Types of Thermocouple Measuring Junction

Types of Thermocouple Measuring Junction

There are four types of thermocouple measuring junction constructions :

1. Grounded Thermocouple Junction
2. Ungrounded Thermocouple Junction
3. Exposed Thermocouple Junction
4. Isolated Thermocouple Junction

Grounded Thermocouple Junction

The grounded thermocouple junction is an integral part of the thermocouple sheath tip.

Advantages:

• fast response time in relation to ungrounded and isolated junctions.
• protects the wires from environmental chemicals and corrosives.
• prolongs the operational life of the thermocouple. Longer lifespan than the exposed junction thermocouple.
• it is recommended for high pressure applications.
• it is the least expensive construction.

Disadvantages:

• thermal expansion of sheath material may differ from element to cause mechanical stress and work hardening of metals.
• ground loops may cause interference with instruments.
• faults in insulation are more difficult to detect.

Ungrounded Thermocouple Junction

The ungrounded thermocouple junction is electrically insulated and electrically isolated from the outer sheath material. In a dual ungrounded thermocouple, one common junction is electrically insulated from the outside sheath.

Advantages:

• the thermocouple junction is isolated from the ground.
• defects in the MgO insulation can be detected by measuring resistance from loop to sheath.
• long term drift under cycling conditions is minimized.

Disadvantages:

• response time is usually slower than grounded thermocouples.
• more expensive than grounded thermocouples.

Exposed Thermocouple Junction

The exposed thermocouple junction extends beyond the protective metallic sheath.

Advantages:

• recommended for measurement of noncorrosive static gas, or air.
• very fast response time, faster than grounded junction.

Disadvantages:

• cannot be used in an environment with a high percentage of solids, high pressure, or flowing material since the junction is exposed to this environment.

Isolated Thermocouple Junction

Isolated thermocouple junctions are used in a dual or triple thermocouple when the junctions are isolated from the outer sheath material as well as from each other.

Advantages:

• the elements are insulated from ground.
• performs better than ungrounded or grounded junctions in a thermal cycling environment.

Disadvantages:

• slower response time than a grounded dual thermocouple.

Reference : gilsoneng.com