Liquid Flow Meter Types & Functions

A liquid flow meter measures the flow rate of a liquid moving through a pipe or channel by calculating the volume or mass of liquid passing through per unit of time and is commonly used in industries like water treatment, oil and gas, food processing, and pharmaceuticals.

You’ll find many liquid flow meter types in the market and selecting the right one is critical for optimal performance and efficacy.

Liquid Flow Meter Types

Liquid flow meters can be divided into two major categories, each containing several types, of which we’ll look at a few. The two classifications are volumetric flow meters and mass flow meters.

Volumetric Flow Meters

Volumetric flow meters measure the volume of a fluid passing through a system over time using liquid velocity. Here are the various volumetric flow meters in use:

Differential Pressure Flow Meters

One of the most commonly used flow measurement devices, differential pressure flow meters work using Bernoulli’s Principle which states that pressure decreases as the liquid velocity increases. 

The flow rate can be determined by measuring the pressure drop across a constriction in the pipe made using a basic body or a primary element. 

Pressure sensors such as differential pressure transducers or secondary elements measure upstream and downstream values. 

Differential pressure flow meters are highly used in power, gas, oil, and chemical industries and the two main types are:

Orifice Plate Flow Meters

This differential pressure flow meter measures flow by creating a pressure drop across a thin metal plate with a hole or orifice on the plate placed in the flow path creating a constriction. 

Different types of plates exist such as concentric, eccentric, segmental, and quadrant edge orifice plates, each suitable for specific liquids. Orifice plate flow meters are simple, cost-effective, and reliable.

Venturi Flow Meters

A venturi flow meter is another differential pressure flow meter that uses the Venturi effect to measure fluid flow. It narrows the pipe, reducing the cross-sectional flow area to create a pressure drop. 

Why Differential Pressure Flow Meters Are Beneficial

• They require low maintenance. 
• They provide standardized measurements. 
• They’re powerfully built to handle large flow rates and to function with high-temperature fluids.
• They’re long-lasting.
• They consume low energy.
• They have a high degree of accuracy.

A Significant Limitation

Pressure drops are higher than other flow meters, increasing flow maintenance energy costs.

Magmeters or Electromagnetic Flow Meters

An electromagnetic flow meter is a non-invasive flow measurement device using Faraday’s Law of Electromagnetic Induction to measure the flow of conductive liquids, i.e. fluids containing charged particles. 

The voltage change due to flow is proportional to the flow rate measuring fluid velocity with the help of electrodes. 

It is highly reliable, precise, and durable, and no moving parts means it requires minimal maintenance. Magmeters are widely used in water treatment, power generation, chemical processing, and food production. 

Magmeters are made of: 

• Electrodes in the tube that detect voltage produced by fluid in the magnetic field. 
• A magnetic coil that produces a magnetic field. 
• A flow tube through which liquid passes.

Advantages

• Magmeters measure slurries and corrosive liquids without contact, thereby ensuring longevity. 
• There’s less wear and tear due to the absence of moving parts.
• They’re extremely accurate and reliable for conductive fluids.
• They’re versatile since they measure different liquids and work in caustic and high-pressure environments.
• They’re energy-efficient and sustainable.
• They assimilate easily with other control systems such as digital monitoring programs to track flow instantly, improving operations.

Drawbacks

• A conductive liquid is a must for accurate measurements.
• Magmeters can cost more to buy and install.

Mechanical Flow Meters

A mechanical flow meter uses mechanical components that move with the flowing liquid to determine the flow rate of a liquid. 

They’re in direct contact with the fluid to measure flow and the simplest devices for many industrial and commercial applications such as oil, water treatment, agriculture, food and beverage, chemical, pharmaceutical, and HVAC.

Being used since the 20th century, they’re trusted devices for countless businesses. Turbine and positive displacement flow meters are two of its types.

Turbine Flow Meters 

A turbine flow meter measures the liquid flow rate by detecting the rotational speed of a turbine or rotor placed inside the flow path. 

These meters are widely used in industries that require accurate and repeatable flow measurements, such as oil and gas, water treatment, and chemical processing.

Fluid force, fluid movement, and gravity are crucial to obtain results. The rotor sends electrical pulses to sensors that measure flow based on the number of rotations.

However, here’s why turbine flow meters may not be ideal:

Higher viscosities can generate inaccurate readings due to greater resistance and slower movement of mechanical parts. Flow should be consistent to ensure constant motion.

Caustic or dirty liquids can cause debris or particle buildup that clogs and damages the blades and affects rotation.

Sufficient straight pipe lengths upstream and downstream are required for streamlined flow.

Mechanical parts can wear over time and require periodic maintenance and upkeep.

Low flow rates can minimize efficiency by reducing or stopping rotor movement.

Positive Displacement Flow Meters

Positive displacement flow meters are mechanical flow meters that measure flow by capturing and releasing fixed amounts of liquid through mechanical components and counting the times this volume is filled and emptied. 

The fluids they normally work with are low flow and high viscosity such as petroleum products, industrial chemicals and solvents, syrups, and adhesives. Common examples include helical and oval gear meters. 

For efficient readings, the moving parts must seal tightly to prevent leakage and any liquid that enters must exit.

Advantages

• They can work with different viscosities since they measure small volumes entering and exiting the chambers, which is ideal because temperature and density changes can lead to subtle viscosity modifications.
• Flow conditioning and straight pipe lengths aren’t necessary because pulses are well-tolerated without affecting accuracy or measurement repeatability.
• They don’t need a power supply to operate, reducing energy demands and making them better suited for remote locations.
• They have low-pressure drops across the flow meter. Pressure drops can increase with increased viscosity but are compensated by reduced flow across rotor clearance. 
• They can be installed in horizontal or vertical pipes. 
• Both mechanical counters and digital displays are available.

Disadvantages

• They might give errors with extremely low viscosity such as water due to higher chances of leakage.
• Sudden flow changes may damage internal mechanical parts.
• Moving components may experience deterioration in the long term and require periodic servicing by specialists.
• Regular inspection and routine cleaning of mechanical parts is a must because of the chances of debris collection or buildup which causes obstruction leading to measurement inaccuracies.

Ultrasonic Flow Meters

An ultrasonic flow meter is a high-tech instrument that measures fluid velocity using sound waves. It calculates the time difference between sound pulses traveling between two transducers, a sender and a receiver. 

It’s non-intrusive, great with high-pressure situations and harsh liquids, and a highly accurate option for many industries. 

It’s also versatile because it can work with different fluid types such as clean, slightly contaminated, or viscous. However, liquids with higher viscosity may pose problems. 

Two ultrasonic flow meters, transit-time and Doppler flow meters, are used for liquids such as water, chemicals, oils, acids, and solvents and in settings where flow must not be disrupted at any cost.

Should One Choose A Doppler Or A Transit-Time Ultrasonic Flow Meter?

It depends on your fluid type, how accurate you want your readings to be, and the flow rate range. 

Doppler flow meters work best in liquids containing particles and bubbles off which they can bounce. Transit-time flow meters measure how long sound pulses take to move with or against liquid movement. 

Secondly, Doppler flow meters are great with aerated and dirty fluids while transit-time flow meters must be used with clean fluids.

How Accurate, Suitable, Or Adaptable Are They?

Doppler flow meters use the Doppler effect to measure fluid velocity using ultrasonic signals and suspended particles within the liquid. They’re accurate to ±2% of the measurement. 

Transit-time flow meters use two sensors on either side of the pipe to measure signal travel time between them. Ideally, they’ll offer far superior precision of ±1%.

However, they’re used in different liquids because of their distinct suitabilities and functionalities. Temperature, pressure, and other environmental conditions affect performance. 

Transit-time flow meters, for instance, are influenced by viscosity and density changes needing proper installation and adjustment. Doppler flow meters use the Doppler effect rather than sound’s speed in the fluid and thus aren’t impacted by these changes.

Why Use Ultrasonic Flow Meters

• They’re highly accurate, stable, and suitable for chemicals and fuels.
• Their installation is simple since they clamp on the pipe’s exterior.
• There aren’t any pressure drops when using ultrasonic flow meters.
• They don’t use moving parts or touch the fluid, thus requiring little maintenance and a more cost-effective use.
• They’re useful for food and pharmaceutical industries since they don’t interact with the fluid. They’re versatile, don’t interrupt the flow, and can handle harsh or abrasive liquids in tough industries. 
• They’re more affordable than electromagnetic flow meters.
• They can handle different flow rates and various pipe dimensions.
• Their capacity to measure flow bi-directionally allows for wider use.

Limitation

They’re unusable with liquids having large solid particles due to limited ultrasonic wave access across the fluid.

Mass Flow Meters

A mass flow meter measures the mass of a fluid passing through a point in a system per unit of time rather than its volume. The most common mass flow meter is Coriolis. 

Coriolis Mass Flow Meters 

Coriolis mass flow meters use the Coriolis effect to measure liquid mass flow rates by forcing it across a vibrating tube.

An operational device vibrates a small tube continuously at a natural frequency using a small oscillation force. The deflection is measured using two sensors placed along the tube. The sensors read the same deflection if no fluid passes through.

When there’s liquid in the tube, inertia causes a phase shift which measures the mass flow rate. A larger flow rate will result in a higher phase shift because the two are proportional.

But that’s not all, the Coriolis mass flow meters also measure liquid density independently of mass flow rate. The phase shift calculates the mass flow rate but the frequency measures fluid density. 

Denser liquids emit lower-frequency vibrations compared to those having a lower density.

Their ability to measure density and mass flow rates makes them versatile and useful in major industrial applications such as the life sciences and oil, food, and chemical industries.

A Coriolis mass flow meter is useful because:

• It gives accurate mass flow measurements and has a wide range.
• It can be used with many fluids, including highly viscous ones.
• It works adequately with bi-directional flows. 
• The flow meter isn’t affected by variations in temperature, pressure, density, viscosity, or other fluid properties.
• It can measure two or multiphase mass flows.
• It doesn’t require straight-length pipes since turbulence or swirls aren’t a problem.

However, the flow meter comes with its own set of challenges:

• Since Coriolis devices vibrate at a certain frequency, external matching frequency vibrations can interfere and give inexact readings.
• It’s expensive.

How to Choose The Right Liquid Flow Meter

Choosing a liquid flow meter will depend on your needs and the type of fluid you work with. Here are a few things to consider to reduce potential problems down the line:

Fluid Type

Liquids come in different viscosities and densities, have varying conductivity, and are either clean or contain particles. 

Flow meters must be compatible with the type of liquid you’re working with to ensure meter accuracy. Thus, a thorough fluid analysis is required to find the best flow meter for accurate readings. 

Flow Rate

Check your fluid flow rate and whether the flow meter you’re selecting can match this flux. For instance, turbulent flow or swirling caused by bends and obstacles can give imprecise measurements due to disruptions. Good pipe design should therefore be considered.

Pipe Size And Installation

Proper guidelines must be followed during installation. Ensure your flow meter is compatible with pipeline size and space constraints. 

The meter should be ideally positioned and fit flawlessly where there’s the least obstruction or turbulence. Pipe orientation and pertinent straight lengths should be considered to reduce flow disturbances and measure accurately.

Accuracy Requirements

You may need a higher precision with certain applications which only certain flow meters can provide. 

Flow meters vary in accuracy depending on where and how they’re used. For instance, pairing an ultrasonic flow meter with a dirty fluid will result in a mismatch and imprecise readings. 

Environmental Factors

External conditions such as temperature, pressure, and corrosion can affect flow meter performance. Measurements won’t be precise if the meter isn’t sturdy or not efficient for a particular use.

Magmeters, for instance, can be affected by external magnetic fields and moisture.

Flow Meter Maintenance

As with all objects, flow meters require regular maintenance to function optimally. This includes regular cleaning, inspection, and calibration to minimize errors and ensure long-term stability.

However, some require more thorough and frequent maintenance like flow meters containing mechanical parts and those in contact with the fluid they’re measuring.

Long-Term Benefit

Some flow meters, like the electromagnetic meter, have higher initial costs compared to others, like mechanical flow meters. 

However, they’re a better long-term investment because they don’t use mechanical components that can be damaged over time. They also give more accurate readings, ensuring smooth operations. 

Industry-Standard Compliance

Flow meters must be industry-compliant and have adequate certifications such as IEC or ISO. Authentications prove that the device functions optimally. 

Each industry has its certification requirements depending on the type of fluids handled. For instance, dedicated flow meters measure the flow of potable water. Following safety precautions ensures you follow regulatory excellence.

Optimization Techniques

Certain techniques help optimize flow meter performance and ensure smooth long-term functioning. They often accompany extra purchases to maintain flow and ensure durability. Here are the top three:

Flow Conditioning

Flow conditioning tools are available to help streamline flow by reducing turbulence to give precise readings.

Material Selection

Meter components can unfortunately corrode over time if not constructed with proper materials. All parts chosen for its manufacture should be corrosion-resistant and robust to perform optimally and for a long time in the harshest environment. 

Regular Monitoring

Regular diagnostics using efficient and relevant equipment help to detect and solve issues that may arise on time. This reduces downtime so your processes can continue running smoothly and without delay.

The Biggest Advantage Of Certain Flow Meters

As businesses evolve and automation takes over, leaders look for comprehensive solutions that reduce maintenance hassle while providing top-notch operational efficacy and durability. Certain flow meters do just that because they contain no mechanical components.

Mechanical flow meters use moving parts that render the device sensitive to caustic liquids or cause gradual deterioration. This means higher maintenance costs due to the need for regular cleaning or replacements. 

Flow meters like magmeters have no mechanical depreciation and last longer, giving reliable precision throughout. They’re perfect for abrasive, viscous, or dirty fluids for which other meters might not provide accurate readings. 

The non-invasive structure makes these devices efficient and resistant.

The Difference Between A Flow Sensor And A Flow Meter

A flow sensor senses the presence of fluid and its flow rate. This detection produces an electrical signal proportional to the flow rate. They’re used in applications such as domestic plumbing and HVAC systems.

A flow meter is more comprehensive and normally includes a flow sensor and measures and displays the flow rate. A display unit shows the reading measured. They monitor flow rates in industrial settings such as for vehicle fuel or fluids used in medical equipment. 

Liquid flow meters are essential because liquids are used in almost all fields, including for fire hydrant testing. 

Sound knowledge of liquid flow meter types and their uses helps manufacturers understand the subtleties of measuring flow rates effectively and reducing chances of error. If you’re lost or desire more knowledge, our experts are here to assist you. Get in touch for the latest updates in liquid flow measurements and management.