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Understand the Thermocouple Circuit Working Principle and Its Applications

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What is a Thermocouple?

• A temperature sensor made by joining two dissimilar metals.

• When the junctions are at different temperatures, a voltage (emf) is produced.

• Voltage corresponds to the temperature difference between junctions.

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Working Principle

• Based on the Seebeck effect: emf is generated when heat flows from hot to cold junction.

• Thermocouple has:

• Hot Junction – Exposed to the process temperature.

• Cold Junction – Reference point (often 0°C or electronically compensated).

Formula:

emf = b × (T1 - T2)

Where:

• b = Seebeck coefficient (V/K)

• T1 = Temperature at hot junction

• T2 = Temperature at cold junction

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Important Effects

• Seebeck Effect: Voltage from heat difference.

• Peltier Effect: Heat absorbed/released at junction when current flows.

• Thomson Effect: Additional emf in single wire with a temperature gradient; must use homogeneous wires to avoid error.

Cold Junction Compensation

• Traditional: Cold junction in ice bath (0°C).

• Modern: Electronic compensation simulates reference temperature.

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Output Characteristics

• Output is in millivolts.

• Non-linear relationship between voltage and temperature.

• Low sensitivity: ~50 µV/°C.

• Needs amplification and filtering.

Thermowell

• A protective metal tube for thermocouples.

• Functions:

• Shields sensor from damage/corrosion.

• Allows sensor removal/replacement without process interruption.

• Commonly used with high-pressure or hazardous fluids.

Advantages

• Low cost

• Simple and rugged

• Fast response time

• Wide temperature range (-200°C to 1700°C)

• Works in harsh environments

Disadvantages

• Accuracy is lower than RTDs

• Output is not linear

• Sensitive to electrical noise

• Requires cold junction compensation

• May degrade over time, especially in corrosive environments