Thermocouple temperature measurement principle and its application focus:

        1, master thermocouple temperature measurement principle

        2, understanding of the thermocouple measurement circuit and its compensation method

        3, understanding of thermocouple applications

        First, thermocouple profile

        It is widely used to measure the temperature within the range of -200 ℃ ~ 1300 ℃, under special circumstances, can be measured to 2800 ℃ high temperature or low temperature of 4 K. It has a simple structure, Low cost, high accuracy, wide temperature range, etc. Because the thermocouple will convert the temperature into electricity to detect, so that the temperature measurement, control, and the temperature signal amplification is very convenient for remote measurement and automatic control. In the contact temperature measurement method, the thermoelectric thermometer is the most common. In this case,

        Second, the principle of thermocouple temperature measurement:

        1. Definition: A combination of two conductors, the temperature is converted into a thermoelectromotive force sensor is called a thermocouple. 2. Temperature measurement principle: thermocouple temperature measurement based on the thermoelectric effect. Conductors A and B of two different materials are connected in series to form a closed loop. If the temperature of the two contacts 1 and 2 is different, if T> T0 (as shown in Figure 12-1, the thermoelectric effect) Electromotive force, in the loop to produce a certain size of the current, a phenomenon known as the thermoelectric effect. Thermal electromotive force recorded as EAB, conductor A, B, known as the hot electrode. Contact 1 is usually welded together, the measurement will be placed in the temperature measurement site feel the measured temperature, it is called the measurement side (or work side, hot side). Contact 2 requires a constant temperature, called the reference (or cold) side. 3. Thermoelectric effect: conductor A and B thermocouple closed circuit composed of two contacts in the eAB (T) and eAB (T0) two contact potential, and because T> T0, conductor A and B also in the There is a temperature difference potential. Therefore, the closed-circuit total thermoelectromotive force EAB (T, T0) should be the contact electromotive force and the temperature difference potential algebraic sum, namely: 4. closed loop total thermoelectromotive force For the selected thermocouple, when the reference temperature is constant, the total thermal emf (T, T0) = f (T), which is the single-valued function of the measurement terminal temperature T. This is how the thermocouple measures the temperature. In the actual temperature measurement, the thermocouple closed loop must be introduced into the wire and instrumentation. In this case,

        Third, the basic principles of thermocouple temperature measurement

        A closed loop composed of a homogeneous conductor does not produce thermoelectromotive force regardless of the cross-sectional area, length, and temperature distribution of the conductor. If the thermocouple's two hot electrodes consist of two homogeneous conductors, the thermoelectromotive force of the thermocouple depends only on the temperature of the two contacts, independent of the temperature distribution of the thermocouple. If the thermocouple is an inhomogeneous electrode, Temperature gradient of the temperature field, will have an additional potential, if only from the thermocouple thermoelectric power to determine the size of the temperature will cause errors. 1, the principle of homogeneous conductor: 2, the middle conductor of the rules: the thermocouple circuit in the third material access to the conductor, as long as the two ends of the same temperature, the conductor access will not affect the thermocouple loop thermoelectric power . According to this principle, the thermocouple can be a contact disconnect the third conductor, a thermocouple can also be a conductor disconnect access to a third conductor, as long as the temperature of each conductor at both ends of the same, Do not affect the circuit's total thermal emf. In the actual temperature measurement circuit, there must be a connecting wire and display instrument, if the connecting wire and display instrument as a third conductor, as long as they are the same temperature at both ends, it does not affect the total thermal emf. 3, the reference electrode rule: the two conductors A, B, respectively, and the reference electrode C (or standard electrode) thermocouple, if they are generated by the thermoelectromotive force is known, A and B bipolar pair of thermoelectromotive force available Can be seen, as long as the two conductors are known with the reference electrode thermocouple thermocouple when the composition of the thermocouple can be calculated according to the reference electrode to determine the thermocouple when the two conductors thermoelectric force. Thus simplifying the thermocouple matching work. As the physical and chemical properties of platinum stable, high melting point, easy to clean, so people often use high purity platinum as a reference electrode. In this case,

        Fourth, commonly used thermocouple

        Suitable for the production of thermocouple materials are more than 300 kinds, which are widely used in 40 to 50 species. The International Electrotechnical Commission recommends 8 kinds of thermocouples as standardized thermocouples to the countries in the world, and there are 8 standardized thermocouples in China. Platinum Rhodium 13 - Platinum (R), Platinum Rhodium 30 - Platinum Rhodium 6 (B), Nickel Chromium - Nickel Silicon (K), Nickel Chromium - Constantan (E), iron-constantan (J), copper-constantan (T) and nickel-chrome-nickel silicon (N). The following briefly describes several of them

        5, several commonly used thermocouple

        1, platinum and rhodium 10 - platinum thermocouple 1. Composition: φ0.5mm by the pure platinum wire and the same diameter of platinum and rhodium wire, indexing S platinum and rhodium wire is positive, pure platinum wire is negative. 2. Features: thermoelectric properties, and strong oxidation resistance, should be in the oxidizing, inert atmosphere for continuous use. Long-term application of the temperature of 1400 ° C, more than this temperature, even in pure platinum wire in air will recrystallization and grain growth. Short-term use temperature is 1600 ℃. Of all the thermocouples, the highest accuracy level, usually used as standard or high temperature thermocouple measurement, the use of a wide temperature range (0 ~ 1600 ℃), homogeneity and interchangeability. In this case,

        The drawback is expensive, thermoelectric potential is small, with a high sensitivity display instrument.