1. thermocouple measurement principle is what?
The working principle of thermocouple is based on the Seebeck (seeback) effect, that is, two different components of the conductor ends are connected to form a loop, such as the two connecting ends of the temperature is different, then in the loop to produce a thermal current of physical phenomena.
A thermocouple consists of two different wires (heat electrodes), which are welded to each other at one end to form the measurement end of the thermocouple (also called the working end). It is inserted into the medium whose temperature is to be measured; the other end of the thermocouple (the reference or free end) is connected to the display instrument. If there is a temperature difference between the measuring end of the thermocouple and the reference end, the display meter will point out the thermoelectric potential generated by the thermocouple.
2. What is the measuring principle of RTD?
RTD is the use of metal conductors or semiconductors have a temperature change in its own resistance changes with the characteristics of the measurement of temperature, the heated portion of the RTD (temperature sensing element) is a fine wire uniformly wrapped around the skeleton made of insulating material or formed in the substrate through the laser sputtering process. When the measured medium has a temperature gradient, the measured temperature is the average temperature of the medium layer within the range of the sensing element.
3. How to choose thermocouples and RTDs?
According to the choice of temperature range: 500 ℃ above the general choice of thermocouple, 500 ℃ below the general choice of RTD;
According to the choice of measurement accuracy: high precision requirements for the choice of RTD, the precision requirements for the choice of thermocouple;
According to the choice of the measurement range: thermocouple measurement generally refers to the "point" temperature, RTD, the measurement range is the average temperature of the medium layer in the range of the sensing element.
In the IEC1515 standard name for "mineral insulated thermocouple cable", that is, inorganic mineral insulated thermocouple cable. The heat electrode, insulation and sheath through the whole system and formed, the outer surface seems to be covered with a layer of "armor", so it is called armored thermocouple. Compared with the general assembly thermocouple, with high pressure resistance, good bending performance, good oxidation resistance and long service life and other advantages.
5. Thermocouple graduation number has which kinds? What are the characteristics?
Thermocouple graduation number there are mainly S, R, B, N, K, E, J, T and so on several kinds. Among them, S, R, B belongs to the precious metal thermocouple, N, K, E, J, T belongs to the cheap metal thermocouple.
S index number is characterized by strong oxidation resistance, suitable for continuous use in oxidizing, inert atmosphere, long-term use temperature 1400 ℃, short-term 1600 ℃. In all thermocouples, S scale number of the highest accuracy level, usually used as a standard thermocouple;
R scale number compared with the S scale number in addition to the thermal electromotive force is about 15% larger, other properties are almost identical;}B scale number at room temperature thermal electromotive force is very small, so in the measurement of the general do not need to compensate for the wire. Its long-term use temperature of 1600 ℃, short-term 1800 ℃. It can be used in oxidizing or neutral atmosphere, and can also be used in vacuum conditions for short-term use.
N index number is characterized by 1300 ℃ high-temperature oxidation resistance, long-term stability of the thermoelectric potential and short-term thermal cycling of the reproducibility of good, nuclear irradiation and low temperature resistance performance is also good, can be partially replaced by the S index number of thermocouples;
K index number is characterized by strong resistance to oxidation, suitable for oxidative and inert atmosphere for continuous use, the long-term use of the temperature of 1000 ℃, the short-term 1200 ℃. Short-term 1200℃. In all thermocouples in the most widely used;
E index number is characterized by the common thermocouple, its thermal electric potential is the largest, that is, the highest sensitivity. Suitable for continuous use in oxidizing, inert atmosphere, use temperature 0-800 ℃;
J index number is characterized by both oxidizing atmosphere (use temperature upper limit of 750 ℃), can also be used for reducing atmosphere (use temperature upper limit of 950 ℃), and resistant to corrosion of H2 and CO gases, mostly used in refining and chemical industry;
T index number is characterized by the highest level of accuracy among all the cheap metal The T scale is characterized by the highest accuracy level among all the cheap metal thermocouples and is usually used to measure temperatures below 300°C.
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6. How many kinds of lead wires for RTD? What are the effects?
There are 3 types of lead wires for RTD: 2-wire, 3-wire and 4-wire.
2-wire RTD wiring is simple, but it is necessary to bring in the additional error of the lead resistance. Therefore, it is not suitable for manufacturing A-level precision RTDs, and the leads and wires should not be too long when used.
The 3-wire system can eliminate the influence of lead resistance, and the measurement accuracy is higher than the 2-wire system. As a process detection element, it is most widely used.
The 4-wire system not only eliminates the effect of lead wire resistance, but also eliminates the effect of this resistance when the connected wires have the same resistance value. The 4-wire system is to be used for high-precision measurements.
7. What are the advantages and disadvantages of N-type thermocouple compared with K-type thermocouple?
N-type thermocouple advantages:
- high-temperature oxidation resistance, long-term stability. K-type thermocouple nickel-chromium in the positive pole of Cr, Si elements of the preferential oxidation of the alloy caused by the composition of non-uniformity and thermal electromotive force drift, etc., in the N-type thermocouple to increase the content of Cr, Si, so that the nickel-chromium alloy oxidation mode from the oxidation of the internal oxidation to the external oxidation, resulting in oxidation reaction only on the surface;
- low-temperature oxidation resistance.
- low temperature short-term thermal cycle stability, and inhibit the magnetic transition;
- resistance to nuclear radiation ability. N-type thermocouple canceled the K-type easy to metamorphosis elements Mn, Co, so that the resistance to neutron irradiation ability to be further strengthened;
- in the range of 400 ~ 1300 ℃, the N-type thermocouple thermoelectric properties of the linearity of the better than the type K.
- N-type thermocouple, the N-type thermoelectric properties of the linearity of the K-type.
Disadvantages of type N thermocouple:
-The material of type N thermocouple is harder than type K, which is more difficult to process;
-Relatively more expensive.The coefficient of thermal expansion of type N thermocouple is 15% lower than stainless steel, so the outer sleeve pipe of the type N armored thermocouple should be made of NiCrSi/NiSi alloy;
-In the range of -200~400℃. Non-linear error is larger.
8. How to choose the right thermal installation sleeve?
The shape of the thermal installation casing is mainly based on the temperature, pressure, density and flow rate of the medium and the required insertion length, which is fully specified in ASME/ANSI PTC19.3. The casing strength analysis software can be used to calculate whether the casing design meets the process requirements. Installed in the field of heat casing need to calculate the strength of the heat casing, the impact of the strength of the sheath tube are mainly the following three points:
1. flow-induced vibration; through the sheath tube of the liquid produces a certain frequency of vortex, known as vortex frequency, the frequency of the flow rate is proportional to the flow rate. If this frequency and the intrinsic frequency of the heat pipe is close to or consistent, will produce *** vibration, so that the absorption of large amounts of heat energy, thus generating high stress and may damage the heat pipe and casing sensors. ASME technical standards require: vortex area frequency and the ratio of the intrinsic frequency of the heat pipe should be less than 0.8.
2. flow-induced stress; fluid flow with the flow rate and density and changes, and the heat pipe exerts a force on the flow rate of the fluid. And in the heat casing exerts a force, this flow-induced stress can be derived by calculation.
3. process pressure; the maximum static pressure that the heat sleeve can withstand can be calculated." The general thermal installation casing is connected by threaded connection type, flange connection type and welding type.
9. How to choose the right bimetal thermometer?
Horizontal installation, select the axial or universal bimetallic thermometer;
Vertical installation, select the radial or universal bimetallic thermometer;
Inclined installation, according to the actual need to select the axial, radial or universal bimetallic thermometer;
If you need to set up a measuring point of the upper and lower limits of the alarm control, you can select the electric contact bimetallic thermometer <
10. What are the advantages and disadvantages of bimetal thermometers?
The advantages of bimetallic thermometers are relatively inexpensive, intuitive readings, the disadvantage of the temperature measurement range is small, relatively low precision. Usually as an in situ measurement, display instrumentation.
11. What are the characteristics of temperature transmitters?
Temperature transmitters are characterized by
- low static power consumption, safe and reliable, no maintenance, long service life.
-Smaller volume, can be integrated with thermocouples, RTDs, not only easy to install, but also save temperature converter installation costs.
-Transmission signal is 4-20mA standard signal, not only anti-interference ability, transmission distance, and can save the more expensive compensation wire.
-Conforms to HART protocol and FF, PROFBUS communication protocol.
12. What is the measuring principle of pressure thermometer?
Based on the law of expansion of liquids, that is, a certain mass of liquid, under the condition of constant volume, the pressure and temperature of the liquid is linear. Gas, steam pressure and temperature is also a function of a certain relationship, so the pressure thermometer scale should be evenly divided. Pressure thermometer is filled with temperature-sensitive medium temperature package, pressure transfer element (capillary tube) and pressure-sensitive components (spring tube).
13. Infrared thermometer measurement principle?
Infrared thermometer by the optical system, photodetector, signal amplifier and signal processing. Display output and other components. Optical system convergence of its field of view of the target infrared radiation energy, infrared energy focused on the photoelectric detector and transformed into the corresponding electrical signals, the signal is then converted into the measured target temperature value.
14. How to choose the right compensation wire or cable?
Compensating wires and cables for thermocouples are mainly used to extend the thermal electromotive force of thermocouples to the secondary instrument or control room. There are mainly extended and compensated two kinds of compensation wire, extended with the hot electrode of the same material, so the accuracy is higher; compensated with the hot electrode of the thermoelectric potential characteristics of the material, so the accuracy is not as high as the extended type.
The above data is provided free of charge by Beijing Aotome Automation Technology Co.