Seebeck voltage

Electrical conductivity (a.c. and d.c.), Hall measurements, and determination of Seebeck voltage. 

Thermocouples are rugged and versatile temperature sensors frequently found in industrial control systems. A thermocouple consists of a pair of dissimilar metal wires twisted or otherwise bonded at one end. The Seebeck effect is the physical phenomena which accounts for thermocouple operation, so thermocouples are known alternatively as Seebeck junctions. The potential difference (Seebeck voltage) between the fi ee ends of the wire is proportional to the difference between the temperature at the junction and the temperature at the fi ee ends. Thermocouples are available for measurement of temperature as low as —270°C and as high as 2300°C, although no single thermocouple covers this entire range. Thermocouples are identified as type B, C, D, E, G, J, K, N, R, S, or T, according to the metals used in the wire. 

The Seebeck voltage can only be measured in terms of the difference in Seebeck coefficients between two conductors as illustrated in Figure 17.7. Junctions are formed by spot welding wires with compositions A and B at each end. Wire B may be broken and connected to a voltmeter with leads of a different composition as long as these two connections are held at the same temperature. The voltage read by the voltmeter is given by... 

Integrating Equation 17.46, we see the Seebeck voltage will be a cubic (or higher order polynomial) of the difference in temperature between the two junctions. There will also be a contact potential at the junctions between the wire B and the meter leads, but if these two junctions are at the same temperature, there will be no potential difference and the effect from these jimctions will cancel. 

Typical thermocouple arrangement with junctions held at Tj and T2- The Seebeck voltage is read by a microvolt meter in series with one of the wires. 

Thermocouples are also used in the safety shut-off valves in gas appliances. The current developed by a thermocouple inserted in the pilot flame flows though a solenoid that holds the gas supply open. If the pilot flame were to go out, the valve would automatically close, preventing the release of gas. It may seem surprising that such a little voltage could produce enough current to activate a solenoid valve, but there is practically no internal resistance in the thermocouple junction. This means that the current that flows is the Seebeck voltage divided by the resistance in the wires of the solenoid, which can be a small fraction of an ohm. 

The Seebeck voltage generated at a moving solidification interface in a directionally solidified alloy has been used to measure the kinetic undercooling and to observe the incipient breakdown of a plane interface to dendritic growth (see Section 13.2.6). 

The Seebeck voltage in pV as a function of absolute temperature is represented bj ... 

---