In almost any industry, measuring fluid flow matters, whether the fluid is a gas, liquid, vapor or solid powder. In transport, trade and industrial processes, accurate flow measurement keeps operations running correctly, smoothly and safely.
Let’s start by understanding what flow measurement is.
Flow measurement is the use of a flow meter to measure the volume or mass of fluid passing through a specific point over a given period of time. Fluids measured include liquids, gases, vapors or solid powders. Measuring liquid flow is necessary to control and check the quality of industrial processes.
Which flow meter you need comes down mostly to the medium:
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What is a flow measurement device?
A flow measuring device is an instrument that can measure fluid flow (volume or mass of a given fluid) per unit time. Simply put, a flow measurement device is a flow meter.
Common Flow Measurement Units
When measuring the flow of fluids, we generally classify the units into two categories: volumetric flow rate and mass flow rate.
Volumetric Flow Rate: This measures how much volume of fluid passes through a given point in a certain amount of time. Common units include:
Cubic Meters per Second (cms): A metric unit, typically used for large volumes of flow, like in rivers or big pipes.
Cubic Feet per Second (cfs): Used primarily in the United States for measuring large-scale flows, like in water treatment plants.
Gallons per Minute (gpm): A unit familiar in the U.S., especially for smaller applications like household plumbing.
Liters per Second (L/s): Another metric unit, versatile for various scales of flow, popular in many countries around the world.
From / To
Cubic Meters per Second (cms)
Cubic Feet per Second (cfs)
Gallons per Minute (gpm)
Liters per Second (L/s)
cms
1 cms
35.3147 cfs
15850.3 gpm
1000 L/s
cfs
0.028317 cms
1 cfs
448.831 gpm
28.317 L/s
gpm
0.00006309 cms
0.002228 cfs
1 gpm
0.06309 L/s
L/s
0.001 cms
0.035315 cfs
15.8503 gpm
1 L/s
Mass Flow Rate: This measures the mass of the fluid passing through a given point per unit of time. It’s like weighing the amount of a flowing substance, such as grains, that passes through a point. Common units include:
Kilograms per Second (kg/s) or Kilograms per Hour (kg/h): These metric units are widely used in industries where the actual weight of the flowing material is crucial.
Pounds per Hour (lb/h): Common in the U.S., particularly in industries like food processing and pharmaceuticals.
The continuity equation in fluid dynamics is a fundamental principle that describes the transport of some quantity, like mass or energy, within a fluid system. The equation is based on the simple concept that the amount of fluid entering a system must equal the amount of fluid exiting the system, assuming there is no accumulation of fluid within the system.
Simply put, water flowing into one end of the pipe must flow out of the other end.
In mathematical terms, the continuity equation for fluid flow can be expressed as:
Flow=Velocity*Area
Here’s what each term represents:
Flow: This is the flow rate, representing the quantity of fluid passing through a cross-section of a pipe or channel per unit time. Like cubic meters per second (m³/s) or gallons per minute (GPM).
Velocity: This is the speed at which the fluid is moving in a certain direction.
Area: This refers to the cross-sectional area of the pipe or channel through which the fluid is flowing.
The significance of the continuity equation in flow measurement is its ability to relate these three critical aspects.
For instance, if you know the area of a pipe and can measure the velocity of the fluid, you can calculate the flow rate. Similarly, if the flow rate and the area are known, you can determine the velocity of the fluid.
For example:
The diameter of the pipe is 0.5 meters (m).
The velocity of water flowing through the pipe is 3 meters per second (m/s).
Flow = Velocity × Area = 3 m/s × π × (0.5 m / 2 )² ≈ 0.588 cubic meters per second (m³/s).
Flow Measurement Methods
Depending on whether the flow is in an open channel or a closed pipe, flow measurement methods can vary significantly.
Open channel flow measurement:
In open channels, fluids are exposed to the atmosphere, such as rivers, canals, and sewers.
Weirs and Flumes:
Principle: These structures alter flow in a controlled manner. The flow rate is determined based on the changes in water level caused by the structure. Applications: Commonly found in environmental monitoring, irrigation systems and wastewater treatment plants.
Area-Velocity Meter:
Principle: These meters combine water level (area) and velocity measurements to calculate flow. Application: Suitable for different flow conditions and channels where weirs or flumes cannot be installed.
Ultrasonic Doppler:
Principle: A Doppler meter measures the frequency shift of an ultrasonic signal as it reflects off particles in a fluid, while a time-of-flight meter measures the propagation time of the signal. Application: Used in channels and rivers where invasive methods are not feasible or where flow rates vary greatly.
Closed Pipe Flow Measurement Methods
In closed pipes, where the fluid flow is completely enclosed and usually under pressure, various methods are used to measure flow. These methods are critical in a wide range of industrial and utility applications:
Differential pressure flow meter (orifice plate, venturi tube, annubar, etc.):
Principle: They create a pressure drop across the constriction and determine the flow rate based on this pressure difference. Application: Commonly used in liquid and gas measurement in different industries.
Electromagnetic Flowmeter:
Principle: These meters calculate flow by measuring the voltage produced when a conductive fluid passes through a magnetic field. Applications: Ideal for water treatment and other applications involving conductive liquids. Electromagnetic flowmeters also have special models that can measure flow in partially full pipes.
Coriolis mass flow meter:
Principle: Direct measurement of mass flow rate through vibration changes in a fluid-filled pipe. Applications: Critical in the chemical processing, food and beverage industries.
Ultrasonic flow meter:
Principle: Measure the time difference between ultrasonic signals sent downstream and upstream. Application: Non-intrusive measurement for various industries.
Turbine flowmeter:
Principle: The turbine or propeller rotates at a speed proportional to the flow rate and converts this rotational speed into flow rate. Applications: Commonly used in the water, petroleum and chemical industries for measuring low viscosity fluids.
Vortex flowmeter:
Principle: These instruments detect eddy currents created by a bluff body placed in a fluid. The frequency of vortex shedding is proportional to the flow rate. Application: Suitable for flow measurement of steam, gas and low viscosity liquid. They are valued for their sturdiness and low maintenance requirements.
Positive displacement meter:
Principle: Flow is measured by capturing a fixed volume of fluid and counting the number of times that volume is filled and emptied. Application: Ideal for accurate measurement of low flow and high viscosity fluids.
Proven on liquid, gas and steam, and scales to large diameters.
Exact pipe size, accuracy, wetted materials and price depend on the model. Check each meter’s datasheet, or tell us your medium, pipe size and accuracy target and we will match one. Talk to an engineer.
Flow Measurement by Industry
Different industries put different demands on a flow meter, from hygiene to chemical resistance. A few common patterns:
Oil and gas: custody transfer and injection monitoring need high accuracy, so Coriolis and ultrasonic meters are common. They handle high pressure and have no moving parts to wear in the field.
Water and wastewater: large volumes of dirty fluid suit electromagnetic meters, whose open bore does not clog during effluent monitoring and distribution.
Chemical and petrochemical: corrosive fluids and acids make material compatibility the priority, so the meter and its wetted parts have to resist chemical attack through batching.
Food and beverage: hygiene rules call for sanitary connections and Clean-in-Place (CIP) designs, with no crevices where bacteria can sit.
Pharmaceuticals: tolerances are tighter still, so mass flow meters are often chosen for sterile, precise dosing of high-value ingredients.
Flow measurement in instrumentation refers to the use of various devices and technologies to determine the flow rate of liquids or gases in a system. It’s an essential part of process control in industries, helping ensure efficient and safe operation.
Fluid flow measurement is the process of quantifying the movement of a fluid, either liquid or gas, through a conduit or over an open channel. It’s vital in many industries for monitoring and controlling the flow of fluids in processes.
The thermal method of flow measurement involves measuring the amount of heat absorbed or dissipated by a fluid as it flows past a heated element. This method is often used for gases and low-flow applications. Read more about Thermal Mass Flow Meter.
The meter factor in flow measurement is a calibration coefficient that corrects the readings of a flow meter to account for deviations from standard conditions or inherent inaccuracies in the meter.
A flow meter measures the volume or mass of a fluid passing through it over a given time. The measurement is typically expressed in terms of volume (like gallons per minute or cubic meters per second) or mass (like kilograms per hour).
Our mechanical flow meters have pointer indicators to indicate flow. Other flows can be configured with electronic displays to display instantaneous flow, accumulated flow and other information.
Basic methods for calibration of flow measurement include using a standard known volume, a weighing method, or a master meter comparison. These methods ensure the accuracy of flow meters.
To use a pitot tube for flow measurement, insert it into a fluid flow with its open end facing the flow. The pitot tube measures the fluid’s pressure, which can be converted into flow velocity and then into flow rate.
Square root is used in flow measurement when dealing with differential pressure devices. As flow increases, the pressure drop increases by the square of the flow rate, so taking the square root of the pressure reading gives the actual flow rate.
In Coriolis measurement, the flow rate affects the amount of twist or deflection in the flow tubes. Higher flow rates result in more twist, directly correlating to the mass flow rate. The technology is very accurate across a wide range of flow rates.
A common way to answer this is four pairs: laminar versus turbulent, steady versus unsteady, uniform versus non-uniform, and compressible versus incompressible. For choosing a flow meter the laminar versus turbulent split matters most, because it changes how accurately each meter reads.
You measure flow with a flow meter on the pipe or channel. First decide if you need volume (m³/h, GPM) or mass (kg/h), then match the meter to the fluid: electromagnetic for conductive liquids, vortex for steam and gas, ultrasonic for large or full pipes, Coriolis for high-accuracy mass. The methods section above covers open-channel and closed-pipe options.
It depends on whether you track volume or mass. Volumetric flow uses cubic meters per second (m³/s), gallons per minute (GPM) or liters per second (L/s). Mass flow uses kilograms per hour (kg/h) or pounds per hour (lb/h). The conversion tables above switch between them.
The advancements in flow measurement technologies have opened up new horizons. Today’s instruments are more reliable, accurate, and adaptable than ever before, capable of handling diverse applications and challenging environments. Whether you’re dealing with liquids, gases, or solid powders, there’s a flow measurement solution that fits your needs.
We, Sino-Inst, are a professional flow meter manufacturer. We have more than 50 types of flow meters, which are widely used in various industries, including water treatment, oil trading, chemical production, etc.
Not sure which meter suits your process? Send us your fluid, pipe size and accuracy target, and our engineers will recommend a model and a price.
Zhang Wei, possesses 20 years of experience as an automation instrumentation engineer, specializing in the research, design, installation, commissioning, and maintenance of automation instruments.
Face to various instrument communication protocols (such as Modbus, Profibus, etc.), with solid hardware circuit design and software programming skills (proficient in C language and PLC programming). Has extensive project experience; projects he has led and participated in have all achieved outstanding results, improving product accuracy, reducing costs, and increasing production efficiency.
Possesses excellent communication and coordination skills and a strong team spirit, enabling him to quickly respond to customer needs and provide high-quality automation instrumentation solutions.
