The requirements of using environment for cast copper electric heater: if you want the working efficiency and service life of explosion-proof electric heater not to be affected, we should pay attention to the usual use environment. Cast iron electric heater should be used in normal environment. In daily use, explosion-proof electric heater has strict requirements for the use environment. In order to avoid affecting the working efficiency and service life of explosion-proof electric heater, the environmental requirements should be paid attention to.
The proportion of cast aluminum electric heater in the mechanical industry is relatively large. When installing the explosion-proof electric heater pump, the technical personnel found that when the explosion-proof electric heater works in a certain pipeline, the actual working state of the explosion-proof electric heater not only depends on the performance curve of the electric heater itself, but also depends on the performance curve of the electric heater itself In the research of the heater, it is found that the working voltage of the explosion-proof electric heater also depends on the characteristics of the whole pipeline of the explosion-proof electric heater.
When purchasing explosion-proof electric heater, it is necessary to have more understanding of the specification of explosion-proof electric heater, so as to purchase safe and qualified products, and have better guarantee in reliability, and the reliability that can be achieved will be better. Therefore, how to choose the explosion-proof electric heater should be started according to the actual situation, and the advantages that can be brought are also very excellent.
According to the specifications of explosion-proof heaters, manufacturers with more professional explosion-proof electric heating belts can provide better quality electric heaters, and their functions and uses will be more obvious. These are the links worthy of our more attention. Therefore, how to choose better is also an important part, and it will be reflected in cost performance.
Of course, an electric heater that can heat, warm and keep warm the flowing or static solid-liquid gas medium is also very obvious. It can be used in explosion-proof occasions or household occasions to provide electric energy into heat energy. Moreover, it has better guarantee on the stability of use, and its advantages will be more obvious.
The development of digital sensor has gradually become a new favorite in the field of weighing technology, to adapt to the advantages of on-site ability. Now the cars are using sensors everywhere, so where are the advantages of digital sensors?
1. The digital sensor has strong anti-interference ability: it adopts integrated A / D conversion circuit, digital signal transmission and digital filtering technology. The signal transmission distance of the sensor is far, up to 1200m, with strong anti-interference ability, and the transmission distance of analog signal in digital sensor is extremely short.
At the same time, the sensor shell (elastomer) itself is a good shielding cover, only these two characteristics determine the advantages of its anti-interference ability, greatly improving the stability of the sensor.
2. Convenient for fault diagnosis: since the digital sensor has the functions of automatic acquisition, pre-processing, storage and memory, and has a unique mark, the status of each sensor can be checked respectively after multiple sensors are connected in parallel to facilitate fault diagnosis.
3. Good confidentiality, with anti cheating function: can effectively prevent remote control cheating, once found will automatically take error alarm, effectively ensure the security and accuracy of data.
Because of its long cycle life, small size, light weight and no pollution, lithium batteries are widely used in mobile phones, notebook computers, electric vehicles, street lamp backup power supply, small household appliances, etc. In lithium battery, thermistor with negative temperature coefficient (NTC) is usually installed. NTC thermistor can prevent the battery from charging at too high or too low temperature, so as to avoid lithium battery overheating and cause accidents.
The use of SMT NTC thermistor has the advantages of high assembly density, small volume and light weight. The volume and weight of SMT NTC thermistor are only about 1 / 10 of that of plug-in thermistor. With SMD NTC thermistor, the volume and weight of electronic products can be reduced by 40% ~ 60% and 60% ~ 80% respectively. The chip NTC thermistor has high reliability, strong anti vibration ability, low solder joint defect rate, good high frequency characteristics, reducing electromagnetic and radio frequency interference, easy to realize automation and improve production efficiency. As a result, the cost can be reduced by 30% ~ 50% and materials, energy, equipment, manpower and time can be saved.
The use of NTC thermistor has the following advantages:
1. Save production and processing time. The traditional manual welding is simplified and the production time is shortened.
2. Avoid the contact displacement of plug-in NTC thermistor. If the contact of NTC thermistor moves to or around the heating device, the detection temperature of power management IC will deviate greatly.
3. Avoid short circuit. When NTC thermistor is damaged or exposed copper when it is supplied or processed, it is easy to short-circuit connect with some devices on PCB board, thus damaging PCB board or cell, and even causing safety accidents.
4. Avoid displacement and fracture, reduce the probability of human damage. The use of chip type NTC thermistor can effectively prevent NTC thermistor from breaking when it is supplied or processed, and prevent the temperature detection error of power management IC.
T-type thermocouple is also known as copper-constantan thermocouple (copper/nickel-copper thermocouple, graduation number T, measuring range -200~+350℃), and it is also the best thermocouple for measuring low temperature and cheap metals. Its positive electrode (TP) is pure copper, and the negative electrode (TN) is a copper-nickel alloy, which is often constantan. It is commonly used with Ni-Cr-Constantan constantan EN, but not with iron-Constantan constantan JN. Although they are called constantan, the measuring temperature range of copper-copper-nickel thermocouples is -200~350℃.
T-type thermocouple
T-type thermocouple has the advantages of good linearity, large thermoelectromotive force, high sensitivity, approximate linear temperature and good reproducibility, fast heat transfer, good stability and uniformity, and low price, especially at -200~0℃ Used in the temperature zone, the stability is better, the annual stability can be less than ±3μV, and can be used as a second-class standard for low temperature value transfer after low temperature verification. The positive electrode copper of the T-type thermocouple has poor oxidation resistance at high temperatures, so the upper limit of the operating temperature is limited.
Since the thermoelectric characteristics of various types of thermocouples are different, it is best to use them within the range of their linear segments. At the same time, each type of thermocouple has the highest temperature limit used.
The highest temperature corresponding to different specifications of T-type thermocouples (the thin thermocouple is easy to break, and the smaller the diameter of the thermocouple, the lower the temperature used):
In lithium batteries, a thermistor with a negative temperature coefficient (NTC) is usually installed. The NTC thermistor can prevent the battery from being charged at too high or too low temperatures. Therefore, the battery has three connection terminals: the positive terminal (BAT+), the negative terminal (BAT-) and the connection terminal of the NTC thermistor (see Figure 1). Please note that some batteries with three connections only have ordinary resistors inside for identification. The value of the ordinary resistance will be constant and will not change with the battery temperature.
Figure 1. Most lithium batteries with three connections have an internal NTC thermistor
When using NTC thermistor, it should be connected between THM pin and ground (connected via BAT). A resistor (R7) is also connected between the THM pin and the reference voltage (VL), which creates a voltage divider. Choose the value of the resistor so that it has the same value as the NTC thermistor at a temperature of +25°C. The voltage on the THM pin at +25°C is equal to 0.5 VL. When the temperature rises or falls, the resistance of the NTC thermistor also falls or rises, and the voltage on the THM pin also falls or rises. The device will only charge when this voltage is between 0.28 VL and 0.74 VL. For contemporary NTC thermistors, this corresponds to a temperature between 0°C and 50°C. If there is no NTC thermistor available, R8 should be added, which will result in a voltage of 0.5 VL on the THM pin.
From a physical point of view, heat is a measure of the energy contained in the body due to the irregular movement of its molecules or atoms. Just as tennis balls have more energy with increasing speed, the internal energy of the body or gas increases as the temperature increases. Temperature is a variable that, along with other parameters such as mass and specific heat, describes the energy content of the body.
The basic measure of temperature is Kelvin degree. At 0 ° K (Elvin), every molecule in the body is at rest and there is no more heat. Therefore, there is no possibility of negative temperature because there is no state of lower energy.
In daily use, the usual practice is to use centigrade (formerly centigrade). Its zero point is at the freezing point of water, which can be easily reproduced in practice. Now 0 ° C is by no means the lowest temperature, because everyone knows from experience. By extending the centigrade scale to the lowest temperature at which all molecular motion stops, we reach – 273.15 degrees.
Man has the ability to measure temperature through his senses in a limited range. However, he was unable to accurately reproduce quantitative measurements. The first form of quantitative temperature measurement was developed in Florence in the early 17th century and relied on the expansion of alcohol. Scaling is based on the highest temperatures in summer and winter. A hundred years later, the Swedish astronomer Celsius replaced it with the melting and boiling points of water. This gives the thermometer the opportunity to zoom in and out at any time and reproduce the readings later.
Electrical measurement temperature
Temperature measurement is important in many applications, such as building control, food processing, and the manufacture of steel and petrochemical products. These very different applications require temperature sensors with different physical structures and usually different technologies
In industrial and commercial applications, measurement points are usually far away from indication or control points. Further processing of measurements is usually required in controllers, recorders or computers. These applications are not suitable for direct indication of thermometers because we know them from everyday use, but need to convert the temperature into another form of device, the electrical signal. In order to provide this remote electrical signal, RTD is usually used. Thermistors and thermocouples.
RTD adopts the characteristic of metal resistance changing with temperature. They are positive temperature coefficient (PTC) sensors whose resistance increases with temperature. The main metals used are platinum and nickel. The most widely used sensors are 100 ohm or 1000 ohm RTDS or platinum resistance thermometers.
RTD is the most accurate sensor for industrial applications and also provides the best long-term stability. The representative value of platinum resistance accuracy is + 0.5% of the measured temperature. After one year, there may be + 0.05 ° C change through aging. Platinum resistance thermometers have a temperature range of – 200 to 800 ° C.
Change of resistance with temperature
The conductivity of a metal depends on the mobility of the conducting electrons. If a voltage is applied to the end of the wire, the electrons move to the positive pole. Defects in the lattice interfere with this motion. They include external or missing lattice atoms, atoms at grain boundaries and between lattice positions. Since these fault locations are temperature independent, they produce a constant resistance. With the increase of temperature, the atoms in the metal lattice exhibit increased oscillations near their stationary positions, thus hindering the movement of the conducting electrons. Since the oscillation increases linearly with temperature, the resistance increase caused by the oscillation depends directly on the temperature.
Platinum has been widely accepted in industrial measurement. Its advantages include chemical stability, relatively easy fabrication (especially for wire manufacturing), the possibility of obtaining it in high purity form, and reproducible electrical properties. These characteristics make platinum resistance sensor the most widely interchangeable temperature sensor.
Thermistors are made of some metal oxides and their resistance decreases with increasing temperature. Because the resistance characteristic decreases with the increase of temperature, it is called negative temperature coefficient (NTC) sensor.
Due to the nature of the basic process, the number of conducting electrons increases exponentially with temperature; therefore, the characteristic shows a strong increase. This obvious nonlinearity is a disadvantage of NTC resistors and limits its effective temperature range to about 100 ° C. They can, of course, be linearized by automated computers. However, the accuracy and linearity can not meet the requirements of large measurement span. Their drift at alternating temperatures is also larger than that of RTD. Their use is limited to monitoring and indicating applications where the temperature does not exceed 200 ° C. In this simple application, they are actually superior to the more expensive thermocouples and RTDs, considering their low cost and the relatively simple electronic circuits required.
The basis of thermocouple is the connection between two different metals, thermistor. The voltage generated by thermocouple and RTD increases with temperature. Compared with resistance thermometers, they have a higher upper temperature limit, with a significant advantage of several thousand degrees Celsius. Their long-term stability is slightly poor (several degrees after a year), and the measurement accuracy is slightly poor (average + 0.75% of the measurement range). They are often used in ovens, furnaces, flue gas measurement and other areas where temperatures are higher than 250 ° C.
The difference between thermocouple, thermistor and RTD
Thermoelectric effect
When two metals are connected together, thermoelectric voltage is produced due to the different binding energy of electrons and metal ions. The voltage depends on the metal itself and the temperature. In order for this thermal voltage to generate current, the two metals must of course be connected together at the other end to form a closed circuit. In this way, a thermal voltage is generated at the second junction. The thermoelectric effect was discovered by Seebeck in 1822. As early as 1828, Becquerel suggested the use of platinum palladium thermocouple for temperature measurement.
If there is the same temperature at both junctions, there is no current flow because the partial pressures generated at the two points cancel each other out. When the temperature at the junction is different, the voltage generated is different and the current flows. Therefore, thermocouple can only measure temperature difference.
The measuring point is a junction exposed to the measured temperature. The reference junction is a junction at a known temperature. Since the known temperature is usually lower than the measured temperature, the reference junction is usually called a cold junction. In order to calculate the actual temperature of the measuring point, the cold end temperature must be known.
Older instruments use thermostatic control junction boxes to control the cold junction temperature at known values such as 50c. Modern instruments use thin-film RTD at the cold end to determine its temperature and calculate the temperature of the measuring point.
The voltage produced by the thermoelectric effect is very small and is only a few microvolts per degree centigrade. Therefore, thermocouples are not normally used in the range of – 30 to + 50 ° C, because the difference between the reference junction temperature and the reference junction temperature is too small to produce a non-interference signal.
RTD wiring
In a resistance thermometer, the resistance varies with temperature. To evaluate the output signal, a constant current passes through it and the voltage drop across it is measured. For this voltage drop, Ohm’s law is obeyed, v = IR.
The measurement current should be as small as possible to avoid sensor heating. It can be considered that the measurement current of 1mA will not introduce any obvious error. The current produces a voltage drop of 0.1V in PT 100 at 0 ℃. This signal voltage must now be transmitted through the connecting cable to the indication point or evaluation point with minimal modification. There are four different types of connection circuits:
The difference between thermocouple, thermistor and RTD – 1
2-wire circuit
A 2-core cable is used for the connection between the thermometer and the evaluation electronics. Like any other electrical conductor, the cable has a resistance in series with a resistance thermometer. As a result, the two resistors are added together and the electronics interpret it as a temperature rise. For longer distances, the line resistance can reach several ohms and produce a significant offset in the measured value.
3-wire circuit
In order to minimize the influence of line resistance and its fluctuation with temperature, a three wire circuit is usually used. It includes running additional wires on one of the contacts of the RTD. This results in two measurement circuits, one of which is used as a reference. The 3-wire circuit can compensate the line resistance in terms of its number and temperature variation. However, all three conductors are required to have the same characteristics and to be exposed to the same temperature. This is usually applied to a sufficient extent to make 3-wire circuits the most widely used method today. No line balancing is required.
4-wire circuit
The best connection form of resistance thermometer is 4-wire circuit. Measurement depends neither on line resistance nor on temperature induced changes. No line balancing is required. The thermometer provides measurement current through a power connection. The voltage drop on the measuring line is picked up by the measuring line. If the input resistance of an electronic device is many times greater than the line resistance, the latter can be ignored. The voltage drop determined in this way is independent of the characteristics of the connecting wire. This technique is usually used only for scientific instruments that require a measurement accuracy of one hundredth.
The difference between thermocouple, thermistor and RTD – 2
2-wire transmitter
By using a 2-wire transmitter instead of a multi wire cable, the problem of a 2-wire circuit as described above can be avoided. The transmitter converts the sensor signal into a normalized current signal of 4-20mA, which is proportional to the temperature. The power supply to the transmitter also operates through the same two connections, using a basic current of 4 mA. The 2-wire transmitter provides an additional advantage, that is, signal amplification greatly reduces the impact of external interference. There are two arrangements for positioning the transmitter. Since the distance between non amplified signals should be as short as possible, the amplifier can be directly installed on the thermometer in its terminal head. This best solution is sometimes not possible due to structural reasons or considerations that the transmitter may be difficult to reach in the event of a failure. In this case, the rail mounted transmitter is installed in the control cabinet. The advantage of improved access is that it is purchased at the cost of a longer distance that the non amplified signal must travel.
Thermistor wiring
The resistance of a thermistor is usually several orders of magnitude greater than that of any lead wire. Therefore, the effect of lead resistance on temperature readings is negligible, while thermistors are almost always connected in a 2-wire configuration.
Thermocouple wiring
Unlike RTDS and thermistors, thermocouples have positive and negative legs, so polarity must be observed. They can be connected directly to the local 2-wire transmitter and the copper wire can be returned to the receiving instrument. If the receiving instrument can accept thermocouple input directly, the same thermocouple wire or thermocouple extension wire must be used all the way back to the receiving instrument.
The great advantage of thermistor is its high sensitivity. They are mainly used for room temperature measurements up to moderate high temperatures. They are popular in research and medical applications, such as electronic medical thermometers.
Thermistor is a kind of thermistor whose resistance varies with temperature. Resistance is measured by passing a small measuring direct current (DC) through it and measuring the resulting voltage drop.
There are two basic types of thermistors, PTC and NTC:
PTC (positive temperature coefficient) devices show an increase in resistance as the temperature increases. PTC thermistor is a temperature dependent resistor made of barium titanate. It should be selected when the resistance needs to be greatly changed at a specific temperature or current level.
The NTC (negative temperature coefficient) device shows a decrease in resistance as the temperature increases. NTC thermistors are made up of semiconductors. They are made from a mixture of oxides of manganese, cobalt, copper or nickel. Their operating temperature ranges from – 150 ° C to 600 ° C. For a temperature of about 700 ℃, devices using zirconia doped with rare earth oxides can be used. NTC should be selected when it is necessary to change the resistance continuously in a wide temperature range. They have mechanical, thermal and electrical stability as well as high sensitivity.
Thermistors are commercially available in various shapes: beads, discs, chips and probes. The most common form is a bead with two wires attached. The bead diameter can be about 0.5mm to 5mm.
The main advantages and disadvantages of NTC and PTC Thermistors
Small size thermistors make them very adaptable and are usually included in electronic circuits. They can be encapsulated in epoxy or glass or painted. Thermistors are simple, robust and very reliable
Temperature dependence of resistance
The resistance of CTN thermistor decreases exponentially with the increase of temperature. It is negative and nonlinear, as shown by the following relation:
Where R 0 is the nominal resistance measured at the absolute temperature t0 (usually measured at 25 ° C) and B is the constant of the specific thermistor material. The nonlinear characteristics of resistors may be undesirable, and this can be offset by using two or more thermistors in series or in parallel, packaged in a single device.
The temperature coefficient of resistance α, expressed in% / K (or% / ° C), is defined as follows:
The coefficient α of PTC thermistor is positive, while that of NTC thermistor is negative.
The Steinhart & Hart equation can be used to determine the temperature from the thermistor:
Where R is the thermistor in ohm, t is the absolute temperature, K, a, B and C are constants, which are usually provided by the manufacturer, but can be determined by calibrating at three different temperatures and solving three simultaneous equations.
This equation is very close to the actual device, but it does not always provide the required accuracy over the entire temperature range. This can be corrected by fitting the Steinhart & Hart equations over a range of narrow temperature ranges and then “stitching” them together to cover the desired range.
Advantages and disadvantages of thermistor
The great advantage of thermistors is that they can be point measured
Compared with other temperature sensors, their high sensitivity allows thermistors to operate in a small temperature range and are very accurate (usually better than 0.05 ° C and 0.1 ° C). The sensitivity of thermistor can be one order of magnitude higher than that of resistance thermometer (RTD)
The most common temperature range is 0 ° C to 100 ° C. At higher temperatures, they drift greatly
The high resistivity of thermistors eliminates the need for a four wire bridge circuit
Thermistors are very nonlinear and not robust, which limits their application
The error source is due to the self heating effect caused by excessive bias current when the thermistor is powered on to provide the output voltage signal. In order to reduce this source of error, a current suitable for measuring resistance must be used.
Due to its performance and moderate cost, thermistors are suitable for temperature measurement and control, temperature compensation, electronics, medical and many other applications.
