Thomas Johann Seebeck accidentally discovered the Thermocouple in 1821. He experimentally determined that a voltage exists between the two ends of a conductor when the conductor’s ends are at different temperatures.
Modern theories of electron behavior within molecular structure provide an analytical explanation of the Seebeck phenomena. Mathematics is very complex and involves quantum theory; however, the fundamental concept is straightforward:
When one end of a conductive material is heated the electrons at the hot end are more thermally energized than the electrons at the cooler end. These more energetic begin to diffuse toward the cooler end. Charge neutrality is maintained; however, this redistribution of electrons creates a negative charge at the cool end and an equal positive charge (absence of electrons) at the hot end. Consequently, heating one end of a conductor creates an electrostatic voltage due to the redistribution of thermally energized electrons throughout the entire material. This is the basic principle of thermocouples or Seebeck Effect.
Figure 1
Definitions for the situation shown in Figure 1 are:
- Tx is the unknown temperature.
- Tc is the temperature at the voltmeter connector (assumed the same for both wires).
- Tv is the internal temperature of all the voltmeter circuit elements.
- Sa is the Seebeck coefficient of thermocouple wire material a.
- Sb is the Seebeck coefficient of thermocouple wire material b.
- Sc is the Seebeck coefficient of the wiring used in the voltage measuring circuit.
- The Voltage Circuit measures thermocouple open circuit voltage (VTC) with a high impedance circuit to minimize loop current.
(Sa-Sb)*(Tx-Tc) = [VTC] Equation 1
[VTC] = Sab * (Tx-Tc) Equation 2
(Values of Sa and Sb determine [VTC] polarity)
Definitions:
(Sa-Sb) = Sab and (Sb-Sa) = -Sab
These terms are defined by industry standards as the Sab Seebeck coefficient signifying a type ab thermocouple made of material a and b.
Observations:
- Seebeck Effect is a phenomenon that depends solely on the material’s internal molecular structure and does not depend on the type of junction between materials. The junction only needs to be a good electrical contact.
- Thermocouples measure temperature difference between the junction end and the open end; they do not measure the individual temperature at a junction.
- Both thermocouple wires at the connection to the voltage measuring circuit establish unwanted thermocouple junctions between the connector at temperature (Tc) and the voltmeter circuit wires at temperature (Tv). If these two parasitic junction connections are both at temperature Tc, and if all elements in the voltmeter circuit are at temperature (Tv), then the effect of these parasitic junctions cancel. This condition becomes a requirement for using thermocouples.
- Thermocouple voltages must be measured with high impedance circuits to keep loop current near zero. Current flow in thermocouples can create errors by disturbing the thermal distributions of electrons.