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.

Industrial Temperature Sensors: Basics of Thermocouples

Industrial thermocouples
(courtesy of Applied Sensor Technology)

Thermocouples are the most widely used industrial temperature sensor found in industrial processes today. They are rugged, relatively inexpensive to manufacture, and provide fairly good accuracy.

Thermocouples operate on the "Seebeck Effect", which is the phenomena whereby two dissimilar metal conductors (wires), joined at two points, with one point kept at a known constant temperature, produce a measurable voltage difference between the two conductors.

Thermocouple types - such a type J, type K, type R, and type S - refer to the alloy combinations used for the conductors and are based on standardized color designations. 

Thermocouples are used widely in industrial processes in industries such as power generation, primary metals, pulp and paper, petro-chemical, and OEM equipment. They can be fabricated in protective wells, and can be housed in general purpose, water-tight, or explosion-proof housings.

The following video provides a basic visual understanding of thermocouple wire, how a T/C junction is determined, and also discusses thermocouple connectors, polarity and some aspects of construction (such as grounded vs. ungrounded vs. open tip).