Thermocouple is one of the most commonly used temperature detecting elements in industry. Its advantages are as follows: (1) High measurement accuracy: because the thermocouple is in direct contact with the measured object, it is not affected by the medium. (2) Wide measuring range: Common thermocouple can be continuously measured from -50~+1600℃, some special thermocouple can be measured as low as -269℃ (such as Fe-Ni-Cr), as high as +2800℃ (such as Tungsten-Rhenium). (3) Simple structure, easy to use. A thermocouple is usually made of two different kinds of wire, and is not limited by size or beginning. It has a protective sleeve, which makes it very convenient to use. The basic principle of thermocouple temperature measurement is to weld A and B, conductors or semiconductors of two different materials, together to form A closed loop.
When there is A temperature difference between two persistent points 1 and 2 of conductors A and B, an electromotive force will be generated between them, thus forming A current of A certain size in the loop. This phenomenon is called thermoelectric effect, and thermoelectric couple works by using this effect. Common thermocouple can be divided into standard thermocouple and non - standard thermocouple two categories. Standard thermocouple refers to the thermocouple that the national standard stipulates its thermoelectric potential and temperature relation, permissible error, and has a unified standard dividing meter, it has its supporting display instrument available for selection.
Non-standardized thermocouple in the scope of use or order of magnitude are less than the standard thermocouple, generally there is no unified grading table, mainly used for some special occasions of measurement. Industrial platinum-rhodium thermocouple (type B) in the air of the highest temperature is 1800 ℃, although, tungsten-rhenium thermocouples used under vacuum or inert gas temperatures may reach above 2200 ℃, however, due to the poor oxidation resistance of metal materials, the oxygen content is relatively sensitive, experiment observed that when inert gases have trace amounts of oxygen, even if the oxygen content in (1 ~ 2) x 10-6, drift tungsten-rhenium thermocouples will occur.
