Basic principle of N type thermocouple temperature measurement will be two different material A conductor or semi-conductor A and B welding together, form A closed loop. When the conductor A and B two persistent point temperature difference between 1 and 2, occurs between the electromotive force, thus forming the size of A current in the circuit, this kind of phenomenon called the thermoelectric effect. N type thermocouple is in using this effect to work. 1. Thermocouple type and structure formation ( 1) Thermocouple types commonly used thermocouple could be divided into two types: standard thermocouple and non-standard thermocouple. Using standard thermocouple refers to the standard specifies the thermoelectric potential and the relationship between temperature, permissible error, and a unified standard score table of thermocouple, it has with matching display instrument to choose from. Non-standard thermocouple or on the order of magnitude less than the range using standardized thermocouple, generally no unified indexing table, mainly used for the measurement of some special occasions. ( 2) The structure of N type thermocouple thermocouple is reliable, steady work, to ensure that the requirements for its structure is as follows (1) of the two thermocouple thermal electrode welding must be strong; (2) two hot electrode should be well insulated between each other, in case of short circuit; (3) compensation conductor should be connected to the free cod of a thermocouple to be convenient and reliable; (4) protect casing thermal electrodes should be able to make sufficient isolation and harmful medium. 2. The thermocouple cold end temperature compensation due to the thermocouple materials are generally more expensive ( Especially when using precious metals) And temperature measuring points are far away the distance to the instrument, in order to save thermocouple materials, reduce cost, usually adopt the compensating conductor (of the cold junction of the thermocouple The free end) Reaches to the relatively stable control indoor temperature and connected to the meter terminals. It must be pointed out that the role of the thermocouple compensation wire only extend thermal electrode, make moves to the control room of the cold junction of the thermocouple meter terminal, it itself does not eliminate the cold end temperature change on the influence of temperature, no compensation. So, still need to take some of the other correction method to compensate of the cold end temperature t0 indicates influence on measuring temperature 0 ℃. Must pay attention to when using thermocouple compensating conductor model match, cannot be wrong polarity, compensation conductor should be connected to the thermocouple temperature should not exceed 100 ℃. For example: S type thermocouple) Platinum rhodium - 10 Platinum thermocouple platinum rhodium - 10 Platinum thermocouple ( S type thermocouple) For precious metals thermocouple. Accidentally wire diameter provision of 0. 5 mm, allowable deviation - 0. 02 mm, the positive electrode ( SP） In the name of the chemical composition of platinum rhodium, contain the rhodium is 10%, platinum-based is 90%, the cathode ( SN) For pure platinum, commonly known as single platinum-rhodium thermocouple. The temperature is 1300 ℃, the long-term use of thermocouple short-term use temperature is 1600 ℃. S type thermocouple in the thermocouple series has high accuracy, good stability, test your area wide, long service life, etc. Its physical and chemical performance is good, the thermoelectric potential stability and oxidation resistance at high temperature performance is good, suitable for oxidizing and inert atmosphere. Because of S type thermocouple has excellent comprehensive performance, in line with the use of S type thermocouple temperature scale, for a long time as a scale interpolation instruments, 'ITS - Although 90 'provisions will not be as within the scale of the instrument, but temperature advisory committee ( 有条件现金转移支付) That S type thermocouple can still be used for approximate scale. S type thermocouple deficiency is thermoelectric potential, thermoelectric potential rate is small, low sensitivity and mechanical strength under high temperature to drop, is sensitive to pollution, precious metal materials is expensive, and one-time investment is larger.