Thermocouple – Types, Working principle & Properties

Thermocouple
Thermocouple

Working Principle

Seeback Effect

When heat is applied to a junction of two dissimilar metals, an e.m.f. (thermo e.m.f.) is generated which causes current to flow from hot junction to cold junction. The thermo E.M.F. generated is proportional to the temperature difference between two junctions.

The thermo e.m.f. generated is,

Where,

  • E = Thermo E.M.F. between junction
  • T = Temperature  at measuring junction
  • L = Temperature  at reference  junction
  • C1, C2 = Constant depending on metals (usually C1 in microvolts and C2 in milivolts)

Note: Larger the value of C2 larger is the nonlinearity in the thermocouple characteristics.

There is an opposite principle of seeing back also exists, commonly known as “Peltier Effect”

Peltier Effect

When a current flow across the junction of two metals heat is absorbed or evolved at the junction when the current flows from one metal to another

  • dQ = Heat absorbed or evolved
  • a = Peltier constant
  • I = Current passing through the junction
  • dt = Time of passing the current

Types of Thermocouples

Sr. No. Type Material Range (0C) Tolerance Sensitivity (µV/0C) Comment Applications
1 B Strip 1: Platinum (70%) + Rhodium (30%)

Strip 2: Platinum (94%) + Rhodium (06%)

600 to

1500

 ± 0.0025 0C 5 to 12 More stable than R, ,S

types at higher temperatures

1)     Temperature measurement of flammable materials/ fluids.

2)     Since, they are chemically less responsive, used to measure temperatures of Acids and strong Bases.

2 R Strip 1: Platinum (87%) + Rhodium (13%)

Strip 2: Platinum (100%)

0 to 1400 ± 1 0C 5 to 12 Most stable in all hazardous zones 1)     Temperature measurement of flammable materials/ fluids.

Since, they are chemically less responsive, used to measure temperatures of Acids and strong Bases.

3 S Strip 1: Platinum (90%) + Rhodium (10%)

Strip 2: Platinum (100%)

 

0 to 1400 ± 1 0C 5 to 12 Stable in all hazardous zones 1)     Temperature measurement of flammable materials/ fluids.

Since, they are chemically less responsive, used to measure temperatures of Acids and strong Bases.

4 J Strip 1: Ferrous / Iron (100%) Strip2: Constantan -200 to

1200

± 1.5 0C 45 to 51 Best for temperature under 600 0C Specially used to measure fluid temperatures which require good accuracy and sensitivity e.g. water temperature measurement
5 K Strip 1: Chromel

Strip 2: Alumel

-200 to

1200

± 1.5 0C 40 to 55 Better in oxidizing

atmosphere.

Mostly used in open atmospheric situations
6 T Strip 1: Copper

Strip 2 : Constantan

-200 to 350 ± 0.5 0C 15 to 60 Oxidation occurs beyond

stipulated range

Specially used when measurement of fluid temperature is made remotely
7 E Strip 1: Chromel

Strip 2 : Constantan

-40 to 800 ± 1.50C  

15 to 60

Works in oxidizing

atmosphere

Mostly used in open atmospheric situations

How to select thermocouple?

Before selecting thermocouple’s material for our desired application, we must consider following guidelines:-

  1. The material of thermocouple must have high thermo E.M.F. per unit temperature change.
  2. Low electrical resistance at the couplings/junctions,
  3. Temperature and thermo emf should be linearly proportional in the given range
  4. The high melting point of the materials of the coupling materials for a wider range.
  5. The material should be pure, homogenous and workable in any shape.
  6. The material of thermocouple must be resistant to corrosion and must be usable over a long time without getting brittle.

Thermal properties of materials

Specific Heat

it is the heat required to increase the temperature of 1 Kg mass by 10

Q=MCpΔT

Where,

  • Q = Heat requires (J)
  • M = Mass (Kg)
  • ∆T = Temperature difference (0K)
  • Cp= Specific heat  (J/Kg0K)

Note: Calorimeters is used to calculate specific heat of material

Thermal expansion coefficient

When a solid is heated, it increases in volume. The increase in the length of solid depends upon original length, temperature and thermal expansion of coefficient.

Where,

  • L0 , Lt= Length at 00C and  t 0C respectively (Meter)
  • α = Thermal expansion coefficient of solid (µm/m)

Thermal Conductivity

It is the rate of heat flow per unit time in a homogeneous material under steady conditions per unit area per unit temperature gradient.

Where,

  • Q = Heat flow per unit time (Watts)
  • K = Thermal conductivity
  • A = Area of material
  • X = Thickness
  • T = Temperature difference (K)

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