Thermocouple: Principle, Construction, Characteristics, Advantages, and Limitations.

A thermocouple is a device that converts thermal energy into electric energy. The amount of electric energy generated can be used to measure temperature.

A thermocouple is a thermoelectric transducer, used as a primary transducer. Changes in temperature are directly converted to electrical signals.

Seebeck Effect: If two wires of different materials are joined together forming a closed circuit, and if two junctions are at different temperatures, an electric current flows around a closed circuit.

If two metals used are copper and iron, the current flows from copper to iron forming a hot junction and Iron to copper to form a cold junction.

Peltier Effect: If two dissimilar metals form two junctions and if an external emf is connected then current flows through the junctions. When the current flows through the copper-Iron junction, in the direction from copper to iron, heat is absorbed making junction T1 hot. When current flows from Iron- Copper junction T2 from Iron to copper heat is liberated to make junction cold.

Why Cold junction compensation ?: The thermocouple temperature is the function of the difference between the hot junction and cold junction. If the cold junction temperature is kept fixed, the thermocouple output is a hot junction temperature. In laboratory work, the cold junction can be kept continuously dipped in the ice bath.

In an industrial environment, the cold junction temperature changes with ambient temperature and result in an error. Cold junction compensation is the process for compensating for this.

 Voltage versus Temperature Relationship:

As shown in the above figure, R, S thermocouples are platinum base thermoelements can measure higher temperature range, the voltage output is less than E, J, K thermocouples. They can operate up to 1700 degrees C. They are expensive.

On the other hand, E, J, K thermocouples are Nickel (Ni) base elements operate at low-temperature measurement.

Pt base Thermoelement

In R type thermocouple: Primary element Pt-13% Rh, Secondary element Pt

In S type thermocouple: Primary element Pt-10% Rh, Secondary element Pt

Ni base Thermoelement

In E type thermocouple: Primary element Ni-10% Rh Secondary element Ni 45%-Cu55%

In J type thermocouple: Primary element Ni 5%, Secondary element Ni 45%-Cu55%

In K type thermocouple: Primary element Ni-10%, Ni 5% in the Secondary element.

Construction of Thermocouple:

Thermocouple junctions are formed in two ways. The two wires of the thermocouple are usually twisted and welded together. Namely twisted weld and butt weld.

In twisted weld, the two wires are twisted together in several tuns and welded together. This type of welding is used for larger sized wires, it gives mechanical strength.

In butt weld, two wires are comparatively smaller in size are fused into the round bend.

Thermocouple junction types:

Thermocouple junctions are basically three types.

  • Grounded Junction Thermocouple
  • Ungrounded Junction Thermocouple
  • Exposed junction Thermocouple

Grounded Junction Thermocouple: It is the most common type of junction in which the thermocouple junction is welded with a sheath. 

Ungrounded Junction Thermocouple: 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 or sheath and does not touch it.  

Exposed Junction Thermocouple: Exposed junction thermocouples are also popularly known as “bare wire thermocouple”. In this type of thermocouple, the junction is outside the protective sheath and the junction is exposed to the process medium.

Thermocouple Characteristics:

E Type Thermocouple: Recommended to use in Oxydizing or inert atmosphere. Limited use in vacuum or reducing atmosphere. Highest EMF change per degree. High sensitivity with 68 microvolts / Degree C.

J Type Thermocouple: Recommended to use in vacuum or inert atmosphere. Limited use in Oxydizing at high temperatures. The sensitivity of 50 Microvolts / Degree C.

K Type Thermocouple: Recommended for use in an oxidizing or completely inert atmosphere. Not suitable in reducing atmospheres. Can not be used in a sulfurous environment. Both elements will rapidly corrode and more chances of early failure.

These are the most common general-purpose thermocouples.

Its sensitivity is 41 Microvolts / Degree C.

R and S Type Thermocouple: Recommended for using continuous use in Oxidizing or inert atmospheres. They should not be inserted directly into metallic primary protecting tubes.

Materials used for Thermocouple:

Thermocouples are made from a number of different types of materials.

Following is the table illustrating the range of temperature measurements in the thermocouples of different materials.

Thermocouple color as per ANSI Standard and IEC Standard:

Advantages of thermocouples:  

  • The thermocouples are rugged in construction.
  • It covers a wide temperature range.
  • Thermocouple offers good reproducibility.
  • Comparatively cheaper.
  • The speed of response is high.
  • The calibration can be easily checked.

Limitations of the thermocouple:

  • The stray voltage pickup is possible.
  • They are less advisable for applications where smaller temperature differences to be measured with high accuracy. For example range 0-100 degree C with 0.1 percentage accuracy. 
  • A Thermocouple produces very little amount of voltage signal (mV), amplification is required.

Author: PSS Bapu Rao

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