Thermocouples can be used to measure temperature by using the so-called Seebeck-Effect.

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1 1.3 Thermocouples (TCs) Thermocouples are made of two distinct metals (typically wires). These are connected at one point. Thermocouples can be used to measure temperature by using the so-called Seebeck-Effect The Seebeck-Effect When two electrical conductors (A and B), which are made from materials, are connected with each other at one point, an electrical voltage can be measured between these two conductors, if there is a. between the connecting point and a comparison point. The created voltage is being called:... (U th ). Sketch: The created thermo-voltage is proportional to: - the temperature difference : Δϑ = ϑ M - ϑ C - the so-called Seebeck-Coefficient : k th_ab U th = Δϑ k th AB 1

2 1.3.2 The Seebeck-Coefficient: k th_ab Every electrically conducting material possesses a so-called thermos-electric-force (k th ). This force is being defined with respect to Platinum. The thermos-electric-force of a metal is a constant property of this material. Mean thermo-electric-forces (k th ) of some 25 C: Material Symbol k th Unit Comment Constantan CuNi µv/k "Constantan" : Cu-Ni-Alloy Nickel Ni µv/k Platinum Pt µv/k fixed arbitrarily Aluminum Al µv/k Copper Cu µv/k Iron Fe µv/k Nickel-Chrom NiCr µv/k Silicon Si µv/k Thermocouples can be created by combining any pair of electrically conducting materials. The Seebeck-Coefficient (k th_ab ) can be calculated by subtracting the respective individual thermo-electric-forces of the used materials: k th AB = k th A k th B where A and B refer to the specific materials being use. Example gratia: Consider a thermocouple, where one leg (A or +) consists of Copper, and the other leg (B or -) consists of Constantan. The Seebeck-Coefficient k th_ab of this thermocouple can be calculated as follows: k th_ab = [+ 8 - (- 33)] µv/k = + 41 µv/k 2

3 1.3.3 Normalized Thermocouples Although thermocouples can be made of any combination of conducting metals, 8 paircombinations have been normalized in the following norms: DE: EC: US: DIN IEC-584 NIST-Monograph-175 The following table lists these 8 pairs of thermocouples, ordered by importance: TC-Type Material mean Seebeck- Coefficient k th_ab 25 C) Usable Temperatur- Range Connector- Colour K NiCr vs. NiAl μv/k C T Cu vs. CuNi μv/k C E NiCr vs. CuNi μv/k C S Pt vs. PtRh (10 %) μv/k C J Fe vs. CuNi μv/k C N NiCr vs. NiSi μv/k C R Pt vs. PtRh (13 %) μv/k C B Pt vs. PtRh (30 %) μv/k C Which thermocouple-type should be used or chosen for a specific application? The answer to this question is: it depends on the application. A type-e thermocouple provides the larges Seebeck-Coefficient. In other words, this kind of TC has the highest sensitivity. However, its use is limited to about 900 C only. A type-k thermocouple is the cheapest. Which is why, this type of TC has become very popular. It also provides a reasonable usable temperature range of up to 1200 C. Which is enough for many applications. For extremely high temperatures, a TC of type R or B should be used. These TCs can be used up to 1700 C or 1800 C, respectively. However, their sensitivity is comparably low. 3

4 1.3.4 Advises for using TCs (1) The wires of a TC need to be from the very same material, until reaching the so-called comparison point. There, the different TC-wires are typically connected to symmetrically further wiring, such as copper wires. Example gratia: Consider a TC temperature sensor of type-k. Leg A of the TC is made of NiCr. Leg B of the TC is made of NiAl. The sensor itself has a cable length of 30 cm only. However, the readout device for this sensor is located about 10 m away. Thus, the cable of the sensor has to be extended until it reaches the readout device. In this case, the so-called comparison point is the readout device. This means, that each leg of the TC has to be made from the very same material until reaching the readout device. This includes the leg A of the TC-sensor itself, the corresponding wire of the extension cable, and all connecting points in between (such as the pins of a connector). For this very reason, one can purchase TC-extension cable for each TC-type in particular. Likewise, there are TC-connectors that are specific for each TC-type. In addition, TC-connectors are designed with different pin-forms and pin-sizes. So that it is not possible to accidentally twist a connector, and connect TC-wires the wrong way. Also, TC-connectors are colour-coded. For examples, see column 5 of the table above. This ensures that only the correct connectors are used with a specific TC-type, and with the correct corresponding TC-extension-cable. Furthermore, the color-coding of TC-connectors is internationally standardized. I.e., the colour-codes of TC-connectors are uniform throughout this world, and even throughout the entire universe. (2) Like for TC-connectors, there is colour-coding for TC-wires. Typically, the isolating material of the two different legs of a TC are colour-coded. In order to identify a certain TC-wire quick and easy. In addition, the two TC-wires are typically surrounded by an additional layer of isolating material. Combining the two different TC-wires into one TC-cable. This outer layer is colour-coded too. In order to identify a certain TC-type quick and easy. However, the international consensus that we saw for the colour-coding of TCconnectors does not apply when it comes to the colour-coding of TC-wires and TCcables. Instead, total chaos reigns. The following table lists the colour-codes for a type-k TC, sorted by countries / regions. 4

5 One might think that it would make sense to color the outer isolation of a type-k TC in yellow. So that the cable-colour matches the standardized colour of the connector. However, people in the EU decided differently. Within the EU, the standardized colour for the outer isolation of a type-k TC is green. For reasons unknown. The positive leg is coloured in green as well, and the negative leg is coloured in white. In Germany, people followed the same reasoning for the outer isolation colour: green. Interesting is, however, the colour-coding of the legs. On one hand, the EC-norm seems to be valid for the entire political region of Europe. On the other hand, Germany appears to prefer own rules. There, the positive leg is coloured in red, and the negative leg is coloured in green. So, in Europe, the green colour of one leg of a type-k TC does not really mean much. The context matters too. In the US, the colour-coding of TC cables might be politically motivated. There, the colour red cannot possibly be labeled positive. Red has to negative. And so it is: the negative leg of a type-k TC is coloured in red. Interesting is the colour of the outer isolation though: yellow. This way, the colour of the TC-cable actually matches the colour of the connector. Now, colour-coding does make sense. Political reasons may also have motivated the colour-coding of a type-k TC in Japan. Given the colours of the Japanese flag: there, TC-legs are coloured in red and white. Sad is the case of France. The country missed entirely its change to weave the Tricolore into the colour-coding of type-k TCs. It would have been so easy. Credit should be given for the yellow outer isolation though, matching the connector s colour. Also, the positive leg is yellow, just like in the US, the UK, and the Czech Republic. One step closer towards internationally recognized standards. However, a great nation must distinguish itself from another great nation. Which is why, the negative leg is coloured slightly different, compared to the US. The currently existing colour-coding of TC-cables and -wires appears to be an organized confusion. In truth, it is not even a contained catastrophe. It is a complete Tohuwabohu. Someone should clean up this mess. 5