How to separate the positive and negative poles of armored thermocouples?


The basic principle of thermocouple temperature measurement is that two different materials of conductors form a closed circuit. When there is a temperature gradient at both ends, there will be current passing through the circuit, and there will be an electromotive force between the two ends - thermoelectric electromotive force, which is called the Seebeck effect. Two homogeneous conductors with different compositions are thermoelectric electrodes, with the working end at the higher temperature and the free end at the lower temperature. The free end is usually at a constant temperature. According to the functional relationship between thermoelectric electromotive force and temperature, a thermocouple graduation table is made; The graduation table is obtained under the condition of a free end temperature of 0 ℃, and different thermocouples have different graduation tables. When a third metal material is connected to the thermocouple circuit, as long as the temperature of the two contacts of the material is the same, the thermoelectric potential generated by the thermocouple will remain unchanged, that is, unaffected by the connection of the third metal to the circuit. Therefore, when measuring temperature with a thermocouple, a measuring instrument can be connected, and after measuring the thermoelectric electromotive force, the temperature of the measured medium can be known. When measuring temperature, a thermocouple requires the temperature of its cold end (the measuring end is the hot end, and the end connected to the measuring circuit through a lead is called the cold end) to remain constant, so that its thermoelectric potential is proportional to the measured temperature. If the temperature of the cold end changes during measurement, it will seriously affect the accuracy of the measurement. Taking certain measures at the cold end to compensate for the impact caused by temperature changes at the cold end is called normal cold end compensation for thermocouples. Special compensating wires are used to connect with measuring instruments. K-type thermocouples are currently the most commonly used metal thermocouples in temperature measurement areas above 500 ℃, and their usage is the sum of other metal thermocouples. Today we will learn how to separate the positive and negative poles of armored thermocouples. First, let's understand what positive and negative poles are. The nominal chemical composition of the positive electrode (KP) is Ni: Cr ≈ 90:10, and the chemical composition of the negative electrode (KN) is Ni: Si ≈ 97. K-type thermocouples have good linearity and high thermoelectric electromotive force, so they can be used in oxidizing and inert atmospheres. However, K-type thermocouples cannot be directly used at high temperatures in atmospheres of sulfur, reducibility, or alternating reduction and oxidation, and are not recommended for use in weak oxidation atmospheres. Its positive electrode is a nickel chromium alloy (KP) containing 10% chromium, and its negative electrode is a nickel silicon alloy (KN) containing 3% silicon. Therefore, how can armored thermocouples be divided into positive and negative electrodes? Based on this characteristic, using a magnet can easily identify the positive and negative poles of armored thermocouples. As for the distinction between the positive and negative poles of other thermocouples, it is also very simple. Firstly, the nickel chromium nickel silicon thermocouple has two distinct colors: one is green, the other is gray, and the green is the positive pole. If it is an armored thermocouple wire, a small magnetic material can be used to place it on the negative pole surface, which is the gray wire. If it is attractive, it is the negative pole.


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