The working principle of a magnetic flow meter, also known as an electromagnetic flow meter or magmeter, is based on Faraday’s law of electromagnetic induction. Here are the basic steps involved in the operation of a magnetic flow meter:
- Electromagnetic Coil: The magnetic flow meter consists of a flow tube or pipe through which the conductive fluid flows. Inside the flow tube, there are two sets of electromagnetic coils, typically mounted on opposite sides of the tube. One coil acts as the excitation coil, while the other serves as the sensing coil.
- Magnetic Field Generation: A steady direct current (DC) is passed through the excitation coil, creating a magnetic field that extends through the flow tube and perpendicular to the direction of fluid flow. The magnetic field lines are evenly distributed across the cross-section of the flow tube.
- Flowing Conductive Fluid: The conductive fluid, such as water or other liquids with sufficient electrical conductivity, flows through the flow tube. As the conductive fluid moves through the magnetic field, it cuts across the magnetic field lines.
- Induced Voltage: According to Faraday’s law of electromagnetic induction, when a conductive fluid moves across the magnetic field lines, a voltage is induced in the fluid proportional to its velocity. This induced voltage is directly related to the flow rate of the fluid.
- Sensing Coil Measurement: The induced voltage is sensed by the sensing coil, which is perpendicular to both the magnetic field and the fluid flow direction. The sensing coil detects the voltage across the fluid and converts it into an electrical signal.
- Signal Processing: The electrical signal from the sensing coil is sent to the flow meter’s electronics for processing. The signal is typically amplified, filtered, and converted into a usable form, such as a linear 4-20 mA current signal or a digital value.
- Flow Rate Calculation: The processed signal is then used to calculate the flow rate of the fluid based on the relationship between the induced voltage and the fluid velocity. The flow meter may have a built-in calibration curve or a programmable equation to convert the electrical signal into a flow rate value.
- Display and Output: The calculated flow rate can be displayed on the flow meter’s local display or transmitted to external devices, such as control systems or data loggers, for further analysis or recording.
Following are key features and advantages of magnetic flow meters:
- Suitable for conductive fluids: Magnetic flow meters are designed to measure the flow of conductive fluids such as water, slurries, acids, bases, and most other liquids that have a minimum level of electrical conductivity. They are not suitable for non-conductive fluids like oils or hydrocarbons.
- High accuracy and wide turndown ratio: Magnetic flow meters offer high accuracy, typically within ±0.5% to ±1% of the measured flow rate. They can maintain this accuracy over a wide range of flow rates, known as the turndown ratio, which can be as high as 100:1 or even higher.
- No moving parts in the flow path: Magnetic flow meters do not have any moving parts in the flow path, which reduces the risk of mechanical wear and minimizes maintenance requirements. This makes them suitable for applications where debris or particles may be present in the fluid.
- Insensitive to fluid properties: Magnetic flow meters are not significantly affected by changes in fluid properties such as viscosity, density, or temperature. This makes them suitable for measuring fluids with varying properties without the need for frequent recalibration.
- Minimal pressure drop: Magnetic flow meters have a relatively low pressure drop compared to some other flow meter types. This reduces energy consumption and minimizes the impact on the overall system.
- Bi-directional flow measurement: Magnetic flow meters can measure both forward and reverse flow, allowing them to be used in applications where flow direction changes or where flow reversal occurs.
- Wide range of pipe sizes: Magnetic flow meters are available in a wide range of pipe sizes, from small diameters to large pipelines, making them suitable for various applications across different industries.
Following are the limitations of magnetic flow meters:
- Electrical conductivity requirement: Magnetic flow meters require the fluid being measured to have a minimum level of electrical conductivity. Non-conductive fluids cannot be accurately measured using magnetic flow meters.
- Installation considerations: Proper installation is crucial for accurate measurements with magnetic flow meters. Straight pipe runs, absence of air pockets, and proper grounding are important factors to consider during installation.
- Cost: Magnetic flow meters can be more expensive compared to some other flow meter technologies, especially for larger pipe sizes or specialized applications.