Concept of the ultrasonic flowmeter and electromagnetic flowmeter
The ultrasonic flowmeter is an instrument for measuring flow by detecting the effect of fluid flow on the ultrasonic beam (or ultrasonic pulse). As the same as the electromagnetic flowmeter, the ultrasonic flowmeter belongs to the flowmeter because there are no obstructions in the instrument flow channel. It is a type of flowmeter suitable for solving the problem of difficult flow measurement, especially in large-diameter flow measurement. In recent years, it is one of the rapidly developing types of flowmeters.The electromagnetic flowmeter is an inductive instrument that measures the volume flow of the conductive medium in the pipe according to Faraday's electromagnetic induction law. It adopts the embedded technology of a single-chip microcomputer to realize digital excitation and adopts the CAN field bus on the electromagnetic flowmeter.
Working principles of the ultrasonic flowmeter and electromagnetic flowmeter
Ultrasonic flowmeter consists of the ultrasonic transducer, electronic circuit, and flow display and accumulation system. The ultrasonic transmitting transducer converts electric energy into ultrasonic energy and transmits it to the measured fluid. The ultrasonic signal received by the receiver is amplified by the electronic circuit and converted into an electrical signal representing the flow for display and totalizer for display and totalization. In this way, the flow detection and display are realized.
The piezoelectric transducer is commonly used in the ultrasonic flowmeter. It makes use of the piezoelectric effect of piezoelectric materials and uses an appropriate transmitting circuit to add electrical energy to the piezoelectric element of the transmitting transducer to generate ultrasonic vibration. The ultrasonic wave is injected into the fluid at a certain angle and propagates. Then it is received by the receiving transducer and turned into electric energy through the piezoelectric element for testing. The transmitting transducer uses the converse piezoelectric effect of piezoelectric elements, while the receiving transducer uses the piezoelectric effect.
The working principle of the electromagnetic flowmeter is based on Faraday's law of electromagnetic induction. In the electromagnetic flowmeter, the conductive medium in the measuring tube is equivalent to the conductive metal rod in the Faraday test, and the two electromagnetic coils at the upper and lower ends generate a constant magnetic field. When a conductive medium flows through, an induced voltage will be generated. Two electrodes inside the pipe measure the induced voltage generated. The measuring pipe is electromagnetically isolated from the fluid and the measuring electrode through a non-conductive lining (rubber, Teflon, etc.). Conductive liquid flows in a non-magnetic measuring tube perpendicular to the magnetic field, and an induced electromotive force proportional to the flow is generated in the direction perpendicular to the flow direction. The direction of the electromotive force follows the "Fleming right-hand rule".
Classification of the ultrasonic flowmeter and electromagnetic flowmeter
The detection method can be divided into different types of ultrasonic flowmeters, such as propagation velocity difference method, Doppler method, beam offset method, noise method, and correlation method. According to the principle of signal detection, the current ultrasonic flowmeter can be roughly divided into the propagation velocity difference method (including direct time difference method, time difference method, phase difference method, frequency difference method), beam migration method, Doppler method, correlation method, spatial filtering method, noise method, etc. The principle and structure of the noise method are easy to measure and carry, and the price is cheap but the accuracy is low. It is suitable for occasions where the accuracy of flow measurement is not required.
Because the basic principles of the direct TDOA method, TDOA method, frequency difference method, and phase difference method reflect the flow velocity of fluid by measuring the difference between the forward and reverse flow transmission velocities of the ultrasonic pulse, they are also called the propagation velocity difference method. Among them, the frequency difference method and time difference method are widely used because they overcome the error caused by the change of sound speed with fluid temperature and have high accuracy. According to the different configuration methods of transducers, the propagation velocity difference can be divided into the Z method (transmission method), V method (reflection method), X method (cross method), etc.The electromagnetic flowmeter is divided into DC excitation, AC (power frequency or other frequency) excitation, low-frequency rectangular wave excitation and dual frequency rectangular wave excitation according to the excitation current mode; According to the system of output signal connection and excitation (or power supply) connection, there are four wire system and two-wire system; Classified according to the assembly mode of converter and sensor, there are divided type and integrated type; According to the connection mode of flow sensor and pipeline, there are flange type, clamping type, sanitary type, plug-in type and threaded connection; According to whether the flow sensor electrode is in contact with the measured liquid, there are contact type and non-contact type; According to the structure of flow sensors, there are short tube type and plug-in type (plug-in electromagnetic flowmeter); According to the classification of use, there are general purpose, explosion-proof, sanitary, water intrusion prevention and submersible (open channel flowmeter) for open channel flow measurement.
Different accuracy
The ultrasonic flowmeter determines the volume flow by measuring the fluid velocity. For liquid, its mass flow should be measured. The instrument measures the mass flow by multiplying the volume flow by the manually set density. When the fluid temperature changes, the fluid density changes. The manually set density value cannot guarantee the accuracy of the mass flow. Only when the fluid velocity is measured and the fluid density is measured, can the true mass flow value be obtained through calculation.
From the experience of the use of ultrasonic flowmeter in the domestic market, the main disadvantage is that the temperature range of the measurable fluid is limited by the temperature tolerance of the ultrasonic energy transfer aluminum and the coupling material between the transducer and the pipe, and the other disadvantage is that the original data of the measured fluid sound velocity under high temperature is incomplete. At present, the ultrasonic flowmeter in China can only be used to measure fluids below 200 ℃.
The measuring medium of the ultrasonic flowmeter is different from that of the electromagnetic flowmeter. The ultrasonic wave is a sound wave with a low frequency, and the ultrasonic frequency is 20KHz~100KHz. The radar uses an electromagnetic wave of 2.4GHz level. The limitation of the ultrasonic waves is relatively large, and it is easy to interfere with other iron objects. In addition, the external frequency is low, the attenuation is large, the measuring range is small, and the application area is narrow, It is commonly used for flow measurement of large diameter water pipelines and liquid level measurement of open channel flowmeter to convert into the flow. It can also be used on solid silos.
The electromagnetic frequency is high, and the attenuation is small. If the guided wave tube is added, the measuring range can be large, which is more used in storage tanks. However, attention should be paid to the dielectric constant. The medium with too small a dielectric constant cannot be measured or the measuring range is very small. Because this sensor must maintain a certain proportion between the resistance in the pipeline and the impedance of the measuring circuit, it is difficult to manufacture. When the conductivity of the measured medium is about 10 Ω/cm, difficulties begin to arise, and when the conductivity is lower, theoretical difficulties arise. When the conductivity is 10 Ω/cm, it reaches the "boundary" between the conductive medium and the dielectric, and the thermal noise level increases significantly with the increase of the internal resistance.
High-precision ultrasonic flowmeters are of multi-channel or pipe section type. Medium and small-caliber pipe section ultrasonic flowmeters are usually calibrated with real flow and accuracy of 0.5%. At present, the nominal accuracy of the widely used domestic mono-channel ultrasonic flow is 1%. However, in practical application, the measurement accuracy will exceed the nominal accuracy a lot due to many factors such as the inability to accurately measure the internal diameter, wall thickness, and roundness of the on-site pipeline. For the measurement of the water supply industry, the actual measurement error of the ultrasonic flowmeter can be controlled within 3%, which is considered high accuracy.
Different installation, maintenance, and verification costs
The ultrasonic flowmeter is applicable to large circular pipelines and rectangular pipelines. In principle, it is not limited by pipe diameter, and its cost is basically independent of pipe diameter. It not only brings convenience to large pipelines but also can be considered the preferred option when real flow verification cannot be realized. An ultrasonic flowmeter can be used for non-contact measurement.
The clip-on transducer ultrasonic flowmeter can be installed without stopping the flow and intercepting the pipe, as long as the transducer is installed outside the existing pipe. This is the advantage of the ultrasonic flowmeter in industrial flow instruments, so it can be used for mobility (i.e. fixed installation at a non-fixed point) measurement, which is applicable to the evaluation and determination of pipe network flow conditions.
The ultrasonic flowmeter is a non-flow obstruction measurement without additional pressure loss. The instrument coefficient of the flowmeter can be calculated from the geometric dimensions of the actual measuring pipe and sound channel, which can be calibrated by the dry method. Except for the type with measuring pipe section, real flow calibration is generally not required.
The ultrasonic flowmeter is mainly installed outside the pipe and inserted, which is simple and convenient. It can be disassembled online. There is no need for process shutdown during maintenance, which will not affect production. The verification cost is low. It is verified every three years according to the national metrological verification regulations.
The installation and commissioning of an electromagnetic flowmeter are more complicated than other flowmeters, and the requirements are stricter. The transmitter and converter must be used together, and two different types of instruments cannot be used between them. During the installation of the transmitter, from the selection of the installation site to the specific installation and commissioning, the requirements of the product manual must be strictly followed.
The installation site shall be free from vibration and a strong magnetic field. During installation, the transmitter and pipeline must have good contact and good grounding. The potential of the transmitter is equal to the measured fluid. During use, the gas left in the measuring pipe must be drained, otherwise, large measurement errors will be caused. The electromagnetic flowmeter needs to be installed under the condition of liquid with conductivity, and the installation of the electromagnetic flowmeter must be cut off in general, but the electromagnetic flowmeter is characterized by high accuracy under the site conditions that meet the conditions. It is troublesome to disassemble the electromagnetic flowmeter, and the process must be stopped. It is troublesome to disassemble and submit it for inspection. If the accuracy is 0.5%, it should be verified every six months according to the national metrological verification regulation.
Different interference sources
If it interferes with the ultrasonic work, it interferes with the work of the ultrasonic flowmeter. The main factors that interfere with the ultrasonic work are the violent change of temperature and the interference of clutter or the swirling flow or structure with a specific angle in the pipeline making the ultrasonic emitted by the flowmeter cannot be effectively recovered.
The electrochemical polarization potential interference is caused by the polarization of the electrolyte on the electrode surface due to the different polarities of the electrode-induced electromotive force at the two poles. Although the use of positive and negative alternating excitation magnetic fields can significantly reduce the order of magnitude of polarization potential, it cannot fundamentally eliminate the interference of polarization potential. Its characteristics are related to the nature of the fluid medium, the nature of the electrode material, and the overall size and shape of the electrode. It has the characteristics of slow change and a small order of magnitude.
Therefore, selecting appropriate electrode materials and designing electrode shape and size is one of the effective methods to reduce polarization potential. In addition, the rectangular wave excitation technology with alternating positive and negative polarity and the synchronous wide pulse sampling technology of the microprocessor is used to subtract the two sampling values before and after the microprocessor operation function to eliminate the polarization potential interference in the flow signal potential.
The power frequency interference noise is caused by the electromagnetic coupling between the excitation winding of the electromagnetic flow sensor and the input circuit of the fluid, electrode, and amplifier. In addition, the power frequency common mode interference at the working site of the electromagnetic flowmeter and the power frequency serial mode interference introduced by the three power supplies are all caused by the principle of electromagnetic induction.
Firstly, the power frequency interference generated by the electromagnetic coupling between the excitation winding of the electromagnetic flow sensor and the fluid, electrode, and amplifier input circuit has an impact on the operation of the electromagnetic flowmeter. Moreover, the form and characteristics of the interference are different under different excitation technologies, so the anti-interference measures are different. To solve the problems in the operation of electromagnetic flowmeter, new HCMOS series chip technology and microprocessor system power supply voltage monitoring technology can be used.
