The difference between thermocouple and thermal resistance
Although they are all contact temperature measuring instruments, their temperature range is different.
Thermocouples are used in higher temperature environments because they have a small output thermoelectric potential in the middle and low temperature zones (see table for a look). When the potential is small, the countermeasures against interference and secondary meters are very high. Otherwise, the measurement is not Precisely, in the lower temperature region, the relative error caused by the change of the cold junction temperature and the change of the ambient temperature is very prominent, and it is difficult to obtain full compensation.
At this time, at low and medium temperatures, the thermal resistance range of 200~500 is generally used, and even lower temperatures can be measured (for example, a low temperature of about 1K can be measured with carbon resistance). Now the platinum thermal resistance Pt100 is normally used. (There are also Pt50, 100 and 50 represent the resistance of the thermal resistance at 0 degrees. In the old indexing number, it is represented by BA1 and BA2. BA1 has a resistance of 46 ohms at 0 degrees, and copper resistance is also used in industry. The indexing numbers are CU50 and CU100, but the temperature range is small, between 50~~150. In some special occasions, there are indium resistors, manganese resistors, etc.)
2. The basic principle of thermocouple measurement of temperature is the thermoelectric effect. The secondary meter is a voltmeter or an electronic potentiometer is used to improve accuracy. The resistance is based on the characteristics of the resistance of the conductor and the semiconductor as a function of temperature. The secondary meter is an unbalanced bridge.
3. According to the thermocouple temperature measurement principle, the measured temperature is a single-valued function relationship with the thermoelectric potential only when the cold junction temperature is constant. In actual use, a thermoelectric characteristic is similar to that of the corresponding thermocouple. Cheap connecting wires (also called compensation wires), so that the cold end of the thermocouple is extended to a relatively constant temperature (preferably 0 degrees), such as copper--constantan as a compensating wire to extend nickel-chromium-nickel Silicon thermal resistance. Therefore, the extension of the thermocouple to the secondary meter is two.
The thermal resistance and the secondary meter are connected by copper wires. In order to reduce the measurement error caused by environmental changes, three-wire connection method is generally adopted, and two wires connect the thermal resistance in series to the adjacent two bridge arms. On the other side, the other wire is the power source. The sum of the resistance value of each wire and the adjustment resistance is guaranteed to be 5 ohms (±0.01).
On-the-spot judgment at work.
1. Thermocouple. Thermocouple has positive and negative poles, and the compensation wire also has positive and negative points. Firstly, the connection is ensured. The configuration is correct. In operation, there are common short circuits, open circuits, poor contact (multiple gauges can be judged) and deterioration (according to Surface color to identify). When checking, make the thermocouple separate from the secondary meter. I judge the method in practice for your reference: short the compensation line on the secondary meter with the tool, the table indicates the room temperature (if not, Table is bad), then short the thermocouple terminal block, the table indicates the ambient temperature of the thermocouple (not, the compensation line is faulty), and then use the multimeter mv file to roughly estimate the thermoelectric potential of the thermocouple (if normal, please check the process).
2. Thermal resistance. No more than short circuit, and open circuit. Can be judged with a multimeter. In operation, suspect short circuit, just remove the wire end of the resistor end, look at the display instrument, such as to the maximum, short circuit of the thermal resistance. Short circuit. To ensure normal connection and configuration, the table value shows low or unstable, the protection tube is likely to enter the water. The display is the largest, the thermal resistance is broken. The display is minimum and short circuit.
Generally, a thermal resistor having a temperature of 300 degrees or less and a thermocouple having a temperature of 300 degrees or more are used. As the temperature changes, the resistance of the thermal resistance changes, and the thermoelectric potential of the thermocouple changes. Thermal resistors currently use copper thermal resistance and platinum thermal resistance. According to different thermal resistance values ​​at 0 degrees, they are divided into different index numbers, such as PT100, PT1000, CU50, etc., PT100 is taken as an example, PT stands for platinum, and 100 represents The resistance of the thermal resistance at 0 degrees is 100 ohms.
Thermocouples currently have K, B, S and other indexing numbers, which represent different materials for different temperature ranges. For example: K type is nickel chromium-nickel silicon material, generally measuring 0-800 degrees, type B is platinum iridium 30-platinum iridium 6, generally measuring 800-1600 degrees.
1. What is the measurement principle of the thermocouple?
The working principle of the thermocouple is based on the Seeb effect, that is, the two ends of the conductors of different components are connected into a loop. If the temperature of the two terminals is different, the physical phenomenon of generating a thermal current in the loop is obtained. The thermocouple consists of two different wires (hot electrodes), one end of which is welded to each other to form the measuring end (also called the working end) of the thermocouple. Insert it into the medium to be tested at the temperature; the other end (reference or free end) of the thermocouple is connected to the display meter. If there is a temperature difference between the measuring end of the thermocouple and the reference junction, the display meter will indicate the thermoelectromotive force generated by the thermocouple.
2. What is the measurement principle of the thermal resistance?
The thermal resistance is measured by the characteristic that the electrical resistance of the metal conductor or the semiconductor changes with the temperature change. The heated portion of the thermal resistor (temperature sensing element) is a skeleton made of a thin metal wire uniformly wound around the insulating material. The substrate is formed on or by a laser sputtering process. When the measured medium has a temperature gradient, the measured temperature is the average temperature of the dielectric layer within the range of the temperature sensing element.
3. What are the advantages of armored thermocouples?
In the IEC1515 standard, the name is "mineral insulated thermocouple cable", that is, inorganic mineral insulated thermoelectric cable. The hot electrode, the insulator and the sheath are formed by integrally drawing, and the outer surface seems to be covered with a layer of "armored", so it is called an armored thermocouple. Compared with general assembled thermocouples, it has the advantages of high pressure resistance, good bendability, good oxidation resistance and long service life.
4. What are the types of thermocouple's indexing numbers? What are the characteristics?
Thermocouple indexing numbers are mainly S, R, B, N, K, E, J, T and so on. Among them, S, R and B belong to precious metal thermocouples, and N, K, E, J and T belong to inexpensive metal thermocouples. The S indexing number is characterized by strong oxidation resistance and should be used continuously in an oxidizing and inert atmosphere. The long-term use temperature is 1400 ° C and short-term 1600 ° C. Among all thermocouples, the S index number has the highest accuracy level and is usually used as a standard thermocouple; the R index number is about 15% larger than the S index number except for the thermal electromotive force, and the other properties are almost identical;} B The indexing number has a very small thermoelectromotive force at room temperature, so it is generally not necessary to compensate the wire during measurement. Its long-term use temperature is 1600 ° C, short-term 1800 ° C. It can be used in an oxidizing or neutral atmosphere or short-term under vacuum conditions. The N index number is characterized by high temperature oxidation resistance at 1300 °C, long-term stability of thermoelectromotive force and reproducibility of short-term thermal cycle, and good nuclear radiation resistance and low temperature resistance, which can partially replace the S index number. Thermocouple; K index is characterized by strong oxidation resistance, suitable for continuous use in oxidizing and inert atmosphere, long-term use temperature of 1000 ° C, short-term 1200 ° C. It is the most widely used in all thermocouples; the E-index is characterized by the highest thermoelectromotive force in the common thermocouples, ie the highest sensitivity. It should be used continuously in an oxidizing and inert atmosphere at a temperature of 0-800 ° C. The J index is characterized by being used in an oxidizing atmosphere (using an upper temperature limit of 750 ° C) or a reducing atmosphere (using a temperature upper limit of 950). °C), and resistant to H2 and CO gas corrosion, mostly used in refining and chemical industry; T indexing is characterized by the highest level of accuracy in all low-cost metal thermocouples, usually used to measure temperatures below 300 °C.
5. How many types of lead wires are there for thermal resistance? What is the impact?
(YH^ The thermal resistance has three types of lead wires: 2-wire system, 3-wire system, and 4-wire system. The 2-wire thermal resistance wiring is simple, but it has to bring in additional error of lead resistance. Therefore, it is not applicable to manufacturing A. The precision of the thermal resistance, and the lead wire and the wire should not be too long in use. The 3-wire system can eliminate the influence of the lead resistance, and the measurement accuracy is higher than the 2-wire system. As the process detection component, it is the most widely used. The influence of the lead resistance can be eliminated, and the influence of the resistance can be eliminated when the resistance of the connecting wire is the same. In the case of high-precision measurement, a 4-wire system is used.
6. What are the advantages and disadvantages of N-type thermocouples compared to K-type thermocouples?
Advantages of N-type thermocouples: - High temperature oxidation resistance and long-term stability. In the positive electrode of K-type thermocouple nickel-chromium, the preferential oxidation of Cr and Si causes uneven alloy composition and thermal electromotive force drift. In the N-type thermocouple, the Cr and Si contents are increased, and the oxidation mode of the nickel-chromium alloy is changed from internal oxidation to external oxidation. Oxidation, so that the oxidation reaction is only carried out on the surface; - low temperature short-term thermal cycle stability is good, and magnetic transformation is inhibited; - strong nuclear radiation resistance. The N-type thermocouple eliminates the enthalpy elements Mn and Co in the K-type, and further enhances the anti-neutron irradiation ability. - The linearity of the thermoelectric characteristics of the N-type thermocouple is better than that of the K-type in the range of 400-1300 °C. Disadvantages of N-type thermocouples: - The material of the N-type thermocouple is harder than the K-type, which is more difficult to process; - The price is relatively expensive. The thermal expansion coefficient of the N-type thermocouple is 15% lower than that of the stainless steel "target=_blank> stainless steel. Therefore, the outer casing of the N-type armored thermocouple should be made of NiCrSi/NiSi alloy; the nonlinear error is larger in the range of -200~400 °C. .
7. How to choose thermocouple and thermal resistance?
According to the temperature range selection: Thermocouples are generally selected above 500 °C, and thermal resistors are generally selected below 500 °C; According to the measurement accuracy: select the thermal resistance with higher precision, and select thermocouple with less precision; select according to the measurement range: Thermocouples generally refer to "point" temperatures, which are generally measured as spatial average temperatures.
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