Classification Method of Automation Instruments
According to the different uses of instruments, they can be divided into testing instruments, display instruments, conversion and transmission instruments, adjustment and control instruments, and actuators.According to the composition of the instrument, there are base instrument, unit combined instrument, assembled electronic integrated control device, and centralized and distributed control system.
According to the energy used, it can be divided into pneumatic instruments, electric instruments and hydraulic instruments.
According to the different measured parameters, it can be divided into pressure gauge, temperature gauge, liquid level gauge and flow gauge.
According to the different systems used, it can be divided into production system testing instruments and safety system testing instruments.
Temperature detection instrument
Related concepts of temperature
The physical quantity that expresses how hot or cold an object or system is is called temperature. In order to quantitatively describe how hot or cold an object or system is, a temperature scale must be determined. The so-called temperature scale is the scale of temperature medicalization, which stipulates the starting point (zero point) of temperature reading and the basic unit of measuring temperature. Currently commonly used temperature scales are the Fahrenheit temperature scale, Celsius temperature scale, thermodynamic temperature scale, and international practical temperature scale.
The Fahrenheit temperature scale stipulates: Under standard atmospheric pressure, the melting point of ice is 32 degrees, and the boiling point of water is 212 degrees. The middle is divided into 180 equal divisions, each division is 1 degree Fahrenheit, and the symbol is.
The Celsius temperature scale stipulates: under standard atmospheric pressure, the melting point of ice is 0 degrees, the boiling point of water is 1 degree, divided into 1 equal division, each division is 1 degree Celsius, and the symbol is °C.
The relationship between the Celsius and Fahrenheit scales is as follows:
C=5/9 (F — 32)The thermodynamic temperature scale is also called the Kelvin temperature scale, which stipulates that the temperature at which molecular motion stops is absolute zero, and the symbol is K.
The principle and structure of the temperature measuring instrument are relatively simple, and the reliability is high. It is widely used in the field of the petrochemical industry. It can be used to measure the temperature of ordinary process pipes and containers. It can also be used in some key occasions, such as reactor temperature, and cracking furnace temperature Measurement.
Temperature measuring instruments are divided into two types: local indication and remote transmission in chemical plants. Local indications include glass thermometers, bimetal thermometers, and pressure thermometers. This type of instrument is simple in structure and reliable in use.
There is also a class of non-contact thermometers, such as optical pyrometers, radiation thermometers, infrared radiation thermometers, colorimetric thermometers, etc. It is rarely used in petrochemical plants.
Remote temperature instruments mainly include thermocouples, thermal resistance, temperature switch, etc. In addition, in some places where the installation space is limited, pressure instruments (temperature bulbs) are also often used, and pressure thermometers are sometimes used in conjunction with pneumatic base instruments.
The thermocouple is one of the most commonly used temperature detection elements in the industry. The working principle of the thermocouple is based on the Seebeck effect, that is, two conductors with different components are connected to form a loop. A physical phenomenon that generates thermal currents. Its advantages are:
1 High measurement accuracy. Because the thermocouple is in direct contact with the measured object, it is not affected by the intermediate medium.
2 Wide measurement range. Commonly used thermocouples can measure continuously from -50 to +16 C, and some special thermocouples can measure as low as -269 C (such as gold-iron-nickel-chromium) and as high as +28 C (such as tungsten- rhenium).
3 The structure is simple and easy to use. Thermocouples are usually composed of two different metal wires and are not limited by size and opening. There is a protective sleeve outside, which is very convenient to use.
(1) Basic principle of thermocouple temperature measurement
Weld two conductors or semiconductors A and B of different materials to form a closed loop, as shown in Figure 1. When there is a temperature difference between the two junctions of conductors A and B, an electromotive force is generated between the two, thus A certain amount of current is formed in the circuit, which is called the thermo-electric effect or Seebeck (Thomas, Seebeek) effect. Thermocouple thermo-electric potential is composed of contact potential and temperature difference potential. The electromotive force generated at the junction of two conductors or semiconductors is called contact potential. The contact potential is formed at the contact due to the difference in free electron density between two different conductors (or semiconductors).
The electromotive force generated along the temperature gradient of a single homogeneous conductor is called thermo-electric potential. The thermo-electric potential is caused by the difference in energy of free electrons at the high and low-temperature ends of the same conductor. A thermo-electric potential is composed of two homogeneous conductors A and B The total thermo-electric potential EAB (t, t0) generated by it is only related to the two materials and the temperature of the two junctions that make up the thermocouple. When the temperature of one junction of the thermocouple (reference junction temperature) is constant, the thermo-electric potential of the thermocouple has a single-valued function relationship with the temperature of the other junction. At this time, as long as the magnitude of the thermo-electric potential is measured, the value of the temperature can be obtained. This is how a thermocouple works.
(2) Types and structure formation of thermocouples
Commonly used thermocouples can be divided into two categories: standard thermocouples and non-standard thermocouples. The standard thermocouple referred to refers to the thermocouple that the national standard specifies the relationship between thermo-electric potential and temperature, allowable error, and has a unified standard graduation table. It has matching display instruments for selection. Non-standardized thermocouples are not as good as standardized thermocouples in terms of range or magnitude of use, generally do not have a unified graduation table, and are mainly used for measurement on some special occasions.
Standardized thermocouples In my country, from January 1, 1988, all thermocouples and thermal resistances were produced according to IEC international standards, and seven standardized thermocouples of S, B, E, K, R, J, and T were designated as my country's unified design type.The structural form of the thermocouple In order to ensure the reliable and stable operation of the thermocouple, its structural requirements are as follows:
1 The welding of the two hot electrodes that make up the thermocouple must be firm;2 The two thermal electrodes should be well insulated from each other to prevent short circuits;
3 The connection between the compensation wire and the free end of the thermocouple should be convenient and reliable;
4 The protective casing should be able to ensure that the hot electrode is fully isolated from the harmful medium.
(3) Compensation wire for thermocouple:
Since the material of the thermocouple is generally expensive, and the distance from the measuring point to the instrument in the control room is very far, in order to save the material of the thermocouple and reduce the cost, the cold end (free end) of the thermocouple is usually extended to the control room by using a compensation wire on the meter terminals. The thermo-electric characteristics of the material of the compensation wire should be similar to those of the thermocouple used. Therefore, when using it, care must be taken to match the model, the polarity cannot be reversed, and the temperature at the connection between the compensation wire and the thermocouple should not exceed 1 °C.
Thermal resistanceThermal resistance is widely used to measure medium and low temperatures (generally below 5 C). It is characterized by high accuracy; when measuring medium and low temperatures, its output signal is much larger than that of thermocouples, and its sensitivity is high; it can also realize remote transmission, automatic recording, and multi-point measurement.
(1) Working principle of thermal resistanceThermal resistance temperature measurement is based on the characteristic that the resistance value of the metal conductor changes with the change of temperature for temperature measurement. The heating part (temperature-sensing element) of the thermal resistance is a skeleton made of thin metal wire evenly wound on the insulating material Above, when there is a temperature gradient in the measured medium, the measured temperature is the average temperature in the medium layer within the range where the temperature sensing element is located.
(2) Types of thermal resistance:Copper and platinum are the most widely used in industry at present. The platinum thermal resistance with graduation number Pt1 is the most used, and its resistance value at 0 C is 1 Q.
The relationship between Pt and Cu resistance and temperature:
Pt10, Pt50, Pt1, resistance ratio R1/R0 is 1.385 ±0.1
Cu50, Cu1, the resistance ratio R1/R0 is 1.428 ±0.2
(3) Structural features of thermal resistance:Thermal resistance can directly measure various production processes from -2. . Temperatures of liquid, vapor, and gaseous media and solid surfaces in the range of + 6 C.
Assembled thermal resistance: It is usually composed of main components such as temperature sensing elements, installation fixtures, and junction boxes. It has the advantages of high measurement accuracy, stable and reliable performance, etc. In practical application, Pt1 platinum thermal resistance is the most widely used.
Armored platinum thermal resistance: Armored thermal resistance is a solid body composed of temperature-sensing elements, leads, insulating materials, and stainless steel sleeves. It has the following advantages: slender body, fast thermal response time, anti-vibration, long service life Long, and other advantages.
Flameproof RTD: Flameproof RTD uses a junction box with a special structure to confine the explosion of the explosive mixed gas inside the junction box due to the influence of sparks or arcs on the junction box, and the production site will not cause an explosion.
End-face thermal resistance: The temperature-sensing element of end-face thermal resistance is made of specially treated resistance wire, which is closely attached to the end face of the thermometer. Compared with ordinary axial thermal resistance, it can reflect the actual temperature of the measured end face more accurately and quickly. Suitable for measuring surface temperature.
Pressure detection instrument
Pressure is one of the important parameters in industrial production. In fact, it is the pressure in the physical concept, that is, the force acting vertically on the unit area. In pressure measurement, there is often absolute pressure, gauge pressure, negative pressure, or vacuum degree. The so-called absolute pressure refers to the total pressure of the measured medium acting on the unit area of the container, expressed by the symbol pj. Instruments used to measure absolute pressure are called absolute pressure gauges.
Atmospheric pressure on the ground is expressed with the symbol pq. The difference between absolute pressure and atmospheric pressure is called gauge pressure, which is represented by the symbol pb, namely: pj-pq=pb. When the absolute pressure value is less than the atmospheric pressure value, the gauge pressure is a negative value (negative pressure), and the absolute value of this negative pressure value is called the degree of vacuum, which is represented by the symbol pz.
Classification of pressure instruments
Common pressure measuring instruments can be divided into three categories according to the principle of pressure measurement.(1) According to the balance method of gravity and measured pressure, directly measure the size of the force per unit area. For example, liquid column pressure
Force gauges and piston gauges.
(2) According to the method of balancing the elastic force and the measured pressure, measure the elastic force generated by the deformation of the elastic element after compression, such as the spring tube pressure gauge, bellows pressure gauge, diaphragm pressure gauge, and bellows pressure gauge.
(3) Utilize the physical characteristics of some substances related to pressure, such as resistance change when pressed, voltage change when pressed, etc. For example, pressure sensors such as capacitive, resistive, inductive, strain gauge, and Hall plates.
Spring tube pressure gauge
The sensitive element of the spring tube pressure gauge is an elastic C-shaped tube with an oval cross-section. When the C-shaped tube bears the pressure p, the free end of the C-shaped tube uniquely drives the pointer to indicate the pressure.
(1) Ordinary pressure gauge is mainly used as an ordinary gauge suitable for measuring the pressure of liquid, steam, and gas media that do not crystallize, do not solidify, and have no corrosion effect on steel and copper alloys.
(2) Precision pressure gauges are mainly used as calibration instruments for ordinary pressure gauges.
The pressure transmitter is mainly composed of a load cell sensor, a measurement circuit, and a process connection. It can convert the received gas, liquid, and other pressure signals into standard current and voltage signals 4~20mADC, which can be supplied to secondary instruments such as indicator alarms, recorders, regulators, etc. for measurement, indication, and process adjustment.
1) Capacitive pressure transmitter
The two pressures of the measured medium of the capacitive pressure transmitter enter the high and low-pressure chambers, act on the isolation diaphragms on both sides of the sensitive element and transmit them to both sides of the measurement diaphragm through the isolation diaphragm and the filling liquid in the element.
The capacitive pressure transmitter is composed of a measuring diaphragm and electrodes on both sides of the insulating sheet to form a capacitor. When the pressure on both sides is inconsistent, the measuring diaphragm will be displaced, and the displacement is proportional to the pressure difference, so the capacitance on both sides is not equal. Through the oscillation and demodulation link, it is converted into a signal proportional to the pressure.
Capacitive The A/D converter of the pressure transmitter converts the current of the demodulator into a digital signal, and its value is used by the microprocessor to determine the input pressure value. A microprocessor controls the operation of the transmitter. Additionally, it performs sensor linearization. Resets the measurement range. Operations such as engineering unit conversion, damping, square root, sensor fine-tuning, etc. and diagnostics and digital communications.
There is 16-byte program RAM in the pressure transmitter microprocessor, and there are three 16-bit counters, one of which performs A/D conversion. The D/A converter fine-tunes the data from the microprocessor and the corrected digital signal, which can be modified by the transmitter software. The data is stored in EEPROM, even if the power is cut off, it will be completely preserved.
The digital communication line provides the transmitter with a connection with the external equipment (205 intelligent communicators or the control system using the HART protocol). This line detects the digital signal superimposed on the 4-20mA signal and transmits the required information through the loop.
Accuracy: 0.075% turndown ratio 1:1;
Gauge pressure: calibration range from 2.5inH2O to 20psi;
Absolute pressure: calibration range from 0.167psia to 40psia;
Process isolation diaphragm: stainless steel, Hastelloy CR, Monel R, tantalum (CD, CG only), and gold-plated Monel;
The design is compact, strong and lightweight, easy to install.
2) Diffused silicon pressure transmitter
The diffused silicon pressure transmitter utilizes the anisotropy of single-crystal silicon, selects different crystal orientations on the n-type substrate, diffuses into a P-type resistor, forms a Wheatstone bridge, and uses the P-n junction to realize the connection between each resistor. Electrical isolation between. A stress cup of a certain geometric shape is machined on the non-resistive surface of the substrate. When pressure acts on one side of the stress cup, the Wheatstone bridge excited by a constant voltage or constant current source outputs a certain voltage value. Thereby realizing the perception of pressure.
The diffused silicon pressure transmitter has the advantages of reliable operation, stable performance, convenient installation and use, small size, lightweight, high-cost performance, etc., and can be widely used in various positive and negative pressure measurements. Diffused silicon pressure transmitter adopts advanced The diffused silicon or ceramic core is used as a pressure detection element, and the sensor signal is converted into 0-10mA by a high-performance electronic amplifier. Or 4-20mA unified output signal. The pressure transmitter can replace the traditional remote pressure gauge, Hall element, and differential transmitter, and has the performance of DDZ-II and DDZ-III transmitters. Diffused silicon pressure transmitters can be used with various types of moving coil indicators, digital pressure gauges, and electronic potentiometers, and can also be used with various automatic adjustment systems or computer systems.
Accuracy grade: 0 075 basic error ± 0.25%;
Non-linear error: 0.3 level <±).3%FS;
Lag error: < work. 3%FS;
Constant current output internal resistance greater than 10M Q;
Two-wire 4-20mA output: standard power supply DC24V;
Explosion-proof mark: Exiall CT4-6;
Flow detection instrument
Flow rate is the amount of fluid flowing through a certain section per unit of time. The flow rate can be expressed by volume flow rate and mass flow rate, and the units are respectively m3/h, L/h, Kg/h, etc. A flowmeter refers to an instrument for measuring fluid flow. It can indicate and record the flow value of a certain instantaneous fluid. Flowmeters include throttling devices, rotameters, target flowmeters, etc., and the most commonly used throttling devices have orifice plates. , Nozzle, Venturi tube.
Differential pressure flowmeter
A differential pressure flowmeter is an instrument that measures flow based on the differential pressure generated by the flow detection device installed in the pipeline, known fluid conditions, and the geometric dimensions of the detection device and the pipeline. The differential pressure flowmeter consists of a primary device (detection part) and a secondary device (differential pressure conversion and flow display instrument). Differential pressure flowmeters are usually classified by the type of detection parts, such as orifice flowmeters, Venturi tube flowmeters, and uniform velocity tube flowmeters.
Positive displacement flowmeter
A positive displacement flowmeter, also known as a positive displacement flowmeter, is the most accurate type of flow meter. It uses mechanical measuring elements to continuously divide the fluid into a single known volume and measures the total flow volume according to the number of times the metering chamber fills and discharges the volume of fluid successively and repeatedly. Positive displacement flowmeters generally do not have a time reference, and an additional device for measuring time is required to obtain instantaneous flow values.
Vortex flow juice is to place one (or more) bluff bodies in the fluid, and the fluid is separated alternately on both sides of the bluff body to release two series of regular vortices, and the vortices are separated within a certain flow range. The frequency is proportional to the average flow velocity in the pipeline, and the flow rate of the fluid can be calculated by measuring the vortex frequency with various detection elements.
The basic principle of the electromagnetic flowmeter is Faraday's law of electromagnetic induction, that is when the conductor cuts the magnetic force line in the magnetic field, an induced electromotive force is generated at both ends of the conductor, and the conductive liquid flows in the non-magnetic measuring tube perpendicular to the magnetic field, in the direction perpendicular to the flow direction Generate an induced potential proportional to the flow. The electromagnetic flowmeter is composed of a flow sensor and a converter. The sensor measuring tube is equipped with an excitation coil. After the excitation current is passed, a magnetic field is generated through the measuring tube, and a pair of electrodes are installed in the measuring tube. The wall is in contact with the liquid, and the induced potential is drawn out and sent to the converter. The excitation current is provided by the converter.
Ultrasonic flowmeters are classified according to the measurement principle: ① propagation time method; ② Doppler effect method; ③ beam shift method: ④ correlation method; ⑤ noise method
The rotameter is a volume flow meter that measures the flow area between the floats in the vertical conical tube as the flow changes and changes the flow area, also known as the rotameter.
The flow detection element of the rotameter is composed of a vertical tapered tube that expands from bottom to top and a float group that moves up and down along the axis of the tapered tube.
The working principle is shown in Figure 1. When the measured fluid passes through the annular gap 3 formed by the tapered tube 1 and the float 2 from bottom to top, the upper and lower ends of the float will generate a differential pressure to form the upward force of the float. When the upward force on the float is greater than that immersed in the fluid When the weight of the float is medium, the float will rise, the area of the annular gap will increase accordingly, the flow rate of the fluid in the annular gap will drop immediately, the differential pressure between the upper and lower ends of the float will decrease, and the lifting force acting on the float will also decrease until the lifting force is equal to that immersed in the fluid When the weight of the float is medium, the float is stable at a certain height. There is a corresponding relationship between the height of the float in the conical tube and the passing flow.
Liquid level (level) instrument
Level detection refers to the height of the liquid medium in the container (open or closed) (called liquid level), the height of the interface between two liquids (called interface), and the accumulation height of solid blocks and granular substances called material level). The instrument used to detect the liquid level is called a liquid level gauge, the instrument used to detect the interface is called an interface gauge, and the instrument used to detect the solid material level is called a level gauge, and they are collectively called a level gauge.
Level detection plays an important role in the modern industrial production process. Through level detection, the storage capacity of the measured medium in the container can be determined to ensure the material balance in the production process, and also provide a reliable basis for economic accounting; through level detection and control, It can maintain the material level within the specified range, which is of great significance for ensuring the output and quality of products and ensuring safe production.
In industrial production, the object of level detection includes liquid level and material level, etc. There are large containers with a height of tens of meters and micro containers with a few millimeters, and the characteristics of the medium are even more varied. Therefore, there are many methods for level detection to meet various detection requirements.
The common and most intuitive level detection is the direct reading method, which is to open some windows on the container for observation. For liquid level detection, a glass tube or glass plate connected to the measured container can be used to display the liquid height in the container. This method is reliable and the result is accurate, but it can only be used in the measured object whose container pressure is not high and only needs on-site indication. In addition, the commonly used level detection methods can be divided into the following types.
(1) Static pressure level detection. According to the principle of hydrostatics, the difference between the static pressure of a certain point in the static medium and the pressure of the free space above the medium is proportional to the height of the medium above the point, so the differential pressure can be used to detect the liquid level, this method is generally only used in the detection of liquid level.
(2) Buoyancy-type level detection uses the position of the floater floating on the liquid surface to change with the liquid level, or the buoyancy of the substance partially submerged in the liquid changes with the liquid level to detect the liquid level. The former is called the constant buoyancy method, and the latter It is called the variable buoyancy method, both of which are used to detect the liquid level.
(3) Electrical level detection. The sensitive element is made into electrodes of a certain shape and placed in the measured medium, then the electrical parameters between the electrodes, such as resistance, capacitance, etc., will change with the change of the level. This method can be used for both liquid-level detection and material-level detection.
(4) Acoustic level detection uses the propagation speed of ultrasonic waves in the medium and the reflection characteristics between different phase interfaces to measure the level. Both liquid-level and material-level detection can use this method.
(5) Radiation-type level detection. The radiation emitted by radioactive isotopes (such as radiation, gamma rays, etc.) passes through the measured medium (liquid or solid particles) and is weakened due to its absorption, and the degree of absorption is related to the material level. Using this method can realize the non-contact detection of material level.
In addition, there are microwave methods, optical methods, heavy hammer methods, etc.
Static pressure level detectionDetection principle: The static pressure level detection method is based on the principle that when the liquid level changes, the static pressure generated by the liquid column also changes.
Buoyancy type level detection instrument
The basic principle of buoyancy level detection is to measure the displacement of a float (also known as a buoy) floating on the liquid surface to be measured as the liquid level changes or to use a buoy (also called a sinker) immersed in the liquid to be measured The relationship between the buoyancy and the position of the liquid level is used to detect the liquid level. The former is generally called constant buoyancy detection, and the latter is called variable buoyancy detection.
The sound wave is a kind of mechanical wave, which is the propagation process of mechanical vibration in the medium. When the vibration frequency is more than ten Hz to more than 10,000 Hz, it can cause human hearing, which is called an audible sound wave; lower frequency mechanical wave is called an infrasonic wave; above 20kHZ Frequency of mechanical waves is called ultrasound. Ultrasonic waves are generally used for level detection.
Ultrasound is a discipline that has been around for decades and has a wide range of applications. Ultrasound is not only used for the detection of various parameters but is also widely used in processing and processing technology. Ultrasound is used for level detection mainly using its following properties.
(1) Like other sound waves, ultrasonic waves can propagate in gases, liquids, and solids, and have their own propagation speeds. For example, the speed of sound in air at normal temperature is about 334m/s, the speed of sound in water is about 1440m/s, and it is about 50m/s in steel. The speed of sound is not only related to the medium but also related to the state (such as temperature) of the medium. For example, the sound velocity of an ideal gas is proportional to the square root of the absolute temperature T. For air, the main factor affecting the sound velocity is temperature. In many solids and liquids, the sound velocity generally decreases with the increase in temperature.
(2) The sound wave will be absorbed and attenuated when propagating in the medium, the gas absorbs the strongest and the attenuation is the largest, the liquid is second, the solid absorbs the least and the attenuation is the smallest, so for a sound wave of a given intensity, the distance it propagates in the gas will be obvious Shorter than the distance traveled in liquids and solids. In addition, the degree of attenuation of the sound wave when it propagates in the medium is also related to the frequency of the sound wave. The higher the frequency, the greater the attenuation of the sound wave, so the attenuation of the ultrasonic wave is more obvious than other sound waves when they propagate.(3) The directionality of the sound wave propagation becomes stronger with the increase of the frequency of the sound wave, the emitted sound beam is sharper, and the ultrasonic wave can be approximated as a straight line, with good directionality.