Thermocouples are simple temperature sensors used throughout science and industry. They consist of two wires of dissimilar metals joined together at a single point or junction, which is usually welded for ruggedness and reliability.
At the open circuit ends of these wires, a thermocouple generates a voltage in response to the junction temperature, the result of a phenomenon called the Seebeck effect, discovered in 1821 by German physicist Thomas Seebeck.
Types of Thermocouples
Any two wires of different metals in contact will produce a voltage when heated; however, certain combinations of alloys are standard because of their output level, stability and chemical characteristics.
The most common are “base metal” thermocouples, made with iron or alloys of nickel and other elements, and are known as Types J, K, T, E and N, depending on composition.
“Noble metal” thermocouples, made of platinum-rhodium and platinum wires for higher temperature use, are known as Types R, S and B. Depending on the type, thermocouples can measure temperatures from about -270 degrees Celsius to 1,700 C or higher (about -454 degrees Fahrenheit to 3,100 F or higher).
Limitations of Thermocouples
The advantages and disadvantages of thermocouples depend on the situation, and it is important to first understand their limitations. The output of a thermocouple is very small, typically only around 0.001 volt at room temperature, increasing as temperature rises. Each type has its own equation to convert voltage to temperature. The relationship is not a straight line, so these equations are somewhat complex, with many terms. Even so, thermocouples are limited to accuracies of about 1 C, or about 2 F, at best.
To get a calibrated result, the voltage of thermocouple must be compared to a reference value, which once was another thermocouple immersed in an ice water bath. This apparatus creates a “cold-junction” at 0 C, or 32 F, but it is obviously awkward and inconvenient. Modern electronic ice-point reference circuits have universally replaced ice water and enabled the use of thermocouples in portable applications.
Because thermocouples require the contact of two dissimilar metals, they are subject to corrosion, which can affect their calibration and accuracy. In harsh environments, the junction is usually protected in a steel sheath, which prevents moisture or chemicals from damaging the wires. Nevertheless, care and maintenance of thermocouples are necessary for good long-term performance.
Advantages and Disadvantages of Thermocouples
Thermocouples are simple, rugged, easy to manufacture and relatively inexpensive. They can be made with extremely fine wire to measure the temperature of tiny objects such as insects. Thermocouples are useful over a very wide temperature range and can be inserted in difficult locations like body cavities or abusive environments like nuclear reactors.
For all these advantages, the disadvantages of thermocouples must be considered before applying them. The millivolt level output requires the additional complexity of carefully designed electronics, both for the ice-point reference and amplification of the tiny signal.
In addition, the low voltage response is susceptible to noise and interference from surrounding electrical devices. Thermocouples may need grounded shielding for good results. Accuracy is limited to about 1 C (about 2 F) and may be further reduced by corrosion of the junction or the wires.
Applications of Thermocouples
The advantages of thermocouples have led to their incorporation in a wide range of situations, from controlling household ovens to monitoring the temperature of airplanes, spacecraft and satellites. Kilns and autoclaves use thermocouples, as do presses and molds for manufacturing.
Many thermocouples can be connected together in series to create a thermopile, which produces greater voltage in response to temperature than a single thermocouple. Thermopiles are used to make sensitive devices for detecting infrared radiation. Thermopiles can also generate power for space probes from the heat of radioactive decay in a radioisotope thermoelectric generator.