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Thermocouple sensor
Dec 21, 2017

Thermocouple sensor

Thermocouple temperature measurement instruments commonly used in temperature measurement components, is composed of two different components of the conductor at both ends of the junction into the loop, when the two junction temperature of the thermocouple is different, it will generate thermal current in the loop. If there is temperature difference between the working end and the reference end of the thermocouple, the indicating instrument will indicate the temperature value corresponding to the thermoelectric power generated by the thermocouple. Thermocouple thermoelectric heating with the measurement of temperature increases, its size and only the temperature of the material and the two ends of the thermocouple, and the length of the hot electrode diameter has nothing to do. The various thermocouples are often very different in appearance, but their basic structures are generally the same. They are usually made up of the main parts, such as the thermopotentiator, the insulation sheathing tube, and the junction box. The thermocouple is usually connected to a display instrument, a recording instrument and an electronic regulator Supporting the use of equipment.

basic introduction

Thermocouple is a temperature sensing element, is a meter. It directly measures the temperature, and the temperature signal into a thermoelectromotive force signal, through the electrical instrument (secondary instrument) into the measured medium temperature. The basic principle of thermocouple temperature measurement is composed of two different components of the material composition of the closed loop conductor, when there is a temperature gradient at both ends, the circuit will have a current through, this time between the two sides there is electromotive force - thermal EMF, which The so-called Seebeck effect (Seebeck effect). Two different components of the homogeneous conductor for the hot electrode, the higher the temperature end of the work side, the lower end of the temperature free end, the free end is usually at a constant temperature. According to the thermoelectromotive force and temperature as a function of the thermocouple indexing table; table is the free end temperature at 0 ℃ under the conditions obtained, different thermocouples with different indexing table.

In the thermocouple circuit access to a third metal material, as long as the temperature of the two contacts the same material, thermocouple thermoelectric power generated by the same will remain unchanged, that is not affected by the third metal access circuit. Therefore, the thermocouple Temperature, you can access the measuring instrument, measured thermoelectromotive force, you can know the measured medium temperature.

Thermocouple

Thermocouple temperature required to measure the cold side (measurement side of the hot side, through the lead wire and the measurement circuit connected to the end of the cold side) as the temperature remains unchanged, the size of its thermoelectric power was measured with a certain proportion of temperature. If measured, the cold end of the (ambient) temperature changes will seriously affect the measurement accuracy. In the cold end to take some measures to compensate for the cold junction temperature changes as the impact of the cold junction compensation as normal thermocouple.

Annex: thermocouple cold junction compensation calculation method: from mV to temperature: measuring the cold side temperature, converted to the corresponding millivolt value, coupled with the millivolt value of the thermocouple, convert the temperature. From the temperature to mV: measured the actual temperature and the cold side temperature, respectively, converted to millivolt value, subtracted after the millivolt value, which was temperature

Features

◆ simple assembly, easy to replace

◆ Compression spring type temperature sensing element, good seismic performance

◆ measuring range (-200 ℃ ~ 1300 ℃, under special circumstances -270 ℃ ~ 2800 ℃)

◆ High mechanical strength, good pressure resistance

◆ high temperature up to 2800 degrees

structure

Thermocouple structure In order to ensure reliable and stable operation of the thermocouple, its structural requirements are as follows:

① thermocouple composition of the two hot electrodes welding must be strong;

② two hot electrodes should be well insulated from each other to prevent short circuit;

③ compensation wire and the free end of the thermocouple connection to be convenient and reliable;

④ protective sleeve should be able to ensure adequate separation of the hot electrode and harmful media.

working principle

Two different components of the conductor (called thermocouple wire or hot electrode) into the circuit at both ends of the junction, when the temperature difference between the two joints in the circuit will produce electromotive force, a phenomenon known as the thermoelectric effect, and This electromotive force is called thermoelectric power. Thermocouple is the use of this principle for temperature measurement, in which the direct use of the measurement medium temperature is called the work side (also known as the measurement side), the other end is called the cold side (also known as the compensation side); cold side and display Instrument or instrumentation connected, indicating that the thermocouple thermometer will be generated by the thermoelectric power. Thermocouple is actually a kind of energy converter, it will be converted to thermal energy, with the thermoelectric power generated by measuring the temperature, the thermocouple thermoelectric power, should pay attention to the following questions: 1: thermocouple thermoelectric power is thermoelectric Even the working end of the temperature difference between the two ends of the function, rather than the cold end of the thermocouple and working end, the temperature difference between the two ends of the function; 2: thermocouple thermoelectric power generated by the size of the material when the thermocouple is uniform, Even the length and diameter have nothing to do, only with the thermocouple material composition and the temperature difference between the two ends;

3: When the thermocouple of the two thermocouple wire material composition is determined, the thermocouple thermoelectric power of the size, only with the thermocouple temperature difference; if the temperature of the thermocouple cold side to maintain a certain, which into the thermocouple thermoelectric power is only Single - valued function of working temperature. Two different materials of the conductor or semiconductor A and B welded together to form a closed loop, as shown. When there is a temperature difference between the two attachment points 1 and 2 of the conductors A and B, an electromotive force is generated therebetween, thereby forming a current of a large magnitude in the circuit. This phenomenon is called a thermoelectric effect. Thermocouple is to use this effect to work.

Category Editing

Commonly used thermocouple can be divided into standard thermocouple and non-standard thermocouple two categories. The standard thermocouple is called the national standard provides the relationship between the thermoelectric power and temperature, allowing the error, and a unified standard table thermocouple, which has its supporting display instrument available. Non-standardized thermocouple in the use of the scope or magnitude are less than the standardization of thermocouples, generally there is no uniform indexing table, mainly for some special occasions. Standardized thermocouple China from January 1, 1988 onwards, thermocouple and thermal resistance all according to IEC international standard production, and specify S, B, E, K, R, J, T seven standardized thermocouple for our unified design type Thermocouple.

Commonly used thermocouple type thermocouple thermocouple indexing electrode material temperature (℃)

S platinum and rhodium alloy (rhodium content of 10%) pure platinum 0-1600

R platinum and rhodium alloy (rhodium content of 13%) pure platinum 0-1600

B Rhodium alloy (rhodium content 30%) Platinum and rhodium alloy (rhodium content 6%) 0-1800

Nickel Chromium nickel silicon 0-1300

Pure copper - copper - nickel

J iron, copper and nickel 0- + 500

Nickel Chromium Silicon Silicon Nickel 0 - + 800

Nickel - chromium - copper - nickel

Thermocouple type: thermocouple assembly, armored thermocouple, the end of the thermocouple, compression spring fixed thermocouple, high temperature thermocouple, platinum and rhodium thermocouple, corrosion thermocouple, wear thermocouple, high pressure thermocouple, Hand-held thermocouple, micro-thermocouple, precious metal thermocouple, fast thermocouple, tungsten rhenium thermocouple, single-core armored thermocouple, and so on.

In theory, any two different conductors (or semiconductors) can be prepared into a thermocouple, but as a practical temperature measurement components, its requirements are manifold. In order to ensure the reliability of engineering technology, and adequate measurement accuracy, not all materials can be composed of thermocouple, the general thermocouple electrode material, the basic requirements are: (1), within the temperature range, thermoelectric properties stable , Does not change with time, there is sufficient physical and chemical stability, not easy to oxidation or corrosion; (2), the resistance temperature coefficient is small, high conductivity, small specific heat; (3), thermoelectric power generation should be large, and (4), the material replicability is good, high mechanical strength, the manufacturing process is simple, the price is cheap. (2) the thermoelectric power and temperature linear or nearly linear between the single-valued function;

Selection criteria

Select the thermocouple according to the use of temperature range, the required accuracy, the use of atmosphere, the performance of the measured object, response time and economic considerations.

1, the choice of measurement accuracy and temperature measurement range

The use of temperature 1300 ~ 1800 ℃, the required accuracy and relatively high, the general selection of B-type thermocouple; required accuracy is not high, the atmosphere allows the use of tungsten rhenium thermocouple, tungsten rhenium thermocouple is generally higher than 1800 ℃; 1000 ~ 1300 ℃ required accuracy and relatively high available S-type thermocouple and N-type thermocouple; generally below 1000 ℃ with K-type thermocouple and N-type thermocouple, generally lower than 400 ℃ with E-type thermocouple; 250 ℃ And negative temperature measurement is generally used with T-type galvanic couple at low temperature when the T-type thermocouple stability and high accuracy.

2, the use of the atmosphere of choice

S-type, B-type, K-type thermocouple suitable for strong oxidation and weak reducing atmosphere, J-type and T-type thermocouple suitable for weak oxidation and reduction atmosphere, if the use of airtight protective tube, The atmosphere is less stringent requirements.

3, durability and thermal response of choice

Large diameter thermocouple durability, but some of the slower response, large heat capacity of the thermocouple, the response is slow, large gradient temperature measurement, the temperature control in the case, the temperature difference. Requires fast response time and requires a certain degree of durability, select the armored even more appropriate.

4, the nature and state of the object to measure the choice of thermocouple

Temperature requirements of moving objects, vibrating objects, high-pressure containers High mechanical strength, chemical pollution of the atmosphere requires a protective tube, the case of electrical interference requires relatively high insulation.

Selection process: Model - sub-degree number - Explosion-proof grade - Accuracy class - Mounting fixed form - Protection tube material - length or insertion depth

Product Selection and Ordering Information:

1, in the selection and ordering, please specify:

2 product model

2 sub-degree number

2 protection tube material and diameter

2 protection tube length L and placement depth I

2 fixed device type

2 The actual measuring range of the product

2, the type of screw-type device in the order are not marked are fixed external thread M27 × 2, (the rest of the thread fixed type are required to indicate)

3, because of the special needs of users with the above model does not match those who need specialized manufacturing products, please specify your special technical requirements, special orders.

Temperature compensation of cold junction of thermocouple sensor

Because thermocouple materials are generally more valuable (especially when using precious metals), and the temperature measurement point to the instrument are far away, in order to save the thermocouple material, reduce costs, usually using the compensation wire to the cold end of the thermocouple (free End) to a relatively stable temperature control room, connected to the instrument terminal. It must be noted that the role of thermocouple wire compensation only play the role of extension of the hot electrode, so that the cold end of the thermocouple to the control room of the instrument terminal, which itself can not eliminate the temperature change of the cold end of the impact of temperature compensation. Therefore, the need to use other correction methods to compensate for the cold side temperature t0 ≠ 0 ℃ on the temperature measurement. In the use of thermocouple compensation wire must pay attention to the model match, polarity can not take the wrong, compensation wire and thermocouple junction temperature can not exceed 100 ℃.


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