Flow sensors – Advantages and disadvantages explained

Flow sensors are devices designed to measure the flow rate of gases, liquids, or solids flowing through a pipe or conduit over a certain amount of time. This flow measurement is important for controlling many industrial processes and applications to ensure machinery’s efficient and optimum performance.

For instance, flow sensors in automobiles measure the engine air intake and adjust fuel delivery to the fuel injectors to provide optimum fuel. Flow sensors can also be found in medical ventilators that ensure the correct air and oxygen supply to patients for respiration.

Flow sensors can be partially mechanical or electronic, using ultrasonic detection of the flow. They are further categorized into differential pressure flowmeter, positive displacement flowmeter, variable area flowmeter, magnetic flowmeter, vortex flowmeter, thermal flowmeter, multivariable flowmeter Airflow sensors. The choice of the most suitable sensor type depends on various factors such as nature, fluid viscosity, flow rate range, and the required accuracy of the measurement.

All different flow sensors generally fall into three categories: positive displacement flow sensors, mass flow sensors, and velocity flow sensors. This article will discuss these flow sensors, including how they operate and their advantages and disadvantages.

1. Positive displacement

Positive displacement flow sensors are unique among other flow sensors since they perform direct measurement of the volume of the fluid passing through the device. Other sensors do not directly measure flow rate but instead measure a different parameter like pressure and then use it to derive the flow rate.

In a positive displacement flow sensor, a known fluid volume is trapped and moved through the rotating parts. It rotates the components, and when it completes a full revolution, it is estimated that a known fluid volume has passed through the sensor. Counting the rotations which take place per unit time establishes the fluid volume passed per unit time. Since the fluid flow directly moves the components, the rotational velocity is directly proportional to the flow rate.

These sensors can function over a wide range of fluid viscosities, have low maintenance requirements, and provide a mechanical or electronic interface. Their applications include measurement of oils, gasoline, hydraulic fluid, and home installed metering of water and gas.

Pros:

  • High precision components that have minimum clearances
  • High repeatability and accuracy, limited only by the amount of slippage.
  • Low maintenance and cost-effective
  • Provide mechanical or electronic interface.
  • Require no power
  • Can handle high pressures, entrained gases, and suspended solids.
  • Relatively easy to design

Cons:

  • Expensive to install and maintain owing to moving parts.
  • Susceptible to corrosion by water-based fluids
  • Relatively complex design increases cost
  • Require clean fluid, and some meters can actually block the flow if a larger particle is trapped in the wrong place.
  • If the flow rate increases, the size of the sensor seems to increase.

2. Mass flow

A flow sensor is a device used to measure the flow rate or quantity of a liquid or gas moving through a pipe by determining the mass per unit time (e.g., kilograms per second).

Sometimes called mass airflow sensors, they operate on the “Coriolis principle.” They are widely found in automobiles, where they are used to measure the air entering the air intake system of an internal combustion engine.

They operate by measuring the transfer of energy from a heated surface to a flowing fluid. There are several methods to accomplish this: (1) introduce thermal energy and measuring the temperature change; (2) maintain a constant temperature and measure the amount of energy needed to do so; (3) introduce electric current to a resistive wire and measure the current needed to maintain temperature. There are two types of mass flow sensors available today: thermal and Coriolis flowmeter.

Pros:

  • Directly measure liquid flow with high accuracy
  • Used for a wide range of measurable fluids, including highly viscous liquids
  • Bidirectional flow measurement.

Cons:

  • Poor zero stability.
  • Cannot measure liquids with low density
  • Highly sensitive to vibration interference.

3. Velocity flow

Velocity flow sensors detect flow rate by determining the velocity of fluid flowing through the sensor. These flow sensors come in several varieties, including mechanical (e.g., turbine, propeller, and paddlewheel), electromagnetic, and ultrasonic.

  • Mechanical: Fluid flow is measured by a paddlewheel movement detected by a magnetic coil or infrared sensor. A rotating mechanical device such as a paddlewheel sits directly in the flow path. As the fluid moves, the paddlewheel rotates, and this rotation is detected by a magnetic coil or infrared sensor. The flow sensor’s electronics then converts the rotations into an output signal, programmed to represent a given volume output per unit time. Mechanical sensors are commonly used in water/waste treatment plants. They are cost-effective, compact, and need very little energy to operate. They can detect a wide variety of fluids. However, moving parts are subject to wear and a minimum amount of fluid needed to move the paddle wheel. It can also build-up contamination due to the flow of dirty fluids.
  • Electromagnetic: Electromagnetic flow sensors (EMF flow sensors or magnetic flow sensors) operate on Faraday’s law of induction. They have a coil that induces a magnetic field in the fluid and uses a set of electrodes to measure the voltage that results from the conductive fluid flow through the magnetic field. The voltage will be proportional to the velocity of the fluid in the pipe. The velocity measurement can then be converted into a volumetric flow rate, knowing the velocity and dimensions of the pipe’s cross-section. Electromagnetic flow sensors require a minimum level of conductivity on the fluid being measured. Although they are suitable for fluids with some degree of contamination, they may not function with non-conductive fluids like oil, steam, or gas. Electromagnetic flow sensors are also not well suited for use in vacuum conditions, with fluids that may have abrasive characteristics and fluids with the presence of ferromagnetic particles. They are widely used in chemical manufacturing, petrochemical industries. They can measure liquids with some degree of contamination, and pressure drop is not induced in the pipe. These sensors do not function with non-conductive fluids and are not suitable for vacuum conditions. They require fluids to have some level of minimum conductivity.
  • Ultrasonic: In an ultrasonic sensor (also called transit time ultrasonic flow sensor), a pair of ultrasonic transducers generate a signal directed into the fluid flow, and each signal is directed back to the receiver using a set of mirrors. The transducers and mirrors’ orientation is set so that one signal is traveling for part of its path with the fluid flow while the other is traveling against the fluid flow. The receiver measures the transit time of each signal and computes the time difference between the two. When the fluid is not in motion, the transit times are the same for each signal – when the fluid is moving, the signal moving with the flow will have a shorter transit time. Therefore the difference between the two signal transit times reflects the velocity of the fluid. This sensor is used in facilities management, aquafarms, pulp, and paper manufacturing. Using both conductive and non-conductive fluids can handle high temperatures and pressures and can be non-wetted. However, fluids with air bubbles cannot pass through ultrasonic energy; and high vibrations can cause difficulty in reading and high cost.

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