroft® ITT1 Temperature Transmitter
Ready to learn more?
Now that you understand why selecting the correct RTD and thermocouple extension cables—or considering the use of transmitters— is so important for long-distance applications that require accurate temperature measurements. If you still have questions, please contact us anytime to talk to a product expert.
Until then, here are a few other relevant articles that may be of interest to you:
- Which thermocouple lead wire is best for my application?
- How an RTD works
- When to use an RTD or Thermocouple
Or, download our guide to learn about RTD and Thermocouple Temperature Probes.
