Thermocouples are most widely used sensors for measuring temperature. They are based on seebeck effect that occurs in electrical conductors. Thermocouples are basically consists of two dissimilar metal wires joined together at one end, which produce a small unique voltage.

A thermocouple half circuit is formed by joining two dissimilar metals together, it is done usually by welding the tip of two wires together is called thermocouple junction. The voltage potential developed that can be measured in mV across the ends of opposite of the junction.

Actually, there are 4 types of junctions in the Thermocouples

1. Grounded junction thermocouple

2. Ungrounded Junction Thermocouple

3. Exposed junction Thermocouple

4. Isolated Thermocouple junction

1. Grounded junction thermocouple

It is the most common type of junction in which the thermocouple junction and sheath both are welded together. The three metals, two thermoelements, and sheath alloy form a single block of metal at this point. So, the hot junction will have a greater measuring area.

Grounded junctions are used where electromagnetic induction or radiofrequency is present and can interfere with thermocouple signal or measurement.

Advantages:

1. Very good heat transfer from the target and quick response.

2. It is a preferred junction type for high-temperature applications.

Disadvantages:

1. Susceptible to induced noise from ground loops.

2. Due to the effect of ground loops, less accurate.

2. Ungrounded Junction Thermocouple

 In this case, thermocouple junction is welded to form a junction but not welded to the sheath. The sensing junction of an ungrounded thermocouple is electrically isolated from the probe wall and does not touch it.   

Advantages:

1. They eliminate ground loop problems.

2. Used primarily for isolating the control system from the sensor and to prevent a ground loop.

3. This construction is useful where high levels of electrical noise are present.

4. Conductive sheath protects the thermocouple junction.  

5. Provides more accurate reading.

Disadvantages:

1. Response time is long.

3. Exposed junction Thermocouple

Exposed junction thermocouples are also called “bare wire thermocouple”. In this type of thermocouple, the junction is outside the probe wall and exposed to the target medium. Sheath and insulation are removed leaving the bare thermocouple wires exposed for a specific length.

The thermocouple itself is actually unprotected and exposed (hence, the name) to the temperature being measured. These are used for low or normal room temperature, low oxidizing atmosphere.

Advantages:

1. They have the best heat transfer and quickest response.

2. Wherever quick response time is essential, and exposed junction thermocouple is the best choice.

Disadvantages:

1. It is not intended for a pressurized and corrosive atmosphere.

2. No chemical protection.

3. Most are prone to physical damage.

4. Isolated Thermocouple junction

Isolated Thermocouple consists of dual thermocouples that are insulated from the sheath and each of the elements is insulated from one another.

Advantages:

1. Help provide in the event of one sensor fails.

Disadvantages:

1. Diameter may be more than a single sensor thermocouple.

Author: Kireeti Samuel