what is a thermocouple?
When there are two different conductors or semiconductors A and B form a loop, its two ends are connected to each other, as long as the two junctions at different temperatures, one end of the temperature is T, known as the working end or the hot end, the other end of the temperature is T0, known as the free end (also known as the reference end) or the cold end, the loop will produce an electromotive force, the direction and magnitude of the electromotive force and the conductor's material and the two junctions of the temperature. This phenomenon is known as the "thermoelectric effect", the two conductors of the circuit is called "thermocouple", the two conductors are called "hot electrodes", the electric potential is called The electric potential generated is called "thermal electromotive force".
Theoretically, any two different conductors (or semiconductors) can be formulated into thermocouples, but as a practical temperature measurement element, the requirements for it are manifold. In order to ensure reliability in engineering and technology, as well as sufficient measurement accuracy, not all materials can be composed of thermocouples, generally on the electrode material of the thermocouple, the basic requirements are:
1, in the temperature measurement range, thermoelectric properties are stable, do not change over time, there is enough physical and chemical stability, not easy to oxidize or corrode;
2, resistance temperature coefficient is small, high conductivity, small specific heat;
3, temperature measurement in the generation of thermoelectric potential to be large, and thermoelectric potential and temperature is linear or near linear single-value function relationship;
4, good material replication, high mechanical strength, simple manufacturing process, inexpensive.
The installation of thermocouples and RTDs, attention should be paid to facilitate accurate temperature measurement, safe, reliable and easy maintenance, and does not affect the operation of equipment and production operations. To meet the above requirements, in the choice of thermocouple and RTD installation parts and insertion depth should pay attention to the following points:
1, in order to make thermocouple and RTD measuring end and the measured medium between the full heat exchange, should be a reasonable choice of measurement point location, try to avoid valves, elbows and pipelines and equipment near the dead center of the installation of thermocouples or RTDs.
2, with a protective casing thermocouple and RTD heat transfer and heat loss, in order to reduce the measurement error, thermocouple and RTD should have enough insertion depth:
1) for measuring the center of the pipeline fluid temperature thermocouple, generally should be inserted into the center of the pipeline at its measuring end (vertical or inclined installation). If the pipe diameter of the measured fluid is 200mm, then the thermocouple or RTD insertion depth should be selected as 100mm;
2) for high temperature, high pressure and high speed fluid temperature measurement (such as the main steam temperature), in order to reduce the resistance of the protective sleeve to the fluid and prevent the protective sleeve in the fluid under the action of the fracture, can be taken to protect the tube shallow insertion or the use of heat-sleeved thermocouple, shallow insertion of thermocouple protective sleeve, its insertion into the main steam piping should be not less than 75mm deep; heat-sleeved thermocouple standard insertion depth of 100mm;
3) If you need to measure the temperature of the flue gas in the flue, although the flue diameter of 4 m, thermocouple or RTD insertion depth of 1 m can be;
4) When the insertion depth of the measuring element exceeds 1 m, it should be installed vertically as far as possible, or installed with a support frame and protective sleeve.
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