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Heterogeneous effect of thermocouple wire of K-type thermocouple
2012-3-1 11:07:49

是目前用量最大的廉金属热电偶,其用量为其他热电偶的总和。 K-type thermocouple is the cheapest metal thermocouple with the largest amount, and its amount is the sum of other thermocouples. K-type thermocouple wire diameter is generally 1.2 ~ 4.0mm. The nominal chemical composition of the positive electrode (KP) is: Ni: Cr = 90: 10, the nominal chemical composition of the negative electrode (KN) is: Ni: Si = 97: 3, and its operating temperature is -200 ~ 1300 ° C. K-type thermocouples have the advantages of good linearity, large thermoelectromotive force, high sensitivity, good stability and uniformity, strong oxidation resistance, and low price. They can be used in oxidizing inert atmosphere. Widely used by users.


不能直接在高温下用于硫,还原性或还原,氧化交替的气氛中和真空中,也不推荐用于弱氧化气氛热电偶,k型热电偶,铠装热电偶,铂铑热电偶,耐磨热电偶,s型热电偶,热电偶保护管,热电偶补偿导线,热电偶测温仪,快速热电偶K型热电偶是工业消费中最常用的温度传感器,制造比拟容易。 K-type thermocouples cannot be used at high temperatures for sulfur, reducing or reducing, oxidizing alternate atmospheres and vacuum, nor are they recommended for use in weakly oxidizing atmospheres. Thermocouples, wear-resistant thermocouples, s-type thermocouples, thermocouple protection tubes, thermocouple compensation wires, thermocouple thermometers, fast thermocouples K-type thermocouples are the most commonly used temperature sensors in industrial consumption, and they are relatively easy to manufacture. The thermoelectric potential signal output by the thermocouple must pass through the intermediate conversion link before it can be input into the embedded system based on the single chip microcomputer. It is a complex single-chip thermocouple-to-digital converter, which mainly includes: low-noise voltage amplifier A1, voltage follower A2, cold-junction temperature compensation diode, reference voltage source, 12-bit AD converter, SPI serial interface, analog switch, and Digital controller. The working principle is as follows: The thermoelectric potential generated by the K-type thermocouple is amplified and buffered by the low-noise voltage amplifier A1 and the voltage follower A2 to obtain the thermoelectric signal U1, which is then sent to the ADC via S4. . For the K-type thermocouple, the voltage change rate is (41 μV / ° C), and the voltage can be approximated by the following formula to the characteristics of the thermocouple.

U1 = (41μV / ℃) × (T-T0)

In the above formula, U1 is the thermocouple output voltage (mV), T is the measurement point temperature; T0 is the ambient temperature.

Before converting the temperature voltage value to the corresponding temperature value, stop compensating the cold junction temperature of the thermocouple. The cold junction temperature is the difference between the temperature around the MAX6675 and the 0 ° C practical reference value. After the cold junction temperature compensation diode, the compensation voltage U2 is generated and input to the ADC converter through S4.

U2 = (41μV / ℃) × T0

Under the control of the digital controller, the ADC first converts U1 and U2 into digital quantities, that is, the data of the output voltage U0 is obtained, and this data represents the practical temperature value T of the measurement point. This is how the MAX6675 stops cold junction temperature compensation and temperature measurement.
2. Pin function

The MAX6675 uses an SO-8 package. It has 8 pins. Pin 1 (GND) is grounded. Pin 2 (T-) is connected to the thermocouple negative pole. Pin 3 (T +) is connected to the thermocouple positive pole. Pin 4 (VCC) is the power terminal. Pin 5 (SCK) serial clock input pin, pin 6 (CS) chip select pin, enable serial data communication, pin 7 (SO) serial data output pin, pin 8 (NC) is not used. Connect a 0.1μF capacitor between VCC and GND.

丝不均质影响(1)热电偶材质自身不均质热电偶在计量室检定时,按规程请求,插入检定炉内的深度只要300mm。 K-type thermocouple wire heterogeneity impact (1) Thermocouple material itself heterogeneous thermocouple inspection time in the measurement room, according to the requirements of the procedure, the depth of insertion into the verification furnace is only 300mm. Therefore, the verification result of each thermocouple can only show or mainly show the thermoelectric behavior of the 300mm long wire from the measurement end. However, when the length of the thermocouple is long, most of the wire is in the high temperature region. If the thermocouple wire is homogeneous, then according to the homogeneous circuit rule, the measurement result has nothing to do with the length. However, the thermocouple wire is not homogeneous, especially the cheap metal thermocouple wire has poor homogeneity and is in a place with a temperature gradient, then a part of it will generate a thermoelectromotive force, which is called a parasitic potential. Errors caused by parasitic potentials are called heterogeneous errors.
In the existing noble metal and inexpensive metal thermocouple verification regulations, there is no rule for heterogeneity of the thermocouple, as long as there is a certain request for the unevenness of the thermocouple wire in the thermocouple wire specification. The unbalanced thermo-electromotive force was obtained by the end-to-end test method for the inexpensive metal thermocouple. The normal rail thermocouple wire consumption plants all requested the products according to national standards to consume products with an uneven thermoelectromotive force.
(2) Heterogeneity caused by the use of thermocouple wires. Even if the thermocouple of the new system meets the requirements of the new thermocouple, the processing distortion caused by repeated processing and bending will lose the homogeneity. Moreover, the thermocouple used in the long-term under high temperature will also cause the thermoelectromotive force change due to the deterioration of the wire. For example, a thermocouple inserted into an industrial furnace will deteriorate along the length of the wire, and the deterioration will increase as the temperature increases. The degraded part is located in a place with a temperature gradient, and the parasitic electromotive force is also superimposed on the total thermoelectromotive force to present a measurement error.
It is found in the theory that some thermocouples that have passed the inspection by the metrology department (mostly cheap metal thermocouples) are unqualified when they are used in the field. After returning to the metrology department, the test is still qualified, and the main reason is caused by even heterogeneity. The technicians of consumer thermocouples have personally realized that the failure rate of thermocouples also increases with their length. Both are affected by the heterogeneity of the thermocouple wire. In short, the error caused by heterogeneity, that is, parasitic electromotive force, depends on the heterogeneity level of the thermocouple wire itself and the size of the temperature gradient, which makes it extremely difficult to quantify it.