In order to facilitate customers' better understanding of some knowledge about thermocouples, our company has collected 14 common questions about thermocouples for your convenience:
What is the measurement principle of thermocouple?
The working principle of a thermocouple is based on the Seeback effect, which is a physical phenomenon in which two conductors with different compositions are connected at both ends to form a loop. If the temperature of the two connection ends is different, a thermal current is generated in the loop.
A thermocouple is composed of two different wires (thermoelectric poles), one end of which is welded to each other, forming the measuring end (also known as the working end) of the thermocouple. Insert it into the medium at the temperature to be measured; The other end of the thermocouple (reference end or free end) is connected to the display instrument. If there is a temperature difference between the measuring end and the reference end of the thermocouple, the display instrument will indicate the thermoelectric potential generated by the thermocouple.
What is the measurement principle of thermal resistance?
Thermistor measures temperature by utilizing the characteristic that the resistance of a metal conductor or semiconductor changes with temperature. The heated part (temperature sensing element) of a thermistor is formed by uniformly winding thin metal wires around a skeleton made of insulating material or by laser sputtering on a substrate. When the measured medium has a temperature gradient, the measured temperature is the average temperature of the medium layer within the range of the temperature sensing element.
How to choose thermocouples and thermistors?
According to the temperature measurement range, thermocouples are generally selected for temperatures above 500 ℃, while thermistors are generally selected for temperatures below 500 ℃;
Select based on measurement accuracy: choose a thermistor for high accuracy requirements and a thermocouple for low accuracy requirements;
According to the measurement range selection: the temperature measured by thermocouples generally refers to the "point" temperature, while the temperature measured by thermal resistors generally refers to the spatial average temperature;
What is an armored thermocouple and what are its advantages?
In the IEC1515 standard, it is called 'Mineral Insulated Thermocouple Cable',
Inorganic mineral insulated thermocouple cable. The thermocouple is formed by integral drawing of the thermoelectric electrode, insulation material, and sheath, and its outer surface appears to be covered with a layer of "armor", hence it is called an armored thermocouple. Compared with general assembled thermocouples, it has the advantages of high voltage resistance, good bendability, good oxidation resistance, and long service life.
What are the division marks of thermocouples? What are the characteristics?
The division marks of thermocouples mainly include S, R, B, N, K, E, J, T, etc. Among them, S, R, and B belong to precious metal thermocouples, while N, K, E, J, and T belong to low-cost metal thermocouples.
The characteristic of S scale is strong antioxidant performance, suitable for continuous use in an oxidizing and inert atmosphere, with a long-term use temperature of 1400 ℃ and a short-term use temperature of 1600 ℃. Among all thermocouples, the S scale has the highest degree level and is usually used as a standard thermocouple;
Compared with the S scale, the R scale has almost the same performance except for a 15% increase in thermoelectric potential; }
The B division mark has a very small thermoelectric potential at room temperature, so compensation wires are generally not needed during measurement. Its long-term usage temperature is 1600 ℃, and its short-term usage temperature is 1800 ℃. It can be used in oxidizing or neutral atmospheres, as well as for short-term use under vacuum conditions.
The characteristics of the N scale are strong high-temperature oxidation resistance at 1300 ℃, good long-term stability of thermoelectric potential and reproducibility of short-term thermal cycles, and good resistance to nuclear irradiation and low temperature. It can partially replace the S scale thermocouple;
The characteristic of K scale is strong antioxidant performance, suitable for continuous use in an oxidizing and inert atmosphere, with a long-term use temperature of 1000 ℃ and a short-term use temperature of 1200 ℃. Widely used among all thermocouples;
The characteristic of the E-scale is that it has the highest thermoelectric potential and sensitivity among commonly used thermocouples. Suitable for continuous use in an oxidizing and inert atmosphere, with a temperature range of 0-800 ℃;
The characteristic of the J division mark is that it can be used in both oxidizing atmospheres (with an upper temperature limit of 750 ℃) and reducing atmospheres (with an upper temperature limit of 950 ℃), and is resistant to H2 and CO gas corrosion. It is commonly used in refining and chemical industries;
The characteristic of the T-scale is that it has the highest grade among all low-cost metal thermocouples and is usually used to measure temperatures below 300 ℃.
How many ways are there to lead out thermal resistors? What impact does it have?
There are three ways to lead out thermal resistors: 2-wire system, 3-wire system, and 4-wire system.
The wiring of 2-wire heating resistors is simple, but it requires additional errors in the lead resistors. Therefore, it is not suitable for manufacturing Class A precision thermistors, and the leads and wires should not be too long during use.
The 3-wire system can eliminate the influence of lead resistance and has higher measurement accuracy than the 2-wire system. As a process detection component, it has the widest application.
The 4-wire system can not only eliminate the influence of lead resistance, but also eliminate the influence of the same resistance when connecting wires with the same resistance. When measuring with high precision, a 4-wire system should be used.
What are the advantages and disadvantages of N-type thermocouples compared to K-type thermocouples?
Advantages of N-type thermocouple:
-Strong high-temperature antioxidant capacity and long-term stability. The preferential oxidation of Cr and Si elements in the positive electrode of K-type thermocouple nickel chromium causes uneven alloy composition and thermoelectric drift. Increasing the Cr and Si content in N-type thermocouple changes the oxidation mode of nickel chromium alloy from internal oxidation to external oxidation, resulting in oxidation reaction only occurring on the surface;
-Low temperature short-term thermal cycling has good stability and suppresses magnetic transformation;
-Strong resistance to nuclear radiation. The N-type thermocouple eliminates the easily degradable elements Mn and Co in the K-type, further enhancing its resistance to neutron irradiation;
-The linearity of the thermoelectric characteristics of N-type thermocouples is better than that of K-type thermocouples in the temperature range of 400~1300 ℃.
Disadvantages of N-type thermocouples:
-The material of N-type thermocouple is harder than that of K-type and more difficult to process;
-The price is relatively expensive. The thermal expansion coefficient of N-type thermocouples is 15% lower than that of stainless steel, so the outer sleeve of N-type armored thermocouples should be made of NiCrSi/NiSi alloy; -The nonlinear error is relatively large within the range of -200~400 ℃.
8. How to choose a suitable hot installation sleeve?
The shape of the hot installation sleeve is mainly determined by the temperature, pressure, density, flow rate, and required insertion length of the medium. ASME/ANSI PTC19.3 has made sufficient provisions for this, and the use of casing strength analysis software can calculate whether the casing design meets the process requirements. The thermal sleeve installed on site needs to be calculated
The strength of the pipe affects the strength of the protective sleeve mainly in the following three aspects:
1. Vibration caused by flow; The liquid passing through the protective casing generates a vortex of a certain frequency, called the vortex zone frequency, which is proportional to the flow velocity. If this frequency is close to or consistent with the natural frequency of the thermal sleeve, resonance will occur, causing the absorption of a large amount of thermal energy, resulting in high stress and potentially damaging the thermal sleeve and the sensors inside the sleeve. ASME technical standards require that the ratio of vortex zone frequency to the natural frequency of the heat pipe should be less than 0.8.
2. Stress caused by flow; The fluid flow varies with velocity and density, and exerts a force on the thermal casing. The pressure caused by this flow can be calculated.
3. Process pressure; The maximum static pressure that the hot sleeve can withstand can be calculated. '
The general connection methods for hot installation sleeves include threaded connection, flange connection, and welding.
How to choose a suitable bimetallic thermometer?
When installing horizontally, choose axial or universal bimetallic thermometers;
When installing vertically, choose a radial or universal bimetallic thermometer;
When installing at an angle, choose axial, radial, or universal bimetallic thermometers according to actual needs;
When setting upper and lower limit alarm control for measuring points, an electric contact bimetallic thermometer can be selected
What are the advantages and disadvantages of bimetallic thermometers?
The advantages of bimetallic thermometers are relatively low price and intuitive readings, while the disadvantages are small temperature measurement range and relatively low accuracy. Usually used as on-site measurement and display instruments.
What are the characteristics of temperature transmitters?
The characteristics of temperature transmitters are low static power consumption, safety and reliability, no need for maintenance, and long service life. -Small in size, it can be integrated with thermocouples and thermal resistors, making it not only easy to install, but also saving installation costs for temperature transformers.
-The transmission signal is a 4-20mA standard signal, which not only has strong anti-interference ability and long transmission distance, but also can save expensive compensation wires.
-Can provide communication protocols that comply with the HART protocol and FF/PROFBUS bus.
What is the measurement principle of pressure thermometer?
According to the law of liquid expansion, a certain mass of liquid has a linear relationship between pressure and temperature under constant volume conditions. The pressure and temperature of gases and vapors also exhibit a certain functional relationship,
Therefore, the scale of the pressure thermometer should be evenly divided. A pressure thermometer is composed of a temperature bulb filled with a temperature sensing medium, a pressure transmitting element (capillary tube), and a pressure sensitive element (spring tube).
What is the measurement principle of infrared thermometer?
The infrared thermometer consists of an optical system, a photodetector, a signal amplifier, and signal processing Composed of display output and other components. The optical system gathers the infrared radiation energy of the target within its field of view, and the infrared energy is focused on the photodetector and converted into corresponding electrical signals, which are then converted into the temperature value of the measured target.
14. How to choose suitable compensating wires or cables?
The compensation wires and cables of thermocouples are mainly used to extend the thermoelectric potential of thermocouples to secondary instruments or control rooms. There are mainly two types of compensation wires: extension type and compensation type. The extension type uses the same material as the thermoelectric electrode, so the accuracy is higher; The compensation type uses materials that are in phase with the thermoelectric potential characteristics of the thermoelectric electrode, so the accuracy is not as high as the extension type.