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What Is the Difference Between Grounded and Ungrounded Thermocouples?

Thermocouples are devices used to measure temperature. The thermocouple works by using two dissimilar metals that produce an electrical current. The changes in temperature affect the voltage output of the thermocouples. These sensor junctions come in a variety of configurations, including grounded and ungrounded. The differences between grounded and ungrounded thermocouples are significant in design, functionality and in their use among different environments.

Design
- The measuring junction is the junction that is placed into the material to measure the temperature. The grounded junction has wires inside a protective metal sheath. The wires are physically attached to the inside of the probe wall. This provides good heat transfer from the material to the probe and thermocouple. In the ungrounded junction, the wires and the probe wall are physically separated and do not touch; the ungrounded junction thermocouple response times are slowed compared to a grounded junction.
Advantages
- The grounded thermocouple is the most common type of thermocouple used in industry according to the NPH Heaters website. This is because of the fast response times due to how the junction is wired. The grounded thermocouples also have long service lives in corrosive conditions. The ungrounded thermocouple keeps the wires and the sheath separated; it is more reliable and rugged of the two thermocouples. The ungrounded thermocouple offers long life under vibration shock and corrosive environments.
Where They Are Used
- The grounded thermocouple junction is recommended for measuring static or flowing corrosive gas and liquid temperatures. This junction type is also used in high-pressure conditions. The ungrounded thermocouple is used when the instrumentation is not electrically isolated internally. These junctions are used in corrosive environments where the rate of response time is not as critical, as compared to a grounded thermocouple.
Other Junction Types
- A third major type of thermocouple is the exposed junction. The wires in this junction extend outside and are not protected by a sheath. These offer the fastest rate of response because the wires are directly exposed to the materials being measured, but they are not able to withstand the corrosive forces like either the grounded or the ungrounded junctions. The flat-end junction, which can be either grounded or ungrounded, is a junction with a flat tip. The reason for the flattened tip is to reduce the mass of the junction and increase the rate of reaction time by increasing the surface area of the junction.

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