WZP-230, WZP-120, WZP-121, WZP-130, WZP-131, WZP-220, WZP-221, WZP-230, WZP-231, WZP-320, WZP-321, WZP-330, WZP-331, WZP-420, WZP-421, WZP-430, WZP-431, WZP-520, WZP-521, WZP-530, WZP-531, WZP-620, WZP-621, WZP-630, WZP-631, WZP-720, WZP-721, WZP-730, WZP-731, WZP-820, WZP-821, WZP-830, WZP-831, WZP-920, WZP-921, WZP-930, WZP-931, WZC-120, WZC-121, WZC-130, WZC-131, WZC-220, WZC-221, WZC-230, WZC-231, WZC-320, WZC-321, WZC-330, WZC-331, WZC-420, WZC-421, WZC-430, WZC-431, WZC-520, WZC-521, WZC-530, WZC-531, WZC-620, WZC-621, WZC-630, WZC-631, WZC-720, WZC-721, WZC-730, WZC-731, WZC-820, WZC-821, WZC-830, WZC-831, WZC-920, WZC-921, WZC-930, WZC-931 thermal resistance is the most commonly used temperature detector in the mid-low temperature zone. Thermistor temperature measurement is based on the fact that the resistance value of a metal conductor increases as the temperature increases. Its main features are high measurement accuracy and stable performance. Among them, platinum thermal resistance has the highest measurement accuracy, and it is not only widely used in industrial temperature measurement, but also made into a standard reference instrument. Most of the thermal resistances are made of pure metal materials. Currently, platinum and copper are the most widely used. In addition, thermal resistances such as nickel, manganese and tantalum have been used. Metal thermal resistors commonly use a variety of temperature sensing materials, the most commonly used is platinum wire. In addition to platinum wire, metal thermal resistance materials for industrial measurement include copper, nickel, iron, iron-nickel, and the like.
The temperature measurement principle of the thermal resistance is based on the characteristic that the resistance value of the conductor or the semiconductor changes with temperature to measure the temperature and the temperature-related parameters. Most of the thermal resistors are made of pure metal materials. Platinum and copper are the most widely used. Now, thermal resistance is made of materials such as nickel, manganese and tantalum. Thermal resistance usually requires the resistance signal to be transmitted through a lead to a computer control device or other secondary instrument.
For the installation of the thermal resistance, attention should be paid to the accuracy of temperature measurement, safety and reliability, and convenient maintenance, and does not affect the operation and production operation of the equipment. To meet the above requirements, pay attention to the following points when selecting the mounting location and insertion depth of the RTD:
1. In order to ensure sufficient heat exchange between the measuring end of the thermal resistance and the measured medium, the position of the measuring point should be reasonably selected to avoid installing thermal resistance near the dead angle of the valve, elbow and pipeline and equipment.
2. The thermal resistance of the protective sleeve has heat transfer and heat loss. In order to reduce the measurement error, the thermocouple and the thermal resistance should have sufficient insertion depth:
1) For the thermal resistance of the fluid temperature in the center of the pipe, the measuring end should normally be inserted into the center of the pipe (vertically or tilted). If the diameter of the pipe of the fluid to be tested is 200 mm, the insertion depth of the RTD should be 100 mm;
2) For the temperature measurement of high temperature and high pressure and high speed fluid (such as main steam temperature), in order to reduce the resistance of the protective sleeve to the fluid and prevent the protective sleeve from breaking under the action of the fluid, the protection tube may be inserted in a shallow insertion manner or a hot sleeve type. Thermal resistance. The shallow insertion type thermal resistance protection sleeve shall be inserted into the main steam pipe to a depth of not less than 75 mm; the thermal insertion type thermal resistance shall have a standard insertion depth of 100 mm.
3) If it is necessary to measure the temperature of the flue gas in the flue, even though the flue diameter is 4 m, the thermal resistance insertion depth is 1 m.
4) When the measurement original insertion depth exceeds 1m, it should be installed as vertical as possible, or the support frame and protective sleeve should be installed.
The principle of a thermistor thermometer is to take advantage of the fact that the resistance of a conductor or semiconductor changes with temperature.
The main advantages of the thermal resistance thermometer are: high measurement accuracy and good reproducibility; a large measurement range, especially in low temperature; easy to use in automatic measurement, and also convenient for long distance measurement. Similarly, thermal resistors are also defective, with poor accuracy at high temperatures (greater than 850 ° C); easy to oxidize and not resistant to corrosion.
At present, the materials used for thermistors are mainly platinum, copper, nickel, etc. These materials are mainly linear in the ratio of temperature to resistance in the usual temperature range. Here we mainly introduce platinum resistance thermometers.
Platinum is a precious metal. Its physicochemical properties are very stable, especially its oxidation resistance. It is easy to purify and has good processability. It can be made into a very fine platinum wire, compared with metals such as copper and nickel. It has high resistivity and high reproducibility. It is an ideal thermal resistance material. The disadvantage is that the temperature coefficient of resistance is small, it is easy to become brittle in the reducing medium, and the price is relatively expensive. The purity of platinum is usually expressed by the resistance ratio: W(100)=R100/R0
R100 represents the resistance value at 100 °C; R0 represents the resistance value at 0 °C
According to the IEC standard, the platinum resistance of W(100)=1.3850 initial resistance value R0=100Ω (R0=10Ω) is the industrial standard platinum resistance, and the platinum resistance thermometer of R0=10Ω is thicker, mainly used for measurement. Temperature above 600 °C. The resistance and temperature equation of a platinum resistor is a piecewise equation:
Rt=R0[1+At+Bt2+C(t-100°C)t3] t is expressed at -200～0°C
Rt=R0(1+At+Bt2) t represents 0 to 850 °C
Solving this equation, the temperature value can be known according to the resistance value, but in actual work, the thermal resistance index table can be checked to determine the temperature value according to the resistance value.
According to the standard, platinum thermal resistance is divided into Class A and Class B, Class A temperature measurement tolerance ± (0.15 ° C + 0.002 | t |), Class B temperature measurement tolerance ± (0.3 ° C + 0.005 | t |).
The thermal resistance used in the field is generally an armored thermal resistor. It is composed of a thermal resistor body, an insulating material and a protective tube. The thermal resistor body and the protection tube are welded together, and the insulating material is filled in the middle, so that the thermal resistor body can be well protected. , impact resistant, shock resistant and corrosion resistant.
Platinum thermal resistance has two-wire system, three-wire system and four-wire system. The two-wire system has large errors in measurement and is not used. Now industrial use is generally three-wire system, and laboratory use is generally four-wire system. Here we mainly introduce the wiring of the lower three-wire platinum RTD. The three-wire platinum thermal resistance is connected in parallel with a c-end at the a-side of the resistor, so that the resistors lead out the three terminals a, b, and c. Thus, the resistance of the measuring wire itself introduced by the b-wire can be compensated by the c-wire. The influence of the lead resistance error introduced by the lead resistance not changing with temperature is much reduced. Three-wire platinum thermal resistance, in the secondary instrument, there are variable resistance bridges. According to the range of the platinum thermal resistance, the platinum thermal resistance in the bridge of the secondary instrument can be fine-tuned. Make more accurate measurements.
New method for thermal resistance thermometer indexing:
The industrial platinum resistance thermometer is a widely used temperature measuring instrument. For a long time, the CVD equation calculation method has been widely used in the relevant standards or technical specifications at home and abroad for verification and indexing. However, the industrial platinum resistance thermometer using the CVD equation to determine the indexing accuracy is not high, the stability is low, the uncertainty is large, and it cannot be used as a transmission standard.
For this reason, most industrial temperature measurement fields or less demanding laboratories can only use standard platinum resistance thermometers with higher precision as the traceability standard. However, due to various conditions in the actual industrial temperature measurement field, standard platinum resistance thermometers cannot be used. The temperature measurement and traceability are not realized in these places, and the actual measurement and calibration work cannot be carried out.
The feasibility of verifying the industrial platinum thermal resistance thermometer is compared with the temperature-resistance relationship calculated by the commonly used CVD equation, and then the difference between the two is given. The establishment of a precision industrial platinum resistance thermometer is discussed. Ways and means of passing standards. Through the research and analysis of different industrial platinum thermal resistors manufactured by different models and different manufacturers in different temperature zones, the experimental results, data curves and measurement errors caused by the indexing of two different methods are given.
Experiments show that the ITS-1990 international temperature scale interpolation method is feasible for industrial platinum thermal resistance thermometers, and has better accuracy and consistency than the CVD equation used in the calculation method of industrial platinum resistance verification index. Previously, the national metrology institutes in Italy and Canada conducted work on the industrial platinum resistance indexing method using the international temperature scale interpolation formula.
The traditional methods to improve the accuracy and stability of industrial resistance temperature measurement are all in the component purity, packaging technology, and production process; then a new idea is given from the calculation method, which is the transmission of precision platinum resistance and industrial platinum resistance in temperature. And the foundation of the traceability system has laid the foundation, which can be widely used in the field of temperature measurement of industrial platinum resistance.