Thermocouples are widely used in the science and industry. Thermocouples are one of the easiest temperature sensors. This equipment was developed on the basis of Seebeck effect which occurs in electrical conductors that experience a temperature gradient along their length. Thermocouples are simple to use, need no batteries and can measure over very wide temperature ranges.



In 1821, the German?Estonian physicist Thomas Johann Seebeck discovered that when any conductor is a small cuticle (like a metal) is subjected to a thermal gradient, it will generate a voltage. This is now known as the thermoelectric effect or Seebeck effect. Any attempt to measure this voltage necessarily involves connecting another conductor to the "hot" end. This additional conductor will then also experience the temperature gradient, and develop a voltage of its own which will oppose the original. Luckily, the magnitude of the effect depends on the metal in use. Using a dissimilar metal to complete the circuit creates a circuit in which the two legs generate different voltages, leaving a small difference in voltage available for measurement. That difference increases with temperature, and can typically be between 1 and 70 microvolts per degree Celsius for the modern range of available metal combinations. Certain combinations have become popular as industry standards, driven by cost, availability, convenience, melting point, chemical properties, stability, and output. This coupling of two metals gives birth to the name thermocouple.

Thermocouples measure their own temperature. The user must conclude the temperature of the object of interest by being certain there is no heat flow between them when the measurement is taken.

Thermocouples can make mistakes in reading their own temperature, especially when the system is being used for a while, or if the insulation between the wires loses its resistance due to moisture or thermal conditions, or any chemical, nuclear radiation or mechanical effects with the immediate surroundings.

Be careful of the electrical hazards when using thermocouples. Not only they are electrical conductors but also are refractory oxide sheathed models at high temperatures; if they get into contact to another source of electricity, it could electrocute the person to death.

If you want to use thermocouples, you need to have some way to interpret their small and non-linear output voltages. There are lots of ways to do it, the simplest being a measurement of the output voltage and looking up the value in a table of millivolts dc versus temperature and correcting for the cooler junction not being at 0 ?C, or 32'F.

The high end is to hook the thermocouples up to a modern readout display or a DAQ module plugged into a PC and read. However, there are many subtle things about thermocouples and their uses that have made well-intentioned engineers, who thought they understood them, look like fools. The hooking up and reading out are the easy parts of a measurement.

The selection, installation details and the conditions of use play a big role in gaining a measurement that is accurate and reliable. It's like many measurement subjects, the devil is in the details and "simple" thermocouples have a lot of details!