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K-type thermocouple is the most commonly used temperature sensor in industrial consumption
2012-2-24 10:42:54

是工业消费中最常用的温度传感器,制造比拟容易。 K-type thermocouple is the most commonly used temperature sensor in industrial consumption, and it is easier 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.

1 working principle

MAX6675 is a complex monolithic thermocouple-to-digital converter. Its internal structure is shown in Figure 2. It 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.

产生的热电势,经过低噪声电压放大器A1和电压跟随器A2放大、缓冲后,得到热电势信号U1,再经过S4送至ADC。 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.

. 1 Influence of heterogeneous thermocouple wire (1) The thermocouple material itself is heterogeneous. The thermocouple is inspected in the measuring room. According to the requirements of the regulations, 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.
丝经运用后产生的不均质关于新制的热电偶,即便是不平均热电动势能满足请求,但是,重复加工、弯曲致使热电偶产生加工畸变,也将失去均质性,而且运用中热电偶长期处于高温下也会因偶丝的劣化而惹起热电动势变化,例如:插入工业炉中的热电偶,将沿偶丝长度方向发作劣化,并随温度增高,劣化加强,当劣化的局部处于具有温度梯度的场所,也将产生寄生电动势叠加在总热电动势中而呈现丈量误差。 (2) Heterogeneity caused by the use of K-type thermocouple wire. 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. In addition, the thermocouples in use during long-term high temperature will cause changes in the thermoelectromotive force due to the deterioration of the wire. For example, a thermocouple inserted into an industrial furnace will degrade along the length of the wire, and the deterioration will increase as the temperature increases. When the degraded part is in a place with a temperature gradient, a parasitic electromotive force will also be superimposed on the total thermoelectromotive force and a measurement error will appear.
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.