Temperature Measurement

Laws of Thermocouple


         A thermocouple is a junction made of two distinct metals that generates a voltage in response to a change in temperature. Thermocouples are a form of temperature sensor that is frequently used for measurement and control, as well as for converting heat into electric power.

Thermoelectric effect

  • The thermoelectric effect is the conversion of temperature variations into electrical voltage and vice versa.
  • When there is a temperature difference on either side, a thermoelectric device produces voltage. When a voltage is applied, a temperature difference results.

Three effects of Thermocouple

1. The Seebeck Effect.

2. The Peltier Effect.

3. The Thomson Effect

Seebeck Effect

Seebeck effect
  • When the connections of two dissimilar metals are kept at different temperatures, Seebeck discovered that an emf (potential difference) develops in the closed circuit.
  • The thermoelectric current is the flow of electricity caused by theemf created. The thermocouple consists of two dissimilar metals joined to make two connections. The direction of current also reverses if the hot and cold junctions are switched. As a result, the effect can be undone.
  • EMF generated in thermocouple is depends on the nature of the metals making up the pair and the temperature difference at the connections.

Application of Seebeck effect

  1. Thermoelectric generators employ the Seebeck effect (Seebeck generators). These thermoelectric generators turn waste heat into energy in power plants
  2. In order to improve fuel efficiency, this phenomenon is used in automobiles as automotive thermoelectric generators.
Example of seebeck effect

Peltier Thermal Effect

         Heat is produced at one junction of a thermocouple circuit and is absorbed at the other junction when an electric current flows through it. The Peltier effect is this.

Peltier Thermal effect

Thomson Effect

         It speaks of the warming or cooling of a conductor that carries current via a temperature gradient. Depending on the material, any current-carrying conductor with a temperature differential between two places will either absorb or expel heat.

         In a conductor, Thomson demonstrated that if two places are at different temperatures, the density of electrons at those points will vary, creating a potential difference between those points. The Thomson effect can be reversed.

Thomson effect

Laws of Thermocouple

  • Law of Homogeneous Circuit
  • Law of Intermediate Temperatures
  • Law of Intermediate Metal

Law 1: Law of Homogeneous circuit

         The thermal emf produced by two thermocouples at T1 and T2 is independent of and unaffected by any temperature variation down the wires. Regardless of how its cross section varies, a thermoelectric current cannot be maintained in a circuit of a single homogenous material by the application of heat alone.

Law 1 thermocouple

Law 2: Law of Intermediate Metals

         If the two junctions created by the third metal are at the same temperature, inserting an intermediate metal into a thermocouple circuit will not impact the net emf. This law enables the use of extension wires and the installation of measuring devices into the circuit.

2nd Law of thermocouple

         According to the law of intermediate metals, if the emf of materials AB and BC are known, the e.m.f. of materials AC can be predicted.

Law 3: Law of Intermediate Temperatures

         A thermocouple will develop an emf equal to the algebraic sum of the emfs E1 and E2, when the temperatures at its junctions are T1 and T3, if it develops an emf E1 when the temperatures at its junction are T1 and T2 and an emf E2 when the temperatures at its junction are T2 and T3.

3rd Law thermocouple

Law 4

         A single homogenous metal cannot generate an electric current simply by applying heat to it.

Law 5

         Law 5 states that an electric field produced by a circuit having two or more thermocouples operating at the same temperature has an algebraic sum of zero.

5th Law of thermocouple

Construction of Thermocouple

Construction of thermocouple

The heater element, thermocouple, and an indicator device like a PMMC are the three main parts of thermocouple instruments (Permanent Magnet Moving Coil Instrument)

Working of Thermocouple

  • The heating element is opened to allow the current whose magnitude has to be measured to pass through.
  • As a result, the heater element generates heat, which induces a thermo-electric emf in the thermocouple’s output terminals.
  • The thermocouple is crossed by the PMMC instrument, which measures the emf whose magnitude is proportional to the current’s rms value and induces deflection in the instrument for current measurement.

Advantages of Thermocouple

  • Voltage and current are immediately expressed as their rms values.
  • Unaffected by the stray magnetic field.
  • used to measure many different currents.
  • Excellent sensitivity and precision.
  • Used in combination with the standard cell to calibrate the potentiometer.
  • It is suitable for a variety of frequencies.

Disadvantages of Thermocouple

  • Due of the instrument’s square-law response, it has a low overload capacity.
  • Large currents generate a lot of heat, pushing the temperature to the upper limit of its range.
  • Ineffective thermal conversion.
  • high losses of power.
  • Instruments used in the PMMC must be handled carefully.

Application of Thermocouple

  • Used as the temperature sensors in thermostats in offices and homes
  • Used in industries for monitoring temperatures of metal.
  • Used in gas machines for detecting the pilot flame
  • Used in the food industry for cryogenic and low- temperature applications.

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