Thermocouple Sensor

Overview of the thermocouple sensor

The thermocouple sensor is a variety of temperature sensor that relies on the fact that when two electrically conductive metals of different alloy composition meet at a junction, they will generate a differential electrical potential in response to heat. The temperature of the conductors at the junction is compared with the temperature at the so-called ‘cold junction’, which is remote from the heat source; the temperature difference between the heated junction of the dissimilar metals and the cold junction generates a small but measurable voltage, which can be converted into a digital temperature scale on the instrument.

Most temperature-measuring devices require an external power source to function properly but thermocouple sensors are unique in that respect in that they are entirely self-powered. A wide variety of these sensors are now available for measuring different temperature ranges in commercial and industrial settings, although different heat ranges require different alloys. They are inexpensive and highly versatile, and can measure temperature at a heat source remote from the main body of the instrument via intermediate extension wires that are considerably less expensive than the sensor itself.

Thermocouple sensors are generally calibrated against the temperature standard of 0ºC.

 

History

The Baltic German physicist Thomas Johann Seebeck established in 1821 that metal conductors would generate a voltage whenever they were subject to a temperature gradient, a phenomenon which became named after him as the Seebeck effect (also more generally known as the thermoelectric effect). To measure the extent of the voltage, however, Seebeck found that another conductor needed to be connected to the heated area. The second conductor undergoes a change in electrical potential as well but in opposition to the other conductor. Because conductors made of different alloys will differ in the extent of the voltage built up in them by heating, the voltage difference between the two alloys, Seebeck discovered, could be measured when the circuit was completed. The idea behind an instrument now used in countless scientific and industrial applications was born.

 

Technical aspects

The voltage difference between the two dissimilar alloy conductors increases in proportion to the heat energy entering them. Typically, the voltage difference for the most widely used combinations of alloys is of the order of 1 to 70 µV per ºC.

The voltage “follows” the temperature gradient and is measured at the ‘cool’ end of the alloy conductors, not the heated point, where they meet at the reference junction, which is maintained at a steady temperature unaffected by the heat source. This thermocouple sensor will only work properly, however, if the wires in the thermocouple region have not been compromised by oxidation or contamination; if their material composition is corrupted in some way, as tends to happen over time due to the aging of the instrument and its exposure to corrosion or oxidation over time, the measurements become considerably less accurate.

Each of the conductors has a property known as a ‘Seebeck coefficient’, which is different for each alloy and is a measure of the electrical power generated in it per unit of heat. Although this ‘thermopower’ (induced thermoelectric voltage) is conventionally measured in volts per degree Kelvin, in everyday practical applications it is usually measured in µV per ºC. Alloys are selected depending on the temperature range the instrument is designed to measure and will demonstrate stable Seebeck coefficients over this thermal span.

Ii is important to note that the voltage differential is not set up at the heated junction or the cold junction, but along the length of alloy between hot and cold junctions. The cold junction of a thermocouple is maintained at a constant and known reference temperature so that it functions as a standard from which to measure the change in heat energy at the heat source. In most practical applications, the cold junction is actually a simulated cold zone fashioned from thermally responsive devices such as diodes, thermistors or resistance thermometers which measure the temperature of the heated terminal and reduce to a minimum the thermal differential between the terminals (a process known as “cold junction compensation”).

The metallurgical composition of the alloys used in thermocouple sensors varies in quality according to the degree of accuracy required, and, in increasing accuracy and cost, is categorised into three grades: extension grade, standard grade and special limits of error grade. With higher grade thermocouple sensors, extension wires connecting it to a remote heat source for measurement must be made of the same metals as used in the sensor itself or additional voltage differentials would be set up, impairing the accuracy of the device. For platinum thermocouple sensors, however, an exception is made and copper alloy is used in the extension wire because platinum extensions would be excessively expensive.

As has already been noted, different alloy combinations are used in different applications. While cost is always a consideration in the choice of metals, other properties such as metallurgical composition, stability, melting point and electrical output are of signal importance. There are accordingly several different types of thermocouple sensor that have become recognised industry standards.

“B” thermocouples conductors are made of platinum-rhodium alloy (each conductor has different amounts of rhodium) and remain stable for temperatures as high as 1800ºC. “R” and “S” thermocouples possess one wire made purely of platinum while the other is a platinum-rhodium alloy (the alloy mix is different for each type). They are stable at temperatures as high as 1600ºC.

Others which have become industry standards are K (chromel, an alloy of nickel and chromium), the most widely used, E (a chromel-constantan alloy) which is used in cryogenic equipment, J (iron-constantan) and N (Nicrosil-Nisil), which is highly resistant to oxidation and extremely stable and can be used to measure an exceptionally broad range of temperatures from -270ºC to 1300ºC.

Other metal combinations include copper-constantan (“T”), different nickel alloys (“M”), different tungsten-rhenium alloys (“C”) and chromel-gold/iron.

 

Industrial applications

While they are used domestically and in offices and other buildings to control thermostats for central heating systems and as safety devices in gas applications, thermocouple sensors are also used widely in industry and science. They are used to measure furnace-level temperatures in industrial kilns, as well as for temperature monitoring in diesel engines, gas turbine exhausts and a huge range of industrial processes,

 

How the thermocouple sensor differs from other sensors

Thermocouples exploit the tendency of electrically conductive metals to generate voltages when heat is applied to them. As such, they differ from all other temperature sensors although their principal limitation is accuracy. It has proven elusively difficult in most practical applications of thermocouple sensors to reduce systems errors below 1ºC.