Industrial K-type thermocouples are used as a temperature sensor. K-type thermocouples are usually used with display instruments, recording instruments and electronic regulators. Assembled K-type thermocouple can directly measure from 0 ℃ to 1300 ℃ in various productions
Range of liquid vapor and gaseous media and solid surface temperatures.
According to national regulations, our company produces assembled thermocouples that conform to the IEC international standard graduation numbers of platinum-rhodium 30-platinum rhodium 6, platinum-rhodium 10-platinum, nickel-chromium-nickel silicon, nickel-chromium- Constantan, and so on.
通常由感温元件、安装固定装置和接线盒等主要部件组成。 K-type thermocouples are usually composed of main components such as temperature sensing elements, mounting fixtures and junction boxes.
The platinum-rhodium 10-platinum thermocouple with s graduation number is commonly known as single platinum rhodium thermocouple or s-type thermocouple. It was made of a precious metal more expensive than gold. At that time, a 1.65m long s-type thermocouple cost about 1800. Yuan, annual consumption of four temperature measurement points
About 30 sticks, with a total value of more than 60,000 yuan; long-term use temperature is in the range of 0 ~ 1400 ℃, short-term 1600 ℃. Its reliable performance and long service life are characterized by strong oxidation resistance. It should be continuously used in an oxidizing and inert atmosphere.
use. Among all thermocouples, s-type thermocouples have the highest accuracy level and are usually used as standard thermocouples; the national standard for the diameter of thermocouple wires is Ф0.5mm, and the weight and service life of precious metals that are lower than the standard will be doubled.
The k-division nickel-chromium-nickel-silicon thermocouple is commonly known as the k-type thermocouple, and its price was only 1/5 of that of the s-type thermocouple at that time. The long-term use temperature is -200 to 1300 ° C and the short-term 1400 ° C. Its reliable performance and long service life are characterized by anti-oxidation
It has strong chemical properties and is suitable for continuous use in an oxidizing and inert atmosphere. It has good linearity, large thermoelectromotive force, high sensitivity, good stability and uniformity. Of all thermocouples, k-type thermocouples are the most widely used.
Accuracy grades are divided into I and II grades. Generally, I will choose a grade I to increase the price, which needs to be ordered by agreement. It is the cheapest metal thermocouple with the largest amount at present. Wire assembly
There are also couples with a diameter of 3.2mm, the diameter of the even wire is higher than the national standard (Ф2.0mm), and the service life of the product is significantly increased.
The Ф25mm double-layer tube protective sleeve material used in s-type thermocouples is double-layer high-aluminum or corundum ceramics; corundum is a ceramic tube suitable for high temperature and corrosion resistance at a temperature of 0 ~ 1600 ° C. Disadvantages: fragile; high aluminum
0 ～ 1800 ℃, high temperature and corrosion resistant ceramic tube. Cons: Fragile.
The most common metal protection tube material for k-type thermocouples produced according to national standards is 1Cr18Ni9Ti, which is a stainless steel tube protection tube with a wall thickness of not less than 2mm; the diameter of the k-type thermocouple material is 3.2mm.
Pipe diameter Ф20mm; all thermocouple test ends are automatically welded using automatic argon arc welding equipment. The weld seam is smooth, firm and free of pits, which can effectively extend the service life of the sensor.
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. . About K type
The thermocouple has a voltage change rate of (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. Cold junction temperature compensated diode
Tube, generating a compensation voltage U2 input 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 how MAX6675 stops 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.
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 test result of each thermocouple can only show or mainly show 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, thermocouple wires are 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 thermoelectromotive force, which is called 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. End-to-end for cheap metal thermocouples
The test method calculates the uneven thermoelectromotive force. 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 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.
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. The main reason is the unevenness of the silk.
Caused. 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.