Water and wastewater professionals rely on precise flow measurement to ensure process operation and regulatory compliance. Selecting the appropriate flow meter for each application is crucial for obtaining accurate flow data.
First, we need to have a detailed understanding of the application and the fluid being measured. Furthermore, we need to understand the characteristics of different flow meter types to determine the best match. Electromagnetic and ultrasonic flow meters are widely used in flow measurement processes. We will examine different aspects of flow measurement, including their principles, applications, and compatible media. Here is a comparison of these two flow meters to help you make the right decision.
What are Magnetic Flow Meters and Ultrasonic Flow Meters?
An electromagnetic flow meter, also known as a magnetic flow meter, is a device used to measure the flow of conductive liquids. It is based on Faraday’s law of electromagnetic induction. And it uses the electrical conductivity of the fluid to measure flow.
An ultrasonic flow meter uses an ultrasonic feature to measure the flow rate of liquids or gases. It transmits wall vibrations into the interior of the pipe and then measures the time delay of the ultrasonic waves caused by the gas/liquid passing through the pipe to determine the flow rate. Ultrasonic flow meters operate using two principles: time-of-flight and Doppler flow. These two principles are introduced in detail below.
Advantages and Disadvantages
Magnetic Flow meter
Advantages
- High accuracy of 0.5%, basically meeting the requirements of most working conditions;
- Electromagnetic flow meters can be used to measure industrial conductive liquids or slurries;
- No pressure loss;
- Wide measuring range;
- The measurement process is not affected by the fluid’s temperature, pressure, density, or viscosity.
Disadvantages
- Cannot measure the flow rate of non-conductive fluid;
- The lining’s heat resistance must be considered under high-temperature conditions;
- Measurement accuracy affected by temperature;
- Scaling or wear in pipes can change the inner diameter, causing measurement errors;
- Susceptible to signal interference, a microprocessor-based converter can be used to control the excitation voltage.
- Selecting the excitation method and frequency based on the properties of the measured fluid can eliminate in-phase and quadrature interference.
Ultrasonic Flow Meter
Advantages
- Non-contact measurement;
- No pressure loss and easy installation;
- Can measure the flow of highly corrosive and non-conductive media;
- Wide measurement range;
- Can measure the flow of various liquids and sewage;
- The measurement process is not affected by the measured fluid’s thermophysical properties. such as temperature, pressure, viscosity, and density;
- It can be manufactured in both fixed and portable configurations.
Disadvantages
- Narrow temperature measurement range, generally only able to measure fluids below 160°C;
- Poor anti-interference ability: Susceptible to ultrasonic noise from bubbles, scaling, pumps, and other sources;
- Strict straight pipe section requirements: 20°C in the front and 5°C in the rear. Otherwise, the flow rate will be poor and the measurement accuracy will be low;
- Scaling in the measuring pipe can seriously affect measurement accuracy;
- Poor reliability and repeatability;
- Low accuracy;
- Not suitable for low flow rates (<0.3 m/s).
Sino-Inst Featured Magnetic Flow Meter and Ultrasonic Flow Meter
Working Principle of Magnetic Flow Meter and Ultronsonic Flow Meter
Magnetic flow meters measure flow based on Faraday’s law of electromagnetic induction by measuring the induced electromotive force generated by a conductive fluid flowing in a magnetic field. When a conductive fluid flows through a magnetic field perpendicular to its direction of flow, an induced electromotive force proportional to the flow velocity is generated at both ends of the conductor.
Read More about: How Does a Magnetic Flow Meter Work？
Ultrasonic flow meters measure flow rate by using the time difference, or phase difference, of ultrasonic waves propagating through the fluid. If there are moving particles or bubbles in the fluid, the frequency of the ultrasonic wave changes. This change is called the Doppler shift. By measuring the Doppler shift, the fluid velocity and flow can be calculated. Ultrasonic flow meters can utilize the time difference principle of ultrasonic propagation. A transmitter sends an ultrasonic pulse into the fluid in a pipe. A receiver receives the reflected ultrasonic pulse. The fluid velocity and flow rate are determined based on the time difference and intensity of the reflected ultrasonic pulse.
Read More about: Ultrasonic Flow Meter Technical Guide
Temperature Impact
The maximum temperature of the medium measured by an ultrasonic flow meter does not exceed 160°C. While that of a magnetic flow meter does not exceed 120°C.
The measurement accuracy of ultrasonic flowmeters is significantly affected by temperature. The accuracy of average flow velocity measurements can be improved by optimizing the flow rate algorithm and increasing the signal sampling rate. Temperature compensation can also be used to improve accuracy.
The measuring tube of a magnetic flow meter is usually made of metal. When the temperature changes, the measuring tube expands and contracts. The electronic components within the flow meter, such as the circuit boards and chips in the converter, are sensitive to temperature. Excessive temperatures can alter the electrical properties of these components. To mitigate the error, an appropriate temperature compensation method should be used.
Therefore, for high-temperature media, such as asphalt and thermal oil, etc., we advise that you choose oval gear flow meters or differential pressure flow meters.
Read More about: PT Compensation of Flow Measurement
Accuracy
Electromagnetic flow meters offer high accuracy, achieving ±0.5% to ±1%. They respond quickly to changes in flow rate and are suitable for continuous flow measurement.
The accuracy of ultrasonic flow meters depends on the measurement mode and installation conditions. The transit-time method generally offers higher accuracy, achieving ±0.5% to ±1%. The Doppler effect method, on the other hand, may have lower accuracy. Ultrasonic flow meters may also be slower to respond to changes in flow rate.
Different Installation of Magnetic Flow Meters and Ultrasonic Flow Meter
Electromagnetic flow meters have a relatively fixed installation method. It is installed in the pipe, with the outer surface of the sensor in contact with the inner wall. There should be a certain distance between the sensor and the pipe. Pipes with a certain horizontal and vertical slope are generally selected. The installation location should ensure that the fluid can fill the measuring tube. There should be no bubbles or solid particles in the pipe. Furthermore, it should not be installed in environments subject to vibration, radiation, or other factors that may be detrimental to measurement.
Read More about: Magnetic Flow Meter Installation Guide
Ultrasonic flow meters offer greater installation flexibility. And it can be installed externally without disrupting the pipe. This makes them particularly advantageous in applications where direct installation of the measuring instrument is difficult. If installed in the pipe, the probe surface should contact the inner wall. During installation, be aware of any obstructions in the pipe to avoid measurement errors.
- Ultrasonic flow meters can be installed using either a clamp-on or insert-on installation method.
- The pipe section selected for ultrasonic flow meters should avoid interference and eddy currents, which can significantly affect measurement accuracy.
- Installation should be avoided near strong magnetic fields and vibration interference. such as pumps, high-power radios, and frequency converters.
- The pipe section should be made of uniform and dense material that facilitates ultrasonic transmission.
- Sufficiently long straight pipe sections are required. The upstream straight pipe length must be greater than 10D (D is the pipe diameter) and greater than 5D downstream. The distance from the pump upstream of the installation point should be 30D.
- The fluid should fill the pipe. There should be ample space around the pipe for easy access by personnel.
Read More about: Portable Flow meters | Clamp-on ultrasonic flow measurement
Different Maintenance
Due to no moving parts and being insensitive to corrosive media, Magnetic flow meters have low maintenance costs. However, if scale builds up on the pipe wall, regular cleaning may be necessary.
Ultrasonic flow meters may have higher maintenance costs because they are more sensitive to corrosive media and temperature fluctuations. In addition, ultrasonic sensors may require regular calibration.
Read More about: Flow Meter Calibration – A Comprehensive Guide
Different Cost
Generally speaking, electromagnetic flow meters are more expensive than ultrasonic flow meters. Flow meter prices are affected by various factors, including measurement range, accuracy, and the measured medium. Specific pricing can be negotiated with the supplier.
Choose a Magnetic Flow Meter or Ultrasonic Flow Meter?
Magnetic flow meters and ultrasonic flow meters are two common flow measurement equipment. They mainly measure the volumetric flow rate of fluids within a pipeline. Magnetic flow meters are used to measure the flow of various conductive liquids. such as corrosive media like strong acids, strong bases, and salts. They are also suitable for measuring the flow of many slurries. such as those containing a uniform suspension of liquid and solid phases, such as mud, ore slurry, and paper pulp.
Ultrasonic flow meters can measure the flow of uniformly filled pipes. such as water, seawater, acid and alkali solutions, cooking oil, gasoline, coal, diesel, crude oil, alcohol, and beer, as long as they can propagate ultrasonic waves.
In short, choosing the right flow meter for your needs requires consideration in many aspects. If you’re still struggling with which flow meter to choose, please feel free to contact us. We can recommend the best measurement solution for you.
Read More about:
V-Cone Flow Meter VS. Venturi Flow Meter
Electromagnetic Flow Meter vs Turbine Flow Meter
Are ultrasonic flow meters accurate?
Ultrasonic flow meters are primarily affected by the various factors:
The effects of machining accuracy and temperature changes on mechanical dimensions. The machining accuracy and temperature stability of mechanical parameters. such as the sound path angle, pipe diameter D, and sound path L, have a direct impact on flow measurement. During the measurement process, these parameters will vary with temperature. This error can be minimized through precision machining, appropriate material selection, and appropriate structural design.
Effect of temperature changes on flow rate.
The propagation speed of ultrasound increases with increasing fluid temperature, which can introduce errors. The impact of fluid temperature changes on accuracy can be automatically compensated using temperature compensation methods, which measure the fluid temperature and calculate a temperature compensation mathematical model.
Errors in electrical characteristics.
Device instability and low counting circuit accuracy can introduce errors in flow measurement. To minimize these measurement errors, we can use a 100MHz, high-precision, and highly stable quartz crystal oscillator as the clock reference signal and a large-scale, high-speed programmable logic device (CPLD) to implement complex logic control and high-precision timing functions.
Errors caused by echo waveform and amplitude variations.
These variations can shift the timing comparison point in the ultrasonic flow meter's binarization circuit, causing waveform errors. This error is difficult to overcome using hardware methods and must be addressed through software methods that incorporate intelligent analysis algorithms.
Do magnetic flow meters need straight runs?
The requirements for the straight pipe sections of Sino-Inst magnetic flow meters are as follows:
| Pipe installation type | Installation diagram | Standard Pipeline | |
| Front straight pipe L | Rear straight pipe S | ||
| Pipe bending | a | 10D | 5D |
| Horizontal pipe | b | 5D | 3D |
| Downstream of the valve | c | 10D | 5D |
| Expansion pipe | d | 10D | 5D |
| Downstream of pump | e | 15D | 2D |
| Contraction pipe | f | 5D | 2D |
| Mixed liquid | g | 30D | 3D |
For other straight pipe sections of magnetic flow meters, please consult with the specific supplier.
How to Choose the Right Magnetic Flow Meter?
Based on Sino-Inst's years of experience, we recommend considering the following factors when selecting a magnetic flow meter.
Magnetic flow meters can only measure conductive liquids, so we should consider the conductivity of the measured medium. In general, fluids with a conductivity greater than 5 μS/cm can be measured.
Second, we need to select the appropriate flow meter diameter based on the actual flow range.
The right accuracy level is also an important consideration. Magnetic flow meters usually have an accuracy of 0.5% to 1.0%, with high-accuracy models reaching 0.2%.
We should also consider the operating pressure of the piping system. In general, we should select a flow meter with a pressure higher than the actual operating pressure. To ensure safety and reliability.
The sensor of a magnetic flow meter is in direct contact with the measured medium. So, a suitable lining material can prevent corrosion and wear. Common lining materials include PTFE, PFA, and polyurethane. Accordingly, you can contact the supplier for a suitable lining recommendation.
Select the electrode material for the magnetic flowmeter based on the corrosiveness of the measured medium. Common electrode materials include stainless steel, Hastelloy, titanium, and tantalum etc.
Electromagnetic flow meters can output various electronic signals. However, when selecting an output signal, compatibility with secondary instruments should be considered.
Different installation environments require different types of magnetic flowmeters. Generally, one with an IP65 or higher protection rating should be selected to ensure proper operation in harsh environments. Different installation methods are required for different environments. Electromagnetic flowmeters offer a variety of mounting options, including flange, threaded, and clamp connections.
Ultimately, choose a product that meets your measurement requirements and is cost-effective. And consider the manufacturer's after-sales service capabilities.
During the selection process, we recommend that you consult with Sino-Inst to understand the product' function.
All in all, magnetic flow meters and ultrasonic flow meters are used in various flow measurement applications. The biggest difference between them is that magnetic flow meters can only measure conductive liquids, while ultrasonic flow meters can measure other media. If you are struggling to decide which one to choose, we recommend contacting us.
Sino-Inst is a flow meter supplier from China. Our products are exported all over the world and have been well received by users. We welcome your inquiries.
