Thermocouple[/caption]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:-
- The material of thermocouple must have high thermo E.M.F. per unit temperature change.
- Low electrical resistance at the couplings/junctions,
- Temperature and thermo emf should be linearly proportional in the given range
- The high melting point of the materials of the coupling materials for a wider range.
- The material should be pure, homogenous and workable in any shape.
- 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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