Thermocouples are used as contact-type meters to measure and control temperature. They are used in conjunction with sensors and display instruments to directly measure and control the temperature of gases, liquids and steam in the production process.
The principle: the different materials of the conductor A, B connected into a closed loop, the end of the contact temperature measurement point known as the measurement (or work side), the other end is called the reference end (or free end). If the temperature and t0 are different between the measuring end and the reference junction, a thermoelectrical potential EAB (t, t0) is generated between the A and B of the circuit. This phenomenon is called the Seebeck effect, ie the thermoelectric effect. EAB size with the conductor A, B and the two ends of the material temperature t and t0 change, this circuit is called the prototype thermocouple. In practice, the A, B, one end of the welding together as the measurement of thermocouple end of the measured temperature t, and the reference junction separately, with wire access display instrument, and to maintain the reference junction temperature t0 stable. The measured potential measured only with the measured temperature and t changes.
Commonly used common industrial thermocouple wire are:
1. Platinum and rhodium 10 a platinum thermocouple: a precious metal thermocouple, platinum cathode is platinum alloy, platinum cathode, short-term working temperature of 1600 ℃, long-term working temperature of 1300 ℃, physical and chemical stability, generally used High accuracy requirements of high temperature measurement. But the material is more expensive, smaller thermoelectric power, sub-degree of S.
2. Nickel-chromium-nickel silicon thermocouple: It is the most stable non-precious metals in a kind of application is very wide, positive for nickel-chromium. Short-term working temperature of 1200 ℃, long-term working temperature of 900 ℃. This thermocouple thermoelectric power than the last kind of large 4 to 5 times, and better linearity, the error is generally (6-8) ℃. But the hot electrode is not easy to do very uniform, more easily oxidized, poor stability. The division number is K.
3. Nichrome a constant copper thermocouple: the cathode is nickel-chromium, short-term working temperature of 800 ℃, long-term working temperature of 60 ℃. It is the largest thermoelectric power of a thermocouple, measurement accuracy is high, but easily oxidized. The division number is E.
4. Copper a constant copper thermocouple: This is very common in the low temperature thermocouple, measuring temperature range (-200 - +200) ℃, good stability, low temperature, high sensitivity and low price. The division number is T.