Ultrasonic Liquid Flow Meter Vs. Magnetic Flowmeter

Ultrasonic liquid flow meter and Electromagnetic Flow Meters are two common types of flow meters, each with its own advantages and disadvantages. The choice between the two should be based on actual requirements. If the liquid flow is small and the environment is harsh, an ultrasonic flow meter can be selected. If the liquid flow is large, an electromagnetic flow meter is a better choice. It is essential to conduct a detailed evaluation before use to ensure optimal performance.

Key Differences Between Ultrasonic and Electromagnetic Flow Meters

Working Principle:

The electromagnetic flow meter operates based on Faraday’s law of electromagnetic induction. When a conductive liquid moves through a magnetic field, it cuts the magnetic lines of flux and generates an induced electromotive force (EMF) on electrodes positioned perpendicular to both the flow and the magnetic field. The induced voltage is proportional to the fluid’s velocity. Because of this principle, EMFMs require the measured medium to have a certain level of electrical conductivity—typically not less than 5 μS/cm.

The ultrasonic flow meter , on the other hand, measures flow based on the propagation characteristics of ultrasonic waves in a fluid. It calculates flow velocity by detecting either the transit time difference (transit-time method) between ultrasonic signals traveling upstream and downstream, or the frequency shift (Doppler method) caused by moving particles or bubbles. The transit-time method is suitable for clean or low-impurity liquids, while the Doppler method relies on acoustic reflectors in the medium and requires a certain degree of homogeneity.

Accuracy:

Electromagnetic Flow Meter: High accuracy (±0.5% to ±0.2%), with excellent repeatability and long-term stability. Measurements are not affected by changes in fluid density, viscosity, or temperature (within sensor limits). However, it is sensitive to electromagnetic interference, so proper grounding and shielding are essential.

Ultrasonic Flow Meter: Slightly lower accuracy (±1% to ±0.5%) and more affected by pipe conditions. Over time, sensor misalignment, fouling, or changes in flow profile can degrade accuracy. UFMs are also more sensitive to turbulence and flow instability.

Measurement Media:

Electromagnetic flow meters cannot measure low-conductivity liquids, such as petroleum products, organic solvents, or ultrapure water, nor can they measure gases, steam, or insulating liquids. The liner material (e.g., rubber, PTFE) must be selected according to the fluid’s corrosion and abrasion properties. EMFMs are capable of measuring liquids with up to 80% solid content, including wastewater, pulp, slurry, and mud.

Ultrasonic flow meters do not require the fluid to be conductive, allowing them to measure non-conductive liquids (e.g., pure water, oils) and, in some configurations, certain gases or steam. However, the presence of excessive bubbles, solid particles, or uneven sound velocity distribution may cause signal distortion or measurement errors.

Installation and Maintenance:

Electromagnetic flow meters require the measuring tube to be completely filled with liquid and need sufficient straight pipe lengths (typically 5D–10D upstream and 2D–5D downstream, where D is the pipe diameter). For flanged installations, the sealing surface must be coaxial with the pipeline, and the sensor body, measuring tube, and flanges must be properly grounded.

Ultrasonic flow meters—especially clamp-on (non-intrusive) types—do not require pipe cutting or shutdown, making them ideal for systems that cannot be stopped. They are compatible with a wide range of pipe diameters (from a few millimeters to several meters). However, probe spacing must be measured accurately, and coupling gel must be evenly applied. Insertion-type ultrasonic meters require drilling and are affected by the condition of the pipe lining.

In terms of maintenance, EMFMs may require periodic inspection of electrodes and liners for corrosion or wear, while UFMs require regular cleaning of the transducer surface and checking the integrity and shielding of signal cables.

Electromagnetic Flow Meter

1. Advantages

(1) Electromagnetic flow meters can be used to measure industrial conductive liquids or slurries. 

(2) No pressure loss. 

(3) Wide measurement range, with diameters ranging from 2.5mm to 2.6m. 

(4) Measures the volumetric flow rate under actual working conditions without being affected by temperature, pressure, density, or viscosity of the fluid.

2. Disadvantages

(1) Limited applicability as it can only measure the flow of conductive liquids and cannot measure non-conductive media, such as gases and well-treated heating water. Additionally, the liner material needs consideration under high-temperature conditions. 

(2) Electromagnetic flow meters determine volumetric flow based on the velocity of conductive liquids. However, for liquid media, mass flow should be measured, which requires considering the fluid’s density. Different fluids have different densities, which also change with temperature. If the converter does not account for fluid density and only provides volumetric flow at normal temperature, the measurement may be inaccurate. 

(3) Scaling or wear inside the pipeline changes the internal diameter, affecting the predetermined flow value and causing measurement errors. For example, a 1mm change in the internal diameter of a 100mm instrument can result in approximately 2% additional error. 

(4) The transmitter’s measurement signal is a small millivolt-level potential signal that includes interfering signals unrelated to flow, such as in-phase voltage, quadrature voltage, and common-mode voltage. To accurately measure flow, interference signals must be eliminated, and the flow signal must be effectively amplified. Improving the performance of the flow converter by using microprocessor-based converters can help control the excitation voltage and select excitation mode and frequency based on the fluid properties to eliminate interference. However, this makes the instrument structure more complex and increases costs.

Ultrasonic Flow Meter

1. Advantages

(1) The ultrasonic liquid flow meter is a non-contact measurement instrument that can measure the flow of fluids that are difficult to access, observe, or large-diameter pipes. It does not alter the fluid flow state, does not cause pressure loss, and is easy to install. 

(2) It can measure the flow of highly corrosive and non-conductive media.

(3) The ultrasonic liquid flow meter has a wide measurement range, with diameters ranging from 2cm to 5m. 

(4) It can measure the flow of various liquids and wastewater. 

(5) The measurement of volumetric flow is not affected by the temperature, pressure, viscosity, or density of the fluid. It is available in both fixed and portable versions.

2. Disadvantages

(1) The temperature measurement range is limited, typically only measuring fluids below 200°C. 

(2) Susceptible to interference from bubbles, scaling, pumps, and other ultrasonic noise sources, affecting measurement accuracy. 

(3) Requires strict straight pipe sections: 20D upstream and 5D downstream; otherwise, dispersion increases and measurement accuracy decreases. 

(4) Uncertainty in installation can cause significant measurement errors. 

(5) Scaling in the measurement pipeline can severely affect measurement accuracy, leading to significant errors or even failure to display flow. 

(6) Lower reliability and accuracy levels (generally around 1.5-2.5 levels) with poor repeatability. The ultrasonic flow meter determines flow by measuring the fluid velocity and multiplying it by the pipe's cross-sectional area. However, it cannot directly measure the internal diameter and pipe roundness, only estimating the cross-sectional area based on the outer diameter and wall thickness, which introduces uncertainties exceeding 1%, limiting accuracy. 

(7) Short service life (typically, accuracy can only be maintained for two years).