Technical & Practical Flow Meter Handbook : Everything You Need to Know About Flow Measurement


Flow measurement is an important process with diverse applications like measuring the flow rate of blood in human beings to measuring and controlling the flow rate in an oil well useful for extraction of oil. It forms an important part of several industries like chemical, wastewater treatment, pulp and paper, oil and gas, and several others. The scope of flow measurement extends far beyond that covered in the following sections. This exhaustive system spans several processes, techniques, and technologies. We have attempted to touch the tip of the iceberg that is flow measurement.

The accuracy of flow measurement determines the functioning of the system. A well-functioning system will provide highly accurate results. Several new technologies are being developed in this realm to support flow measurement systems. These systems as well as direct and indirect processes can help organizations achieve brilliant results.

In this handbook, we will be discussing flow measurement concepts, processes, and systems that are useful to the amateur and professional. They encompass theoretical and practical topics peppered with interesting information and trivia about flow measurement. From the Mesopotamians to Doppler to the most recent group findings, this handbook promises to be an interesting as well as informative read.


As the name suggests, flow measurement is the process of measuring the flow rate and volume of a liquid or gas. This process can be employed to measure the liquid passing through an application (as seen in water purification systems), or stored in an application (as seen in fuel injectors). Flow measurement is a vital function used to monitor and control the rate of liquid flow in applications. This process is used to measure the flow of versatile substances like heavy oils, abrasive chemicals, and light gases. Hence, this process is utilized in applications across various industries.

Flow measurement is employed in critical applications where the flow rate or level of liquid stored needs to be administered regularly. The safe functioning of applications depends on flow meters. In terms of flow measurement, accuracy is of such importance that it can be the determining factor of a company making a profit or loss.


In some applications, the flow needs to be regulated within a specific range. This is achieved by using flow meters. A flow meter is a device used to facilitate flow measurement. Flow meters are broadly classified as:

  1. Differential Pressure
    1. Orifice Plate
    2. Venturi Tube
    3. Flow Tube
    4. Flow Nozzle
    5. Pitot Tube
    6. Elbow Tap
    7. Target
    8. Variable-Area (Rotameter)
  2. Positive Displacement
    1. Reciprocating Piston
    2. Oval Gear
    3. Nutating Disk
    4. Rotary Vane
  3. Velocity
    1. Turbine
    2. Vortex Shedding
    3. Swirl
    4. Electromagnetic
    5. Ultrasonic, Doppler
    6. Ultrasonic, Transit-Time
  4. Mass
    1. Coriolis
    2. Thermal
  5. Open Channel
    1. Weir
    2. Flume


Flow meters can be used to measure the flow rate of liquids or gases. The unit is decided depending on the function and parameters of flow measurement. The unit used varies according to the system of measurement being followed, as well as the material being measured. Dissimilar media need to be measured under diverse conditions and using different units.

Units Used to Measure Flow
The following units are used to measure liquid and gas flow:

      • Liquids are measured based on density: liters per second or gallons per minute
      • Steam is measured based on weight: Tonnes/ hour and kilograms/ minute
      • Gases are measured based on energy content: Joules/ hour and British Thermal Unit/ day
      • Gases are also measured according to STP (Standard Temperature and Pressure) and NTP (Normal Temperature and Pressure) in units like m3/hour and acm/ hour (actual cubic meters per hour). Depending on whether the gas is measured at NTP or STP, the units will include the details. Two examples of the symbol at STP and NTP, respectively are: Std m3/hour and Nm3/hour

The unit of measurement changes in accordance with the medium of material being measured. For example, the units of measurement of liquids, gases, and steam could vary. This is because the change in their density is dependent on different factors. The density of gases is dependent on pressure and temperature. On the other hand, the volume of liquid is independent of pressure. Hence, the units used to measure the different media change accordingly.

Flow Measurement – A Look Back At History

Mapping the history of fluid dynamics and the flow measurement process, this section takes a look at the milestones achieved.

When: 5000 B.C.
Who: Mesopotamians
What: The earliest record of flow measurement can be found in Sumerian cities that were located near the rivers Tigris and Euphrates. The Mesopotamians created channels from the rivers into the city to supply water to every household (an ancient plumbing system, so to say). They used simple methods of flow measurement to monitor the flow rate of water from the rivers into these channels.

When: 3500 B.C.
Who: Ancient Egyptians
What: The Nilometer
The Nilometer is a structure that was built to measure water flow throughout the year. This system helped the Ancient Egyptians predict floods, draughts, and well-balanced water flow throughout the season. It also helped them anticipate and prepare their food and supplies according to the volume of water expected in the upcoming season.

When: 1738
Who: Swiss Physicist Daniel Bernoulli
What: Bernoulli published Hydrodynamica, supporting his theory of conservation of energy in liquid flows. This thought process pioneered the processes used to determine pressure drop in various processes and equipment.

When: 1759
Who: Swiss Mathematician and Physicist Leonhard Euhler
What: Euhler applied Newton’s Second Law of Motion to fluid dynamics. He developed partial differential equations for motion of fluids.

When: 1832
Who: English Scientist Michael Faraday
What: Faraday invented the theory of the dynamo. He has also been attributed to developing the theory responsible for the invention of the magnetic flowmeter.

When: 1842
Who: Austrian Physicist Christian Doppler
What: Doppler discovered and established a relationship between distance and frequency of sound. Almost a century later, his discovery enabled the invention of the Doppler flow meter.

When: 1843
Who: French Civil Engineer Gaspard Coriolis
What: Coriolis is responsible for discovering the drifting of wind and ocean currents caused by the earth’s rotation. This drift varies depending on the location. For instance, the drift is dissimilar at the two poles. The direction of the drift is also dependent on the hemisphere. This has helped further the field of flow measurement greatly.

When: 1845
Who: Irish Mathematician, Physicist, Politician, and Theologian George Gabriel Stokes
What: Based on Claude Navier’s calculations and equations published for incompressible fluids, Stokes derived equations that helped describe the motion of liquids. These equations are known as Navier–Stokes equations. Stokes also developed theories that led him to invent the Stoke’s Law. This law helps calculate drag force in a viscous fluid.

When: 1883
Who: British Engineer Osborne Reynolds
What: He discovered the ‘Reynolds’s Number’, which is a dimensionless ratio. This number helps us calculate the viscosity of a liquid. This is extremely helpful in flow measurement calculation.

When: 1954
Who: Hungarian-American Aeronautical Engineer Theodore von Karman
What: Karman discovered that the vertices formed in water were always constant irrespective of the velocity of water. His discovery facilitated the discovery of the Vortex flow meter. Based on this principle, the first swirlmeter was made available to the public in 1968.

When: 1954
Who: Hungarian-American Applied Mathematician & Physician John Von Neumann
What: Neumann is regarded as the founding father of computational fluid dynamics. His efforts have helped shape major inventions in the field of fluid dynamics in recent times. His theories on artificial viscosity have also enhanced people’s understanding of shock waves.

TRIVIA: What Are Re-discoveries?
Sometimes, when discoveries are made, they are not utilized at that time. There could be several reasons why this happens. Sometimes, people are not able to comprehend the knowledge. At other times, the technology to support the theory has not been developed. Hence, the theory takes a backseat in the minds of people and it can be forgotten over time. When these theories are resurrected, they are known as re-discoveries. For example, when the sub-field of vortex dynamics within the field of fluid dynamics gained momentum, many discoveries and re-discoveries were made.

Why Measure Flow?

It is obvious that wherever needed, flow meters are critical to the functioning of the application. In fact, in some applications, precise measurement of liquid and gases is needed to maintain safety. However, one too many times, flow meters are installed when they are not needed. At other times, the requirements of the application are not assessed correctly. This causes several problems in terms of functionality, not to mention misdirection of company funds.

Flow meters can be used in conjunction with several types of liquids. Various configurations of these devices are available, which allow them to be used with liquids with varying chemical and physical properties. In terms of configurations, the flow meters can be designed with various functionalities, materials, and capacities. The specifications can be customized according to the needs of the application and industry.

For example, specialized flow meters are available for use in wastewater treatment plants. The material used for the construction of the flow meter will vary depending on the pH levels of the water. In addition, the flow meter will have to be designed to accommodate the inflow of the water. In order to ensure maximum accuracy, the capacity of the flow meter should match that of the wastewater flowing through the system.

Factors to Consider When Selecting a Flow Meter

A market survey has claimed that over 75% of industrial flow meters are not performing up to the expected mark. This is mainly caused due to improper product selection. In the initial stages of product selection, buyers can benefit from understanding the basic requirements of their applications. To do this, the right questions need to be asked.

Some tips to help you define your requirements:
Most Important Flow Meter Functions are:

  1. Repeatability
  2. Accuracy
  3. Range
  4. Linearity

Do I Need?

  1. Local or Remote Operation
  2. Local or Remote Output

Is The Liquid Being Measured:

  1. Viscous?
  2. Clean?
  3. Slurry?
  4. Electronically Conductive?


  1. What is the density of the liquid?
  2. What is the expected flow rate?
  3. What will be the operating temperature?
  4. How much pressure is the device expected to handle?
  5. What is your budget?

Flow Meter Features that Increase Efficiency
Look for the following features within your application to ensure maximum efficiency:

  • Construction should ensure:
    • Insusceptibility to vibration
    • Durability
    • Stable output
    • Resistance to corrosion and abrasion
    • Safe operation
    • Small carbon footprint
    • Ease of installation
  • Should feature drainability for:
    • Low maintenance intervals and costs
    • Maximizing uptime
    • Improved accuracy
  • The following feature adds value to the application:
    • Automatic corrosion resistance features, which help in detection of defect or failure in components (like pipes)

Every product has its own advantages and disadvantages. It is important to match your application’s requirements with those of the flow meter. When the features and needs of both are in harmony, the results are outstanding. Manufacturers and suppliers, alike are eager to assist buyers in their quest to finding the perfect flow meter that delivers on all counts of performance and efficiency.

Flow Meter Management: Calibration

Flow meter calibration along with other installation and maintenance procedures is needed to ensure safe operation of plants. Flow meters analyze a very important function. Hence, before purchasing a flow meter, the buyer should consider whether the device can be installed, used, and maintained in the best possible manner.

Why Calibrate?
Flow meters are used in critical applications and functions. Hence, they need to be calibrated to ensure accurate measurements. With constant use, components wear out and flow meters can fall out of calibration. This is true for the most ruggedly constructed devices. The accuracy of the measurement reduces over time. Regular calibration will ensure that all components function efficiently, providing brilliant results.

Common Problems that Demand Regular Calibration
Why should one calibrate? Here are some problems that could occur with a flow meter. These problems disrupt the functioning of machines. However, they can be solved by employing calibration methods. Some of the problems you should watch out for:

  1. Deposits : Dirt, salt, minerals, and foreign materials can be deposited on the interior surfaces of the machine. This disrupts the functioning of the instrument. Even if the machine seems to be functioning well on the outside, internal deposits can cause major problems to the functionality of the flow meter.
  2. Contamination : Several problems can occur if the material within the flow meter is contaminated. For instance, the intricate parts within the flow meter could be blocked causing the entire operation to shut down. Careful testing of the material flowing within the device should be carried out. In some cases, the problem could lie with the device itself. Hence, regular maintenance should be conducted to identify possible areas and reasons for contamination.
  3. Abrasion : When harsh chemicals are used, the surface of the equipment could wear out. You must keep your flow meter safe from chemical attacks.
  4. Natural Wear & Tear : Every product has a life span. Beyond a certain time or magnitude of usage, natural wear and tear will cause aging. Certain components within the flow meter will have to be changed after a certain period. This information will be provided by the manufacturer. Changing the components at the right time will ensure a longer life cycle of the flow meter.
  5. Unsuitable Treatment : If the machine is not used in accordance with the manufacturer’s directives, some parts or the machine as a whole will stop functioning. On a smaller scale, the performance of the machine will be altered. One way or another, the machine should not be abused.
  6. Improper Installation : Some problems associated with the flow meter can be traced to the installation procedures. This will also lead to inconsistencies between the functioning and calibration of the flow meter.
  7. External Influences : The environment of the application and the natural environment both will have some effect on the functioning of the flow meter. The functioning of the flow meter could be affected by electromagnetic radiation, vibrations, temperature and pressure changes, etc.
  8. Difference in Fluid Properties : A flow meter will function optimally when used with the liquid or gas with which it was calibrated. If there is a major inconsistency in the liquid used, the flow meter will fail to provide accurate results.

Best Practices of Flow Meter Calibration at a Micro Level
(Conducted at the Execution Level)
When calibrating flow meters, the following practices will allow you to get the most out of the process:

  1. Accuracy of Standard : It is a good practice to make sure that your standard is extremely accurate. The norm is to keep the accuracy of the standard four times higher than the Unit Under Test (UUT). Depending on the application, this thumb rule could vary.
  2. Traceability of Standard : As with all best calibration practices for most equipment, the standard used to calibrate your equipment should be traceable to a known standard. Traceability is important to verify your measurements. It also helps define the accuracy of your calibration process.
  3. Real Time Calibration : Since the calibration process is conducted in real time, the flow rate of the flow meter should not vary. The flow between the standard and test equipment should be constant throughout the calibration process.
  4. Physical Conditions :The physical conditions during the functioning of the standard and test flow meter should not vary. A slight change in the temperature or pressure conditions can cause a major disruption in the calibration process leading to errors. You must also ensure that there are no leaks, change in volume, or change of medium/ material.
  5. Real Time Conditions: The tests should be carried out in conditions that will be present during the functioning of the flow meter. This will help you accurately match the application’s requirements.
  6. Multiple Testing :Multiple tests should be conducted to verify your initial findings. If there is a major difference in the findings, you will need to validate the accuracy of your standard and other processes and equipment being used.

Industrial Dynamics Tip :
During the calibration process, the common error zone lies in the medium being measured. This zone comes in play when there is some difference between the liquid’s viscosity, density, or heat content at both the stages of testing.
For example, if the density of the liquid is slightly higher during the operation of the standard as compared to the density of the liquid during the operation of the test flow meter, your results will be inaccurate.

Best Practices of Flow Meter Calibration at a Macro Level
(Conducted at a Company Level)
The following practices should be employed by a company on a macro level. It is the responsibility of the managerial function of an organization to put these processes in place:

  1. Scheduled Calibration : A regular calibration schedule should be in place. All flow meters should be calibrated in accordance with the time of operation or life cycle of product.
  2. Accessible Calibration Data : When a flow meter is calibrated, all data should be carefully recorded. This information should be readily accessible to the person in charge. Hence, at one glance, the technician will know when and what changes were made to the device. This will allow them to get an insight into the maintenance procedures implemented on the product.
  3. Certified Laboratory : If using a calibration lab, ensure that they possess the right experience and certifications. You must also not shift from one lab to another as the calibration methods or standards may defer making it difficult for you to draw a comparison between the two.
  4. Reducing Down Time : Down time is a natural occurrence of the calibration process. You can reduce or even diminish this down time by purchasing spare flow meters. Rotating the flow meters will also ensure better functioning and allow tracking comparisons in the functioning of two flow meters.

Although the calibration of most flow meters will fall out at some time due to wear and tear of components, the calibration could also be off due to improper installation or damaged components. Hence, regular calibration will ensure that the flow meter functions smoothly providing precise results.

Industries Benefitting From Flow Meters

Flow meters are used across several industries. Following are some examples of industries and applications, which use flow meters to accurately monitor and measure different liquids:

  1. Industry: Chemical
    Application: Monitoring Flow of Chemicals
  2. Industry: Oil & Gas
    Application: Measuring the Rate of Flow of Crude Oil
  3. Industry: Pulp & Paper
    Application: Measuring Pulp Stock
  4. Industry: Petrochemical
    Application: Measuring Fuel Flow in Commercial Applications
  5. Industry: Food & Beverage
    Application: Wine Filling
  6. Industry: Refining
    Application: Pump Monitoring
  7. Industry: Pharmaceutical
    Application: Production and Packaging of Liquids
  8. Industry: Waste & Wastewater
    Application: Measuring Wastewater Flowing into Water Filtration Systems
  9. Industry: Power & Energy
    Application: Deionised Flow Measurement
  10. Industry: Agriculture
    Application: Monitoring Water Used for Irrigation


Several specifications of the application and flow meter have to be considered when finding the right fit. Several external factors including environmental conditions, budget, etc. also need to be taken into consideration during product selection. Factors such as the media to be measured, viscosity of media, operating conditions (like temperature and pressure), performance expectation, installation conditions, and material of the flow meter need to be paid attention to when selecting a flow meter. The right product will help improve efficiency of the entire process. You can even consult your manufacturer on the best kind of product that will integrate seamlessly with the rest of your system.

The maintenance of the product throughout its service life cycle should be taken into consideration when outlining the budget for the product. This will reduce the surprise factor and help you in planning your finances accordingly. Regular calibrations, among other maintenance procedures are needed to ensure proper functioning of your flow meter. Calibration, as most flow meter owners understand is an important process that helps mitigate any issues associated with performance. Maintaining the accuracy of a flow meter has to be the top most priority of any organization.

Discussion of several such important topics is the need of the hour, as every organization is looking for solutions to flow measurement issues. Understanding the basics will allow you to select, purchase, and handle the instrument better. Flow measurement knowledge is also useful in increasing the efficiency of your products.

Flow measurement is important for environmental sustainability, increased efficiency, safety, and process optimization.