Thermocouples are sensors composed of two metals that generate emfs (electromotive forces) or voltages when there are temperature differences between them. The amount of voltage produced is dependant on these differences. Thermocouples operate by exploiting the Seebeck effect.
The Seebeck Effect
The Seebeck effect was discovered by German physician turned physicist Thomas Johann Seebeck (1770-1831).
Seebeck found that when he produced a series circuit by forming a junction of two different metals, with one metal at a higher temperature than the other, that he was able to generate a voltage. The larger the difference, the higher the voltage, and he found that the results were independent of the shape of the metal.
The emf E for homogeneous wires is measured by
1) E = ∫ (SA – SB) dT
where Tref is the reference temperature, Tj is the temperature at the junction end, and SA and SB are the Seebeck coefficients for the different metals. If the temperature is measured in K (Kelvin), then S is in units of μV/K or microvolts per Kelvin. Seebeck coefficients depend on the temperature and material; a reference table may be found at Electronics Cooling Magazine. For small temperature ranges, equation 1 becomes
2) E = (SA – SB) (Tj – Tref) = SABΔT
A thermocouple is composed of a junction formed by two metal alloys. One portion of the junction is placed on a source whose temperature is to be measured, while the other end is maintained at a constant reference temperature in accordance with the zeroth law of thermodynamics. Older thermocouples use ice water baths as their temperature source, but modern day ones use a solid state temperature sensor, as shown in figure 2
Thermocouples are valuable in science and engineering due to their accuracy, fast reaction time, small size, and ability to measure extreme temperatures. The latter ability is based on the metal combinations used; a nickel-nickel combination can measure -50oC to 1410oC, while a rhenium-rhenium combination can measure 0oC to 2315oC. The most common combinations are iron-constantan, copper-constantan, and chromel-alumel. The disadvantages of thermocouples are that the signals produced may be non-linear, and thus they need to be calibrated carefully.
In part two, we will explore thermocouple applications and explain how you can perform your own experiments at home.
- Physics For Scientists and Engineers by Raymond Serway
- Figure 1 from Radio-Electronics.com
- Figure 2 traditional and modern thermocouples from www.CapGo.com
- Notes on Using Thermocouples by Dr. Robert J. Moffat – http://www.electronics-cooling.com/1997/01/notes-on-using-thermocouples/