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Thermocouples are a very common type of device that have been widely utilized as a temperature sensor. Thermocouples are now available in a wide range of models and technical specifications, which means that they can be used across a wide range of applications. However, because each thermocouple is different, the user needs to understand the basic structure, functionality, and ability of different thermocouples to determine which types of thermocouple is right for their intended application. Here, we look at what thermocouples are, how they work, and what the different types are.
Simply put, a thermocouple is an electronic device which has two electrodes which are dissimilar in nature, with electrical junctions that form at different temperatures. Thermocouples are made of two different metal wires, and it is the difference in wire properties that forms the different junctions—with thermocouples typically having a hot junction and a cold junction. The two different metals are joined at either end to the other metal—which creates the hot junction—and both metals are attached to a copper wire—which forms the cold junction. The creation of a bi-junction system, in turn, creates a thermoelectric circuit that can change under high temperatures, and two junctions are needed to create a thermocouple.
How They Work
Thermocouples have simple working principles, and this is one of the main reasons why they have been so widely utilized in temperature sensing applications. The two junctions play a part. Where the wires of the hot junction are connected, they often form a point, and this point is used as the ‘sensing surface’ and is connected to the point where the temperature is being measured. The cold junction, on the other hand, has a known temperature, and this junction acts as a reference.
Thermocouples work by utilizing the Seebeck, Peltier and Thomson effects. The Seebeck effect states that when two unlike metals are joined at these junctions, an electromotive force is generated (which is different for different metal combinations). The Peltier effect states that two dissimilar metals in these junctions can generate an electromotive force due to the differing temperatures of the junctions, and the Thomson effect states that two unlike metals in these junctions can generate a potential due to the temperature gradient along the length of the circuit.
Thermocouples use all these effects to determine the temperature of an unknown body. When the hot junction is in contact with a hotter object than the reference junction, it generates an electromotive force due to the temperature difference (using the above effects), which in turn generates a voltage. The voltage across the circuit is then recorded and the temperature can be backed out by using the reference junction to calibrate the unknown temperature.
The Different Types of Thermocouple
As mentioned, while thermocouples work using the same basic principles, there are many different types. The different types are generally governed by the temperature range in which they can be used in, and in which environments they can be used—which are often governed by which the usable temperature ranges are, as well as their chemical stability, resistance to abrasion, resistance to vibration, and how easy they are to install in a given environment. To achieve these distinct temperature ranges, different metals are often used, but the same metal classes can be used across different thermocouple types.
For example, it is common to use base metals to construct J, K, E, T, and N thermocouples, whereas noble metals are often used to R, S, C and GB thermocouples. Of all the thermocouple types, J, K, E, and T thermocouples are the most common due to the temperature ranges which they are usable in. J thermocouples are typically used between 0 °C to 750 °C (32 °F to 1382 °F), K thermocouples are usable in -200 °C to 1250 °C (-328 °F to 2282 °F) temperature ranges, E thermocouples are usable in -200 °C to 900 °C (-328 °F to 1652 °F) temperature ranges, and T thermocouples are used between -250 °C to 350 °C (-418 °F to 662 °F).
Aside from the temperature classifications, the tip of the thermocouples can be grounded in a number of different ways depending on the application requirements. There are three different ways in which this can happen—grounded, ungrounded and exposed. Grounded thermocouples have the wire tip physically attached to the wall of a sheath (which encases the tip)—otherwise known as the probe wall—and are often used for measuring the temperatures of static and flowing corrosive gases and liquids, as well as in high-pressure environments. On the other hand, the hot junction in ungrounded thermocouples are detached from the probe wall (while still encased) and are often used in corrosive environments where the thermocouple needs to be electronically isolated from the sheath. In exposed thermocouples, the junction protrudes out of the tip of the sheath, where it is exposed to the environment directly. Exposed thermocouples are generally used to measure static and flowing non-corrosive gases when a fast response time is needed.
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