Basics of Thermocouple Junction Design

Industrial thermocouples (AST)

Thermocouples are simple devices made up of several key components: thermocouple wire, electrical insulation, and the sensing junction. Many thermocouple designs also include a stainless steel sheath that protects the thermocouple from vibration, shock, and corrosion.

A thermocouple has three variations of sensing tip (or junction):

• Exposed junction, where the exposed wire tips and welded bead have no covering or protection.
• Grounded junction, where the welded bead is in physical contact with the thermocouple's sheath.
• Ungrounded junction, where the tip is inside the thermocouple sheath, but is electrical (and somewhat thermally) insulated from the sheath (no sheath contact).

Exposed junction thermocouples respond to temperature change quickly and are less costly, but their signals are susceptible erratic reading caused by induced or conducted electrical noise. Because there is no sheath, they are also prone to mechanical damage and ambient contamination.

Grounded junction thermocouples provide fast response and are mechanically more robust, with a metallic sheath that protects the thermocouple both mechanically and from contaminants. But because their sensing tip is in contact with the external sheath, their signal still can be affected by externally induced or conducted electrical noise.

Ungrounded thermocouples, like grounded, are protected mechanically and from ambient contaminants by their sheath. However, their sensing junctions are kept separate from their metallic sheath, isolating the junction from external electrical  interference. This separation does come at a small cost in temperature sensing responsiveness though.

For safety, precision, and optimum performance, always talk to an applications specialist when applying temperature sensors. A short phone call can prevent major headaches and lost time in  troubleshooting a misapplied thermocouple.

A Specialty Temperature Sensor Specifically for Improving Heat Tracing Applications

Heat Tracing RTD (courtesy of
Applied Sensor Technologies)

A temperature sensor is key to any heat tracing application as it provides temperature feedback about the pipe temperature, which in turn, is used turn on or off the heating system (electric pipe tracing or steam control valve).

The temperature sensor is critical for both categories of heat tracing - process temperature maintenance and freeze protection.  Failure to maintain process temperature in a pipe or vessel could significantly effect product quality, or cause failure of ancillary equipment such as pumps, valves, and compressors. Properly protecting against freezing keeps pipes from bursting or product from blocking the flow. For both situations, product maintenance and freeze protection, accurate and reliable temperature sensing is critical.

There’s a new and very innovative line of RTD temperature assemblies specifically designed for heat tracing applications. The unique, replaceable element concept can save customers both time and money, plus increase overall system reliability and up-time.

A major refining company determined that they save over $1,000 in labor each time they have to replace a sensor and have reduced their repair time from two days to less than one hour.

The design consists of a terminal head and right-angle shaped outer sheath, with a curved weld-pad at the end. The replaceable RTD element assembly is contained in the outer tube and, when installed, presses against the pipe. Heat transfer is excellent and heat conduction away from the element is minimal. Should the element ever need to be replaced, it's a five-minute job to open the terminal head, unwire the sensor, slide it out and slide a new one in.

For more information, contact:

Ives Equipment
(877) 768-1600

www.ivesequipment.com