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Cheap thermocouples for higher temperatures
2012-2-13 10:07:10

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

The MAX6675 is a complex monolithic thermocouple-to-digital converter. Mainly include: 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. . on

The K type thermocouple has a voltage change rate of (41 μV / ° C). 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 cold junction temperature compensation

The diode generates a compensation voltage U2 which is input to the ADC converter via 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 the MAX6675 stop cold junction temperature compensation

And the principle of measuring temperature.
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 6 (CS) chip selection, enable serial data communication, pin 7 (SO) serial data output, pin 8 (NC) is not used. Connect a 0.1μF capacitor between VCC and GND.

. 1 Thermocouple wire heterogeneity effect


(1) Thermocouple material itself is heterogeneous

When the thermocouple is inspected in the measuring room, as required by 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 performance of the 300mm long wire from the measurement end.

Thermoelectric behavior. 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, thermocouple wires are not uniform

Quality, especially the cheap metal thermocouple wire, which 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 first to use cheap metal thermocouples

The tail test method calculates the uneven thermoelectromotive force. The positive rail thermocouple wire consumption plants all request in accordance with national regulations. The K-type thermocouple works the same as other principles. The K-type thermocouple uses nickel-chromium alloy as the positive electrode and nickel-silicon alloy as the negative electrode

Two conductors are welded at one end. The welded ends of these two conductors are called K-type hot electrodes, and the welded end is the hot end and the non-welded end is the cold end. When performing temperature measurement, it will be inserted into the medium of the object under test to make it feel hot.

Subject to the temperature of the measured medium, its cold end is placed at a constant temperature, and the electrical measuring instrument is connected with a connecting wire. Due to the different temperatures at the two ends, thermoelectric potentials will be generated in the circuit, and the temperature of the cold junction will remain unchanged.

The generated thermoelectric potential only changes with its hot-end temperature. Therefore, after measuring the thermoelectric potential value with an electrical measuring instrument, the corresponding temperature value can be obtained.


K-type thermocouple is a cheap thermocouple that can measure higher temperature. Because this alloy has good high temperature oxidation resistance, it can be used in oxidizing or neutral media. K type thermocouple temperature measurement range can measure 1000 degrees

Temperature can be measured to 1200 degrees in the short term. It cannot be used in reducing media, otherwise, it will corrode very quickly. In this case, it can only be used for measurements below 500 degrees. It is much cheaper than the S-type thermocouple, and its repeatability is very good.

The potential is large, about 0.041mV / degree, so the sensitivity is very high, and its linearity is very good. Although its measurement accuracy is slightly lower, it can fully meet the industrial temperature measurement requirements, so it is the most commonly used in the industry.

I级精度要求为正负0.4%t,II级精度要求为正负0.75%t,K型热电偶在火电厂里应用很多,表示热电极材料:镍铬镍硅。 The k-type thermocouple requires Class I accuracy of plus or minus 0.4% t, and the class II accuracy requires plus or minus 0.75% t. K-type thermocouples are used in many thermal power plants, indicating the material of the thermal electrode: nickel chromium nickel silicon.

Charge for uneven thermo-emf products that fit the request.


(2) Heterogeneity of thermocouple wire after use


Regarding the newly manufactured thermocouple, even if the uneven thermoelectromotive energy can meet the requirements, the processing distortion caused by repeated processing and bending will lose homogeneity, and the thermocouple will be exposed to high temperatures for a long time during use.

The thermoelectromotive force changes caused by the deterioration of the couple wire. For example, a thermocouple inserted into an industrial furnace will deteriorate along the length of the couple wire, and the deterioration will increase as the temperature increases.

The parasitic electromotive force generated is also superimposed on the total thermoelectromotive force and presents a measurement error.


The author found in theory that some thermocouples that passed the inspection by the metrology department (mostly cheap metal thermocouples) were unqualified when they were used in the field. After returning to the measurement department, the test is still qualified, and the main reason is unevenness.

Quality aroused. 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, caused by heterogeneity, parasitic electromotive force

The error 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.