What is a thermocouple wire
A thermocouple wire by definition is used from the sensing point to the junction point. The sensing point is the point where you want to measure the temperature (for example the temperature in the oven), and the junction point is the point with the reference temperature (temperature from the room, for example). The thermocouple measures the difference between these two temperatures and gives a feedback signal to the controller to employ the programmed signal (turn off the oven, when the desired temperature is reached). There are different thermocouples available, defined by the metal used for probes: Type J, K, T, E etc. Every one of these types has its own temperature range in which it is used. A thermocouple wire can vary in size. When the application needs high accuracy of measurement, smaller wires are chosen. In applications where high precision is not required, a larger thermocouple wire is used.
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It must be noted that when the thermocouple is used for measuring, the wire should be connected to the instrument through its own resistivity, to avoid measurement errors from transitional resistances.
Standards and types of thermocouple wires
By electrical engineering standards, a thermocouple wire has to be insulated. According to these standards, there are different colors for wires used. These standards and types are described in this section. By United States ASTM standards, in a type J thermocouple, the negative pole is colored red and the positive is white, but the overall insulation is black. For type T, the overall insulation is blue, the negative pole is red, and the positive is blue. In type B, the overall insulation is gray and the positive pole is gray, too. In type K, the overall insulation and the positive pole are yellow. In types R and S, the overall insulation is green, but the positive pole is black. There are also German DIN, French NFE and British thermocouple wire standards, in which different colors are used. These standards are used everywhere in the world.
Performance versus length of thermocouple wires
In practice, the performance of a thermocouple wire is dependant on the length of the wire. The electromagnetic interference is the main factor of a good performance. The other two factors that influence the performance of a thermocouple wire are the total loop resistance and electrical noise reduction. You should always have in mind that the loop resistance should not be greater than 100 ohms (you can calculate this with a simple multiplication of the wire length measured in feet, and resistance per double feet). The electrical noise of a thermocouple wire can be reduced by using shielded wires.