Ungrounded vs Grounded vs Exposed Thermocouple Probes

 

This article explains the difference between ungrounded thermocouples, grounded thermocouples and exposed thermocouples.

Ungrounded thermocouples

An ungrounded thermocouple is not welded directly to the sheath. It means it’s isolated from the protective sheath. Such a setup translates to a more accurate reading when taking up corrosive environments — albeit for a longer time

Advantage: Less susceptible to electrical noise. More reading accuracy. Best for corrosive environments.

Disadvantage:Takes time to read. More expensive than grounded.

Grounded thermocouples

A thermocouple is grounded if the junction has been welded directly into the protective sheath located at the tip of the sensor. Such a design allows the measurement of the temperature to be done quickly as the metal-to-metal connection ushers in a shorter thermal route.

Advantage: Speed in reading temperature. Least expensive of all.

Disadvantage: Susceptible to electric noise. Less accurate.

Exposed thermocouples

Exposed ones have the thermocouple junction out of the sheath protruding and visible.

Advantage: Fastest reading time.

Disadvantage: Not suitable for corrosive environments. Very sensitive to noise.

Typical Stabilization Times for Type K Thermocouples

• Exposed Junction: These can stabilize very quickly, often within a few seconds, because the thermocouple wire is directly exposed to the temperature source.
• Grounded Junction: These usually take a bit longer, from several seconds to a minute, as the junction is enclosed within a protective sheath which must also reach thermal equilibrium.
• Ungrounded Junction: These typically have the slowest response time, sometimes taking up to several minutes, as the junction is isolated from the sheath, reducing thermal conductivity.

For precise applications, it’s often recommended to allow the thermocouple a few minutes to ensure it has fully stabilized, especially if transitioning between significantly different temperatures.