Thermocouples are essential instruments designed to monitor temperature in critical applications involving harsh conditions, extreme weather and high-vibration environments. They are often used in automation, power, pharmaceuticals, life sciences, clean-in-place, refining, and chemical system installations.
When configured to meet the demands of equipment like turbine systems, gas compressors, pumps and other equipment, the main function of the thermocouple is to provide clear and reliable signals to communicate whether a system is working properly, or not. In doing so, these instruments help reduce downtime, lower maintenance costs and increase productivity.
A key part of the configuration process is selection of the correct junction type for optimal sensor performance. My knowledge in this area is based on 20 years of experience working as an engineer specializing in temperature measurement instruments.
In this article, you will learn about the different thermocouple components, including two of the most common types of junctions: grounded and ungrounded. You will also learn the advantages of each junction type, and which is preferable for different applications. When you are done reading, you will have a clear understanding of the best junction to select for your specific needs.
Basic components of a thermocouple.
Industrial thermocouples like the S50 and S80 from Ashcroft have two alloy conductors that are manufactured from different alloys and welded to form a junction at the hot end, also known as the heat source. This is always immersed in the temperature point measurement area.
The cold end also known as the cold junction, is the ambient location where the sensor wires are terminated, typically on a transmitter, PLC, DCS or recorder.
Figure 1: Thermocouple hot end and cold end.
How thermocouples work.
As explained in our article titled, How Does a Thermocouple Work, thermocouples measure the temperature gradient between the cold end junction and hot end junction. Based on the temperature difference between both junctions, the thermocouple circuit generates a millivolt signal. The higher the temperature difference the higher the sensor’s millivolt signal output.
Cold junction compensation complements the millivolt signal to generate the actual hot end temperature measurement. These millivolt signals are small. For example, a type K thermocouple with a temperature gradient of 100 degrees generates only 4.096 millivolts.
The small millivolt signal means care must be taken to select the correct thermocouple junction type to maintain signal integrity and reliability.
Two types of thermocouple junctions.
The most common types of junctions are grounded and ungrounded.
1. Grounded junction solutions.
To form a grounded junction, the conductor alloy weld bead is positioned in direct contact with the sensor’s outer protective metal sheath. This option has several advantages.
Figure 2: Grounded junction.
Precision measurement.
Direct contact with the outer sheath provides precise measurement making grounded sensors the preferred configuration for surface measurement. Applications such as fired heaters that depend on precision measurements of process tube surfaces, specify grounded thermocouple junctions. Furnace teams take advantage of this information and use it to support decoking schedule calculations. As an example, the surface and mid-wall temperatures are used together with API standards to recommend maintenance schedules for process tubes in fired heaters.
Fast time response.
Time-sensitive control loops with final elements such as valves, pumps and heaters depend on fast time responses from all instrumentation including temperature sensors.
Direct contact of the alloy junction with the outer sheath reduces the heat transfer path through metal-to-metal contact. The efficient heat transfer from the process to the sensor results in a faster time response as compared to an ungrounded junction.
Safety applications that require the detection of a small increase in temperature will benefit from a grounded junction.
Figure 3. Grounded junction with heat transfer.
Grounded junction challenges.
Grounded junctions are popular options for many applications, but they present noise and expansion challenges that affect the small millivolt signal.
Noise interference.
Common mode noise from ground loops affects the small thermocouple millivolt. Ground loops occur when the potentials at each earth ground are different, resulting in ground loop currents. Stray currents from outside sources will also introduce noise into the temperature signal. Sources may vary including for example welding around instrumentation installations.
Figure 4. Grounded junction with heat transfer. Figure 5. Noise interference.
Expansion coefficients and grain growth.
Expansion of the outer sheath may place stress on internal conductors in surface measurement applications where the sensor sheath end is welded in place. This stress can lead to grain growth within the conductors. Initially, this results in intermediate signals and eventual failure of the thermocouple’s open circuit.
Figure 6: Conductor separation. Figure 7: Intermittent signal.
2. Ungrounded junction solutions.
Ungrounded Junctions are formed by isolating the Weld Bead junction from the outer sheath per ASTM and IEC Standards. They also offer advantages for certain applications.
Figure 8: Ungrounded junction.
Signal integrity.
Selecting an ungrounded thermocouple with mineral insulation like MgO (Magnesium Oxide) will prevent ground loops through the sensor tip. The gap between the junction and sensor sheath will isolate the sensor from noise interference.
Loop signal isolators and temperature transmitters with isolation features are good options to prevent noise interference. However, a mineral-insulated thermocouple with an ungrounded junction is an optimal method to isolate the sensor circuit.
Insulation resistance quality inspection.
Undergrounded thermocouples also allow inspection of the insulation resistance between the outer sheath and the thermocouple circuit. This is important because a common source of failure of thermocouples is moisture ingress into the mineral-insulated cable. Note: Insulation resistance inspection is not possible with a grounded thermocouple.
Alloy conductor chemical protection.
The complete isolation of the alloy conductors from the outer sheath provides chemical protection to the entire sensor circuit. This is helpful at higher temperatures where high-energy input can drive impurities toward the alloy conductors.
Time response improvements.
To improve the time response of an ungrounded thermocouple the sheath outer diameter can be reduced. In turbine applications, for example, the tip of the sensor may be swaged to reduce the mass at the sensor tip while providing additional strength.
Both junction types are associated with metal sheath sensors and mineral insulation surrounding the conductors. Crushable insulators from Magnesium Oxide (MgO) are the most common, followed by Aluminum Oxide (Al2O3). Depending on the application, both grounded and ungrounded junctions offer advantages.
Figure 11: Grounded vs. ungrounded advantages.
Which thermocouple junction will you choose?
Knowing the differences between grounded and ungrounded junctions and how they work with thermocouples provides a good foundation for answering that question. However, you may want to know more. Our Tech Team is on hand to address any additional inquiries that come up. Feel free to reach out to our specialists anytime.
In the meantime, download our guide for more information on how to find the right temperature and pressure instrumentation for harsh applications.
If you want to learn more about thermocouples and other temperature sensors, check out some of our other blog posts: