What Is A Flow Meter Device? Different Types & Functions

Flow meters or flow sensors are devices used to measure the rate at which a liquid, gas, or vapor flows through a fixture such as a pipe or channel. 

They help monitor fluid movement in terms of linear or nonlinear mass or volumetric flux in various industries such as healthcare, manufacturing, water treatment, and fire hydrant testing.

This blog will detail the best flow meter device for different fields and situations so you can be well-equipped. 

Flow Meter Device Types

Many different types of flow meters are available in the market depending on needs and applications. Here are a few examples based on the purposes they serve. 

Mechanical Flow Meters

These use moving parts such as turbines, impellers, gears, or rotors directly interacting and moving with the fluid to measure its volume or flow rate. 

Mechanical flow meters are commonly used for measuring water, fuel, oil, and other liquids with low to moderate viscosity.

They’re beneficial because they work with fluids as long as the flow is consistent for its moving parts to remain in motion, and are independent of power sources. 

There are several mechanical flow meter types of which three are detailed here:

Turbine Flow Meters 

Turbine flow meters use a rotor or a turbine to check liquid movement as it passes over the rotating part and the rotational speed measures the flow rate. Sensors then convert it into a readable measurement. 

It’s often used in industries using water, fuels, and chemicals. This mechanical flow meter is ideal for low-viscosity fluids that won’t damage the turbine.

Positive Displacement Flow Meters

These flow meters consist of chambers and are unique in how they work. The compartments fill and empty with liquid and total flow is measured based on the number of times the filling and emptying occurs. 

Common meters in this category are piston and oval gear meters, and they’re indispensable for measuring viscous fluids such as syrups and oils.

Rotameter Flow Meters

A rotameter measures liquid or gas flow rates using a float’s position inside a uniformly tapered tube. The device works against gravity in vertical tubes where fluid flows upwards. 

To get the flow rate reading, we read the tube’s measurement scale corresponding to where the float is. The concept is simple, inexpensive, and reliable.

Advantages Of Using Mechanical Flow Meters Over Digital Meters

Despite the advancement of technology and the introduction of digital meters in the market, mechanical meters still hold significant ground. Why? 

• They’re durable and long-lasting and often a one-time investment in many applications.
• They’re not dependent on external power so they work in any setting, be it remote or during power failures.
• They’re cost-effective since they have a lower initial cost which in combination with their durability renders them popular.
• They’re reliable because they’ve been used for many many years.
• They’re simple to operate since they don’t have many electronic components which reduces errors in their readings.

Differential Pressure Flow Meters

Differential pressure flow meters use Bernoulli’s principle which describes the relationship between fluid velocity, pressure, and height, and states that as fluid velocity increases, pressure decreases, keeping fluid height horizontal. 

These meters measure the flow rate using pressure drops across a constriction in the flow path. As the fluid passes through the restriction, its velocity increases and pressure decreases, creating a measurable difference. 

Pressure sensors within the instrument measure pressures before and after the obstruction, giving accurate readings. Different types of differential pressure flow meters exist such as: 

Orifice Plate Flow Meters 

A type of differential pressure flow meter, orifice plates measure the flow rate of a fluid by creating a pressure drop across a thin, perforated plate installed in a pipeline. The flow rate is determined using standard equations that relate differential pressure to fluid velocity. 

They’re one of the simplest and most widely used flow measurement devices in industrial applications. Many orifice plates exist depending on the fluid passing through, such as eccentric and concentric orifice plates.

Venturi Flow Meters

Venturi flow meters measure pressure differences before and at the constriction as fluid velocity increases and the corresponding pressure decreases. The flow rate is calculated with these changes in flow. 

Pressure sensors measure pressure differences between the wide and narrow sections and the flow rate is calculated using equations that relate pressure difference to velocity.

Flow Nozzles

Fluid velocity increases as it passes through the nozzle, creating a pressure drop that is used to determine the flow rate. 

Pressure sensors measure pressure before and after the nozzle to calculate the pressure drop and the flow rate is calculated based on the pressure difference using flow equations. 

Flow nozzles are more stable than orifice plates, especially during high velocities and temperatures. For example, flow nozzles have measured high flow rates in superheated steam. 

However, one disadvantage is that they’re oftentimes more difficult to replace than orifice plates.

Differential Pressure Flow Meters Are Favored In Many Applications

• They’re versatile and can work with steam, liquids, and gases.
• They can be used to measure viscosity. 
• They can work in extreme pressure and temperature conditions.
• The measurement range can be adjusted to cater to various industries.
• They don’t use moving parts which makes them more reliable and robust.
• They can measure flow in any direction.
• They’re available in different sizes and can be used in various pipes.

Electromagnetic Flow Meters Or Mag Meters

These volumetric flow non-mechanical meters utilize Faraday’s Law of electromagnetic induction to calculate fluid velocity. Faraday’s law states that a conductor in a varying electromagnetic field conducts an electrical signal. 

This signal is directly proportional to the fluid’s speed through the magnetic field. The flow creates changes in voltage which in turn measures the speed of the liquid. The quicker the flow, the higher the voltage. 

They accurately measure water-based fluids passing through a pipe. The meters are low-maintenance, accurate, and durable since they don’t have moving parts and are designed to be obstruction-free so there’s no flow hindrance. 

Electrodes embedded in the tube or sensor pick up signals by creating an electromagnetic field through which the liquid passes and voltage is recorded. 

The Advantages Of Mag Meters 

• Mag meters are cost-effective because they produce highly accurate measurements, typically ±0.5% or better accuracy, straightforward installation, and little to no maintenance because they don’t use moving parts.
• They measure flow bi-directionally due to their use of magnetic fields. 
• There are no pressure drops unlike mechanical meters since flow isn’t obstructed.

Ultrasonic Flow Meters

Ultrasonic flow meters are non-invasive devices that measure the flow rate of liquids using high-frequency sound waves. Unlike mechanical meters, they don’t have moving parts nor do they obstruct flow, making them ideal for different kinds of fluids. 

However, they’re ideally used for dirty, corrosive liquid or wastewater applications or where low maintenance, chemical compatibility, and low-pressure drops are necessary and may not work with drinking or distilled water. 

There are two types of ultrasonic flow meters and they function by using different technologies:

Transit-Time Ultrasonic Flow Meters 

These are the most commonly used ultrasonic flow meters and consist of two transducers placed on opposite sides of the pipe that send and receive ultrasonic signals. 

Upstream and downstream calculations are then compared using the time difference of the sound traveling with and against the flow to calculate velocity and flow rate. If there’s no movement, travel time is the same in the two directions. 

Sound will travel faster with the flow than against it. There shouldn’t be a lot of bubbles or solids in the liquid which can reduce the intensity of sound waves traveling across. 

Transit-time flow meters are best used for low-viscosity and clean fluids like chemicals, oils such as crude oils, and water. 

To calculate flowing velocity, the following formula is used:

Vf = K dt/TL

Where K = calibration factor

dt = time differential between upstream and downstream transit times

TL = zero-flow transit time

Doppler Ultrasonic Flow Meters

The Doppler ultrasonic flow meter uses the Doppler effect or the Doppler shift in which frequencies received depend on the motion of the source relative to the sound source. The first transducer sends sound waves that bounce off particles or bubbles in the fluid. 

The ultrasonic beam shifts frequency based on the particle motion and this frequency shift of the reflected sound is measured by the second transducer to calculate flow speed. 

It’s best for dirty, aerated, or slurry liquids such as wastewater, pulp, and mining slurries and can’t be used in clean fluids. When ultrasonic pulses bounce off particles in the fluid, the frequency change is directly proportional to the flow rate of the liquid.

Ultrasonic Flow Meters Are Beneficial 

• They’re low-maintenance and consume less power since they don’t have moving parts, helping their batteries last long.
• No moving parts mean they don’t degrade, create a blockage, or measure less accurately over time. 
• They’re highly accurate if the meter is properly installed.
• Digital options are available, communicating all measurements in real-time.

How To Choose A Flow Meter

There are several factors to consider when choosing an ideal flow meter. Here are the top ones:

Identifying Fluid Type 

The type of fluid you need to measure is one of the most important factors when choosing a flow meter. Different flow meters work better with specific liquids, gases, or slurries due to their properties. 

Liquids are the most commonly measured fluids and they can be clean, viscous or thick, or dirty or particulate-loaded. Gases are also used in applications, such as clean and dry gases or steam. 

Multiphase and slurry fluids contain liquid and solid particles or gas bubbles suitable for certain flow meters such as Doppler ultrasonic flow meters. 

Considering Flow Rate, Pipe Size And Installation

When choosing a flow meter, you must consider factors such as pipe size, fluid flow rate, and pipe installation. Flow rate translates to how much fluid passes through the pipe over time. 

Some meters handle high flow rates, while others work better with low flows. Therefore, measuring the pipe’s internal diameter where the meter will be installed is crucial. 

You must also consider how and where the meter is placed to ensure accurate readings. For instance, Ultrasonic flow meters can be clamped onto pipes without cutting, while others need to be installed in-line. 

Flow rate range is another aspect to be considered. A wrong flow meter can give inaccurate readings if outside its range and cause it to overload.

Lastly, correct installation is key to getting precise flow measurements. You should match your flow meter to the following features:

Pipe Orientation

Pipes can be vertically or horizontally oriented and flow meter selection will depend on pipe installation. Rotameter flow meters, for instance, work best in pipes arranged vertically. 

Straight Pipe Length

Most flow meters require straight pipe arrangements before and after the point of installation to ensure accurate readings.

Turbulence Considerations

Fluids can have laminar, turbulent, or swirling flow. Laminar is a smooth and steady flow that ensures stable readings and is a common phenomenon when fluid speeds are low.

Turbulent flows are chaotic and have high speeds and straight pipes are recommended for the readings to be accurate.

Swirling flow is more frequent in valves, pumps, or in or near turns.

Installation Positions

Keep in mind these rules when installing your flow meter to get the most accurate and trustworthy readings:

Upstream Or Downstream Installation

Upstream installation means installation close to the source where flow is more stable while downstream installation places the flow meter after the system disturbance caused by pumps or valves. 

Location Selection

Most flow meters perform optimally in straight sections because of the higher chances of laminar flow. Flow conditioners improve fluid stability and are available in the market. They reduce turbulence so you can get accurate measurements. 

Assuring that the pipe is full of water and there are no air spaces at places where you install the flow meter is also important. 

Pipe Size And Material 

Large diameters ensure laminar flows and reduce the chances of swirls or turbulence. If you use small pipes, straight pipe sections should be considerable upstream and downstream so flow can stabilize. 

Take into account features like corrosiveness, conductivity, and compatibility with flow meters when deciding on pipe material. 

Environmental Considerations

Environmental factors such as temperature, pressure, external vibrations, dust, and moisture can all impact flow meter performance. Different flow meters are suitable for distinct environments and conditions. 

Check whether the meter you’re installing can withstand extreme temperatures and pressures if that’s what’s needed. 

Secondly, flow rates must be measured for different fluids requiring special flow meters. Electromagnetic meters handle sludgy fluids, but turbine meters clog easily.

Electromagnetic flow meters should avoid these external factors to function optimally:

Strong magnetic fields

Generally, strong magnetic field interference doesn’t happen because magnetic flow meters aren’t placed near them. However, the sensor’s structural design must be considered to avoid future malfunctions. 

The excitation coil housing must be made of steel instead of non-magnetic substances like plastic or aluminum.

Strong Electromagnetic Waves

Interference from strong electromagnetic waves such as radio waves can happen, so it’s best to check whether the flow meter complies with compatibility requirements. 

Moisture Ingress

Drainage and water supply industries often install electromagnetic flow meters underground in enclosures with certified protection ratings such as IP67 and IP68. However, moisture can still seep in after rainwater is allowed to collect and sit for days. 

Terminal box covers for on-the-ground installations should be sealed properly because temperature changes can impact moisture access. Water ingress can cause device damage and give inaccurate readings. 

Ensuring Accurate Readings 

Flow meter accuracy refers to how close the measured flow rate is to the actual flow rate. Precision is one of the most critical factors when selecting a flow meter. The exactness depends on how and where the readings are used. 

They can be for precision dosing in pharmaceuticals, fuel measurements, or general process control in industrial applications. You can use a turbine or other mechanical flow meters for general monitoring. 

Also, remember that meters drift over time. Regular calibration and maintenance when needed ensure continued accuracy. 

Budget, Maintenance, And Cost Efficiency

You’ll find low-cost options in the market such as mechanical meters, though they may be less accurate and require more maintenance. 

Ultrasonic meters are great long-term investments due to their high accuracy, even if they’re generally more expensive. They’re also low maintenance because they don’t have any moving parts and require minimal servicing.

Flow meters are maintained with periodic inspections for corrosion, signs of wear and tear, and physical damage. Replacements are necessary if components no longer function as they should.

Regular cleaning to remove sediment and debris is also essential to keep readings accurate.

You must analyze your requirements to select the best one. 

Mass Versus Volumetric Flow Measurement Units 

Mass flow rate measures how many molecules of a flowing gas pass through a cross-sectional area per unit of time, whereas volumetric flow rate considers how much fluid volume passes through a cross-sectional area per unit of time, keeping temperature and pressure constant. 

Gases are compressible and impacted by temperature changes. Volumetric flow rates can thus change drastically based on temperature and pressure alterations.

When To Use One Or The Other

Which you use depends on your needs and applications. Use mass flow when you focus on the gas molecules instead of the amount of space they occupy. 

For instance, constituent gas concentrations must be kept constant relative to other gases in gas mixing applications. Another example is custody transfer applications where the gas’ mass is more important than the space it occupies. 

Mass flow meters such as the Coriolis flow meter are great examples of flow meters that use mass flow to calculate measurements. They provide precise readings based on mass rather than volume and are often used in chemical applications.

Volumetric flow is important when you focus on the volume occupied such as the volume of fluids in pipes. It’s useful when installing flow meters and in Bernoulli’s equations.

The relationship between the two is simple. The volumetric flow rate equals the mass flow rate divided by the fluid density. In the case of a constant density, the relationship is predictable. The density changes with pressure and temperature changes. 

If you need more details or have further questions, contact us! We understand how tough it can be to choose and install the correct flow meter for your industry.