There are three types of junction which listed below,
1. Grounded Junction
2. Ungrounded Junction
3. Exposed Junction
Thermocouples are available in different combinations of metals. Thermocouple action between two different materials (Seebeck effect) is a material phenomenon that depends only on materials internal molecular structure and does not depend on the type of
junction between materials. The junction only needs to be a good electrical contact.
At the tip of the grounded junction probe, the thermocouple wires are physically attached to the inside of the probe wall. This results in good heat transfer from the outside, through probe wall to the thermocouple junction.
In an ungrounded probe, the thermocouple junction is detached from the probe wall. Response time is slowed down from the grounded type but the ungrounded type offers electrical isolation.
The thermocouple in the exposed junction type the junction of the thermocouple is exposed to the surrounding environment. This type offers the best response time, but is limited in use to non corrosive and non pressurized applications.
The grounded junction is recommended for the measurement of static or flowing corrosive gas and liquid temperature and for high pressure application.
An ungrounded junction is recommended for measurement in corrosive environment where it is desirable to have electrically isolated from and shielded by the sheath. The thermocouple is physically insulated from the thermocouple sheath by MgO powder.
An exposed junction is recommended for the measurement of static or flowing non-corrosive gas temperature where fast response time is required.