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.
The most common metal protection tube material for k-type thermocouples produced according to national standards is 1Cr18Ni9Ti, which is a stainless steel tube material protection tube with a wall thickness of not less than 2mm; the diameter of k-type thermocouple material is 3.2mm, and the metal protection tube diameter is 20mm ; The thermocouple test ends are all automatically welded by automatic argon arc welding equipment. The welding seam is smooth, firm and free of trachoma, which can effectively extend the service life of the sensor. The compensation cable connected to the k-type thermocouple can be used with high temperature and low temperature resistance. Shielding structure with characteristics of acid resistance, alkali resistance, oil resistance, abrasion resistance, aging resistance, water resistance and non-flammability, etc. It has excellent anti-interference performance and low capacitance and inductance performance. Chromium-nickel-silicon thermocouple compensation wire; It has excellent electrical insulation performance. Cables with wire-pair shielding and general shielding can effectively prevent external interference and improve measurement accuracy. KCB conductors are composed of multiple stranded alloy wires, and the positive electrode is copper. The negative electrode is nickel-plated copper, the positive electrode is red, and the negative electrode is blue. PVC insulation, the positive electrode of the insulation layer is red, and the negative electrode is blue. PVC sheath.
． 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.
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 strong anti-oxidation performance, suitable for continuous use in an oxidizing and inert atmosphere, good linearity, large thermo-electromotive force, high sensitivity, good stability and uniformity. Among all thermocouples, k-type thermocouples are the most widely used. The accuracy grades of k-type thermocouples are divided into I and II; generally, I choose to increase the price and need to order by agreement; it is the cheapest metal thermocouple with the largest amount at present. The amount used is the sum of other thermocouples; k-type thermocouples are generally assembled with a Ф2.5mm wire, and some have a diameter of 3.2mm. The diameter of the wire is higher than the national standard (Ф2.0mm), and the service life of the product is obvious. Added. 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 to 1600 ° C. Disadvantages: fragile; high aluminum 0 ～ 1800 ℃, high temperature resistant, corrosion resistant ceramic tube. Cons: Fragile. After collecting and consulting the relevant information mentioned above, according to the actual situation at the site, I redesigned and selected the thermocouples for the temperature measurement of the furnace temperature of the two mixed iron furnaces in our factory: 1． The thermocouple wire adopts k-type nickel-chromium-nickel-silicon k-type thermocouple with k division number, and the accuracy grade is Ⅱ. Based on the principle that the diameter of the wire is higher than the national standard (Ф2.0mm) and the service life of the product is significantly increased, I chose the diameter of the wire of 3.2mm; 2. 2. The thermowell's protective sleeve is made of Ф25mm double-layer pipe material with a temperature of 0 ～ 1600 ℃, high temperature resistance and corrosion resistance, and double-layer high aluminum or corundum ceramic pipe. 3. The junction box is made of general waterproof stainless steel junction box; 4. 4. Compensation cable is made of heat-resistant KCB nickel-chromium-nickel-silicon thermocouple compensation wire with shielding structure with high temperature resistance, aging resistance, water resistance and non-flammability; 5. The secondary instrument was also re-selected, and a paperless recorder with a temperature measurement range of 1000 to 1400 ° C and an accuracy level of 0.5 was selected, which can manually receive signals from s-type thermocouples and k-type thermocouples. After the above modification, after many years of practical application, it has been proved that the k-type nickel-chromium-nickel-silicon thermocouple WRNT-130 can fully meet the process requirements for the normal use of the mixed iron furnace (the temperature of the furnace of the mixed iron furnace usually reaches 1300 ° C). (The highest temperature is 1500 ℃, so it is not used), which has achieved the economic effect of saving more than 50,000 yuan in annual production costs; because the thermocouple used is a cheap metal, naturally there is no management problem of recycling, which serves once and for all At that time, it had attracted the attention of the company's leaders. For this reason, we also gave our company a small change and small leather achievement award. Nowadays, as the value of gold increases, the raw material used in industrial s-type thermocouples, platinum, is currently priced at more than 610 yuan per gram, and the recycling price of waste platinum-rhodium 10-platinum wire has also increased to 420 yuan / G; a small project of the author ten years ago, the use of k-type thermocouples instead of s-type thermocouples, the current price of k-type thermocouples is only 1/10 of s-type thermocouples, for which the accumulation of savings for the company The production cost should reach hundreds of thousands of yuan! It also dispenses with the recovery of platinum-rhodium-platinum 10-platinum wires with complicated management processes.
Nowadays, the current leader of our factory arranges for me to manage the instruments and meters of the hot-rolled sheet line with a higher degree of automation. There are two thin-plate soaking furnaces in the hot-rolling line under my jurisdiction. The furnace temperature of the soaking furnace usually reaches 1100 ° C. The highest temperature is only 1300 ° C. Now the s-type thermocouple is used in the high-temperature measurement area. The model WRPG-1340TsL = 457mm (rated temperature range 0-1300 ° C, class II), with a total of 76 installation points. High procurement cost (at the current stage, the unit price is 3300 yuan, and only those with a diameter of wire less than the national standard can be purchased, limited to Ф0.45mm, and the service life is doubled), its annual consumption is about 30, and the total value is ten More than ten thousand yuan; low-temperature area uses k-type thermocouple. The high temperature area of the soaking furnace is very similar to the process environment of the furnace of the mixed iron furnace. The environment of the high temperature area of the soaking furnace without hot metal splashing is better than that of the mixed iron furnace. 热电势信号的PLC模块进行薄板均热炉温度的测量。 Therefore, based on my past experience of successfully applying k-type thermocouples to measure the temperature of the hearth of a mixed-iron furnace, I made a "Reformation Plan for Measuring the Temperature of Sheet Hot Rolling Soaking Furnace". (Coil wire diameter Ф3.2mm, service life is equivalent to s-type thermocouple) to replace the current s-type thermocouple. Use armored k-type thermocouple with high temperature corundum tube double protection sleeve and compensation cable. High temperature resistance, aging resistance, waterproof and non-flammable shield structure. Heat resistance KCB nickel-chromium-nickel-silicon thermocouple compensation wire and PLC module for receiving k-type thermocouple thermoelectric potential signal. Measurement. In order to ensure the convenience of replacement, the k-type thermocouple core can be made according to the size of the current s-type thermocouple core, which can be directly installed. The original protective sleeve can also use a part. After the core is installed, the compensation wire and the Replace the PLC module and modify the PLC program. It can meet the requirements of the process and save production costs.