——镍铬-镍硅热电偶 K-type thermocouple ——Nichrome-NiSi thermocouple
Nickel-chromium-nickel-silicon thermocouples (K-type thermocouples) are the cheapest metal thermocouples currently in use, and their usage is the sum of other thermocouples. 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. K-type thermocouples cannot be used directly at high temperatures in sulfur, reducing or reducing, oxidizing and vacuum atmospheres, and are not recommended for use in weakly oxidizing atmospheres. Two different metal conductors are welded together to form a closed loop. If a temperature difference occurs at the welding end (that is, the measurement end), a thermoelectromotive force will be generated in the loop. This phenomenon is called Seebeck-effect. ). If the temperature at the other end (ie the reference end) is kept constant (generally 0 ° C), then the thermoelectromotive force of the loop becomes a single-valued function of the temperature at the measurement end. This type of element that measures temperature by measuring thermoelectromotive force, that is, two pairs of metal conductors, is called a thermocouple.
具有线性度好，热电动势较大，灵敏度高，稳定性和均匀性较好，抗氧化性能强，价格便宜等优点，能用于氧化性惰性气氛中。 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. 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, k-type thermocouples, armored thermocouples, platinum and rhodium Thermocouple, wear-resistant thermocouple, s-type thermocouple, thermocouple protection tube, thermocouple compensation wire, thermocouple thermometer, fast thermocouple
In the SMT industry, in order to meet the needs of automated mass production, most companies use a reflow furnace with a continuous tunnel structure. Such reflow ovens generally have at least 3 temperature zones. Since the temperature change on the printed board is much more complicated than the display temperature of the meter, it is difficult for the reflow oven operator to adjust the temperature and transmission speed of this reflow oven to a short time based on experience. Best state. Therefore, the probe of the filamentous K-type thermocouple must be fixed to the monitoring point of the printed board with solder or high temperature adhesive. The temperature recorder and the printed board pass through the furnace with the furnace's conveyor network or conveyor chain. At the same time, the recorder automatically samples the temperature signal of the thermocouple at predetermined time intervals, and saves the time-varying temperature data in the non-volatile memory of the recorder. During this process, the external temperature of the temperature recorder may reach above 270 ℃, and its internal temperature is also about 60 ℃ after taking necessary insulation technology. 的理论冷端温度为纯水冰点温度(0 ℃) ,所以必须对此给予补偿。 The theoretical cold -junction temperature of K-type thermocouple is the freezing temperature of pure water (0 ℃), so it must be compensated. Hardware system solution
Existing products use three methods to measure the cold-junction ambient temperature.
(1) Directly borrow the internal temperature sensor of the CPU, such as CF020 from Cygnal. However, first, the internal temperature field of the recorder is not uniform, and the temperature of the access point of the hot pair compensation line is different from the surface temperature of the CPU.Second, the sensitivity of the integrated temperature sensor is generally 0.1 ℃ and the accuracy is ± 2 ℃, which is difficult to meet the measurement Claim.
(2) Use a new intelligent temperature sensor, such as Maxim DS1626, 12-bit sampling accuracy, 3-wire serial data communication, 0 ℃ to + 70 ℃, 2.7V <VDD <3.0V, sensitivity 0.0625 ℃, maximum error ± 0.5 ℃. However, this method also has the problem of the hysteresis effect of the chip case on the ambient temperature. In addition, the maximum change rate of the ambient temperature inside the instrument may reach 1 ℃ / S, and the chip's electrical characteristics require the sampling period to exceed 0.75S, which is relatively long.