With the continuous development of the instrumentation industry, more and more flow meters are gradually introduced. However, they have not only advantages but also shortcomings. This article briefly presents the advantages and disadvantages of the electromagnetic flowmeter and ultrasonic flowmeter and the main differences.
1. Electromagnetic flowmeter
1.1 Advantages:
1.1.1 Electromagnetic flow meters can be used to measure industrial conductive liquids or slurries.
1.1.2 The electromagnetic flowmeter has no pressure loss.
1.1.3 The range of electromagnetic flowmeter is extensive, and the content of electromagnetic flow transmitters is from 2.5mm to 2.6m.
1.1.4 When the electromagnetic flowmeter is in operation, its volume flow of the measured fluid does not involve the influence of the fluid’s temperature, pressure, density, and viscosity.
1.2 Disadvantages:
1.2.1 The application of electromagnetic flowmeter has certain limitations, it can only measure the liquid flow of conductive media, but it can not measure the flow of non-conductive media. For example, gas and water can be handled well for heating water. In addition, in high-temperature conditions, we need to consider its lining.
1.2.2 Electromagnetic flowmeter determines the volume flow rate in the working state by measuring the velocity of the conductive liquid.
1.2.3 According to the measurement requirements, for liquid media, we should measure the mass and flow rate; measuring the flow rate of the medium should involve the density of the fluid; different fluid media have different densities and change with temperature. Therefore, if the electromagnetic flowmeter converter does not take into account the fluid’s viscosity, only giving the volume flow rate at room temperature is not appropriate.
1.2.4 Electromagnetic flowmeter water supply pipe scaling or wear will change the inner diameter size. It also affects the original flow value, which will cause measurement errors. Such as 100mm caliber instrument inside diameter change of 1mm will bring about 2% additional error.
1.2.5 The measurement signal of the transmitter is a minimal millivolt potential signal. In addition to the flow signal, it is also interspersed with some signals unrelated to the flow, and common-mode voltage. To measure the flow accurately, we must eliminate all kinds of interfering signals and effectively amplify the flow signal.
We should also improve the performance of the flow converter, we had better use the microprocessor type converter to control the excitation voltage, according to the nature of the measured fluid to choose the excitation mode and frequency can exclude the same phase interference and quadrature interference. However, the improved instrumentation is complex in structure and more costly.
2. Ultrasonic flowmeters
2.1 Advantages:
2.1.1 The ultrasonic flow meter is a non-contact measuring instrument that can measure fluids that are not easily accessible and observable. It can also measure the flow of liquids and large pipe diameters. It does not change the flow state of the fluid, it does not cause pressure loss, and it is easy to install.
2.1.2 Ultrasonic flowmeter can measure corrosive solid media, and it can also measure the flow of non-conductive media.
2.1.3 Ultrasonic flowmeter measurement range is wide. Its measuring caliber range is 2cm ~ 5m.
2.1.4 Ultrasonic flowmeter can measure a variety of liquid and sewage flow.
2.1.5 Ultrasonic flowmeter measuring volume flow is not affected by the temperature, pressure, viscosity, and density of the fluid being measured and other thermal parameters. Therefore, it can be made into two forms of fixed and portable.
2.2 Disadvantages:
2.2.1 Ultrasonic flowmeter temperature measurement range is not high, and it can only measure the temperature below 200 ℃ fluid.
2.2.2 Ultrasonic flowmeter poor anti-interference ability. It is vulnerable to bubbles, scaling, pumps, and other sound sources mixed with ultrasonic noise interference, affecting measurement accuracy.
2.2.3 Ultrasonic flowmeter straight pipe section requirements are strict, for the first 20D, after 5D. Otherwise, poor dispersion is poor measurement accuracy.
2.2.4 Ultrasonic flowmeter installation has uncertainty, and it will bring a significant error to the flow measurement.
2.2.5 Ultrasonic flowmeter measurement pipeline scale will seriously affect the measurement accuracy, bringing significant measurement error, even in serious instrumentation without flow display.
2.2.6 Ultrasonic, the electromagnetic flowmeter is not very reliable, accuracy level (generally about 1.5 to 2.5 level), it has poor repeatability. The ultrasonic flowmeter measures the fluid velocity and is then multiplied by the pipe’s internal cross-sectional area to determine the flow rate. However, the flowmeter can not directly measure the inner diameter and pipe roundness, we can only estimate the cross-sectional size according to the outer diameter, wall thickness by a standard circle, so it brings uncertainty has been more than 1%, so its accuracy is limited.
2.2.7 The ultrasonic, electromagnetic flowmeter has a short service life (generally, the accuracy can only be guaranteed for two years).
3. The main differences between ultrasonic flow meters and electromagnetic flowmeters.
3.1 Different media
Ultrasonic flowmeter flow measurement accuracy is almost independent of the measured fluid temperature, pressure, viscosity, density, and other parameters. Therefore, it can be made into non-contact and portable measuring instruments to solve the problems of different types of devices. It can also measure the substantial corrosive, non-conductive, radioactive and flammable, and explosive media flow measurement problems. However, due to the lining material restrictions, a universal electromagnetic flowmeter can not measure higher temperature liquids.
The electromagnetic flowmeter is used to determine the volume flow rate in the operating condition by measuring the velocity of the conductive liquid. Although according to the measurement requirements, for liquid media, the mass flow rate should be calculated, measurement of media flow rate should involve the density of the fluid, different fluid media have different densities, and with the temperature change. If the electromagnetic flowmeter converter does not consider the fluid’s viscosity, only giving the volume flow rate at room temperature is inappropriate.
3.2 Different levels of accuracy
The ultrasonic flowmeter can determine the volume flow rate by measuring the fluid velocity, and it should measure its mass flow rate. However, the instrument measures the mass flow rate by multiplying the volume flow rate by the artificially set density. When the fluid temperature changes, the fluid density is changed, artificially set the density value, we can not guarantee the accuracy of the mass flow rate. We can only measure the fluid velocity simultaneously and measure the fluid density to get the actual mass flow rate value through the calculation.
Ultrasonic flowmeter and electromagnetic flowmeter measurement mediums are different. Ultrasonic is the use of sound waves, and the frequency is shallow, ultrasonic frequency 20KHz-100KHz, radar is the use of 2.4GHz level of electromagnetic waves, ultrasonic is more restrictive, it is easy to be other iron objects interference, in addition to its low frequency, consumption reduction, the measurement range is small, the application of the surface is relatively narrow, commonly used in large diameter water pipeline flow measurement and open channel type flowmeter to measure the liquid level to Converted to flow.
3.3 Different installation, maintenance, and calibration costs
The ultrasonic flow meter is suitable for large circular and rectangular pipes. It is not limited in principle by the diameter of the tube, and its price is largely independent of the diameter of the tube. Not only is this convenient for large pipelines, but we can consider it a preferred option in cases where absolute flow calibration is not possible.
Ultrasonic flowmeters can be used for non-contact measurement. For example, the clamp-on transducer ultrasonic flowmeter can be installed without stopping the flow and cutting off the pipe simply by mounting the transducer on the outside of the established line. This is a unique advantage of ultrasonic flowmeters in industrial flow measurement. In addition, they can be used for mobile (i.e., non-fixed) measurements, and they are also suitable for assessing flow conditions in the pipe network.
Transmitter and converter must be used in conjunction; between the two can not be used with two different models of instruments. In installing the transmitter, from the selection of the installation site to the specific structure and commissioning, we must strictly follow the requirements of the product manual. The installation site must not be subject to vibration or strong magnetic fields.
An electromagnetic flowmeter needs to be installed in liquid conditions with conductivity, and the general installation of an electromagnetic flowmeter must cut off the structure of the tube. Still, the characteristics of the electromagnetic flowmeter are in line with the conditions of the site conditions of high accuracy. Electromagnetic flowmeter dismantling trouble must require the process stop, dismantling the problem of sending inspection. If it is 0.5% accuracy according to the national metrological testing procedures every six months, it must be tested once.
4. Conclusion
Ultrasonic flowmeters and electromagnetic flowmeters are in different environments, and each has its advantages. On small-cost operations, we do not require high measurement accuracy. We use ultrasonic flow meters more; in the installation, maintenance of sufficient funds, the measurement of high accuracy requirements, we should use more electromagnetic flowmeters. Of course, the height and inspection personnel should carefully examine the source of interference on the flowmeter’s working environment and take effective anti-interference measures.
