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Significance of Calibration in Flow Meters

What is flow meter calibration?

Flow meter calibration is a process of verifying and adjusting the accuracy of a flow meter by comparing its measurement performance against a known reference standard. The calibration process involves adjusting the meter's output or correcting its readings based on the comparison with a standard.

Here are the key aspects of flow meter calibration:

Reference Standard:

A reference standard is a calibrated and traceable device or system with a known accuracy that is used as a benchmark for comparison. This standard is often a calibrated flow measurement device with a higher accuracy than the meter being calibrated. The reference standard is typically at least 4x more accurate than the unit which is being tested.

Calibration Facility:

Calibration is typically performed in a calibration facility or laboratory that is equipped with high accuracy flow meters and controlled application conditions. Some manufacturers offer in-house calibration services.

Calibration Procedure:

The calibration procedure involves comparing the measurements made by the flow meter being tested with the measurements made by the high accuracy flow meter used as the standard. The two devices are subjected to the same flow condition as they are both placed within in the same piping system. The differences in their readings are calculated and recorded.

Adjustment or Correction:

If the flow meter being tested is found to have measurements not in alignment with the reference flow meter, adjustments or corrections can be made to the flow meter to align its readings with the reference standard. This may involve modifying the calibration coefficients, updating electronics, or making physical adjustments. If there is damage to the flow meter that is causing the loss of accuracy, certain flow meters may have components that can be repaired.

Calibration Points:

Multiple calibration points at differing flow rates, or various flow conditions, within the device’s stated operating range are used to assess the flow meter’s overall performance.


Traceability is a crucial aspect of calibration. It involves documenting measurements that connects the calibration results to national or international standards. This ensures that the calibration is traceable back to a known and recognized reference.

Calibration Certificates:

Once calibration is completed, a calibration certificate is provided. This includes details about the calibration process, the reference standard used, the calibration points, and the adjustments made. It serves as documentation of the meter's performance and adherence to standards.

Periodic Calibration:

Certain flow meter technology types should undergo periodic calibration to account for any changes in performance over time. Factors such as wear and tear, environmental conditions, and fluid characteristics can affect a flow meter's accuracy.

At KOBOLD we generally test a minimum of 3 measurement points from the unit under test in the last stage of manufacturing. The 3 points are a low, middle, and high percentage of the measuring range of the meter. All 3 measurement points must be within the stated tolerance for the unit to “pass”. These measurements ensure the unit is performing accurately across the entirety of the flow range scale.

It is commonplace in the industry to require proof that the calibration is accurate. When such proof is required, the manufacturer should have traceable certificates for their reference standards or have the unit serviced by a 3rd party accredited laboratory.

What is the difference between flow meter calibration and flow meter certification?

Calibration is the process of adjusting a flow meter to a known reference or standard. Any adjustments must be applied prior to certification.

Certification is the process of verifying a flow meter is working within specifications when compared against a traceable reference or standard. Certifying laboratory procedures are very strict. A unit will either pass or fail. If the unit is adjustable, the lab may attempt to make an adjustment (if the unit is failing) and test it again.

Labs are not obligated to repair a failed meter unless arrangements are made before the service. In some cases, some labs may offer repair services, but at an additional cost. If the meter fails, in most cases, the cost is the same regardless of the result.

What is a NIST traceable calibration?

NIST is an agency of the U.S. Department of Commerce. Its mission is to promote U.S. innovation and industrial competitiveness by advancing measurement science, standards, and technology in ways that enhance economic security and improve our quality of life.

Companies often offer NIST Traceable Calibrations certifying the equipment used is traceable to a known standard. NIST traceable does not determine the competence of the staff or testing facility. It is simply stating the standards used are traceable back to NIST.

How is ISO related to flow meter calibration and certification?

ISO/IEC 17025 is the industry standard for testing and procedures used with flow meter instrumentation. The process is to ensure the competence of the staff and traceability of the certifying equipment back to a known standard, such as NIST.

To issue a Certificate of Calibration in accordance with ISO 17025 the laboratory must be accredited. Accreditation means the procedures, staff, and lab equipment are audited and inspected periodically by a 3rd party organization to ensure the calibration is reliable. National Voluntary Laboratory Accreditation Program (NVLAP) is an example of a 3rd party accreditor.

What is supplied with a NIST traceable calibration?

A NIST traceable calibration certificate will generally only have the flow meter’s unique identifier and a footnote indicating the equipment used for calibration is traceable to NIST. This will assure the user the equipment used is compliant with NIST standards. The NIST traceable document may also have wording indicating “their” lab equipment is certified by an Accredited lab in compliance with ISO 17025 in specific intervals.

If data was requested, you will also receive a table or graph of how well your unit performed when compared to the NIST standard.

An accredited calibration lab will perform a NIST Traceable Calibration with ISO 17025 and will also have all the information inherent to NIST standards. However, the certificate will also include an uncertainty error and the procedure number for the type of calibration performed on the flow meter.

Why does flow meter calibration matter?

Calibration is essential to ensure that the flow meter provides accurate and reliable measurements over its operational range.

It is especially crucial for industries and applications where precise flow measurements are essential for people’s safety and wellbeing. Examples of such industries include pharmaceuticals, aerospace, energy, water treatment, and manufacturing processes.

Inaccurate flow measurement can also lead to profit loss for a wide variety of applications where consumption and loss of fluids in process has more direct outcome on costs and profits.

Inaccurate flow meters can also lead to equipment damage, such as cooling applications that protect expensive equipment from damage from heat and friction.

Inaccurate flow meters that are used to ensure that the system does not have any leaks may not provide accurate enough readings to diagnose any leaks to the system or malfunctioning equipment, such as found in the oil and gas industry or municipal water applications.

Certain industries and applications may also require regular calibration to remain in compliance with regulations.

What types of flow meters typically need to be calibrated?

Many types of flow meters should undergo calibration to ensure accurate and reliable measurements, especially if the application requires high accuracy. Please note that calibration recommendations are based not only on technology type, but also on the industry and the application variables.

These types of flow meters are generally ones built on mechanical principles that are subject to component degradation over time, which causes decreased accuracy.

Common types of flow meters that are more likely to require more frequent calibration:

Differential Pressure (DP) Flow Meters

Differential Pressure (DP) Flow Meters

All types of DP flow meters, such as Orifice plates, Venturi tubes, flow nozzles, and others require calibration. The accuracy of DP flow meters is sensitive to factors such as fluid properties, temperature, and pressure.

Differential Pressure (DP) Flow Meters
Positive Displacement Flow Meters

Positive Displacement Flow Meters

Positive displacement flow meters, including oval gear types, screw type, and piston type, and others benefit from calibration to ensure the accuracy. Wear and tear that happens over time on the moving mechanical parts can affect the meter's performance as it ages.

Positive Displacement Flow Meter Technology
Turbine Flow Meters

Turbine and Paddle Wheel Flow Meters

Turbine flow meters, which measure flow based on the rotation of a turbine or paddle placed within the liquid stream, require calibration to address changes in liquid properties and to ensure continued accuracy.

Variable Area Flow Meters (Rotameters)

Variable Area Flow Meters (Rotameters)

Variable area flow meters may require calibration to account for variations in fluid properties and ensure accurate visual or electronic readings.

Variable Area Flow Meters (Rotameters)

What type of flow meter technologies are less likely to need regular calibration?

There are certain flow meter technologies that, under specific conditions, may require minimal or no calibration. Most are based on newer flow meter technologies that do not have moving parts that are subject to wear and tear.

While many newer technologies that do not employ mechanically moving parts as part of their operating principle are more reliable and less susceptible to drift, they can still be affected by the process or environment they are located in.

Here are some flow meter technologies that are less likely to experience accuracy degradation.

Magnetic Inductive Flow Meters

Magnetic Inductive Flow Meters

Magnetic flow meters offer stability and accuracy over a wide range of liquid conditions. While they may not require frequent calibration, the accuracy and operation of magnetic flow meters can be influenced by factors such as changes in liquid conductivity and temperature.

There are also models of magnetic flow meters that are insertion installations, like out PIT Insertion Magnetic Flow Meter. These offer an advantage when it comes to calibration as the meter can be more easily removed from the system for testing than inline meters which require breaking into the piping.

Magnetic Flow Meter Technology
Ultrasonic Flow Meters

Ultrasonic Flow Meters

Ultrasonic flow meters can provide accurate measurements without frequent calibration. However, factors such as changes in liquid properties and pipe conditions may impact performance over time.

Ultrasonic flow meters are available as clamp-on models, such as our DUC Clamp-on Ultrasonic Flow Meter, where the flow meter components are strapped to the outside of the piping. There is a clear advantage here. Removal for calibration is as simple as undoing a strap.

Clamp-on ultrasonic flow meters are also sometimes used for on-site calibration. This approach requires no system downtime. Certain companies offer rentals for clamp-on ultrasonic flow meters so verification can be informally made in-house. In this instance, there is no certification issued. But this approach is useful when it is desirable to check-up on the accuracy without a certification requirement.

Companies with multiple lines can also purchase a clamp-on ultrasonic flow meter to have in-house that can test many different process lines as needed.

Ultrasonic Flow Meters
Vortex Flow Meters

Vortex Flow Meters

Vortex flow meters offer stable and accurate performance. Calibration may still be recommended in certain situations, especially when there are changes in the media.

Vortex Flow Meters
Thermal Mass Flow Meters

Thermal Mass Flow Meters

Thermal mass flow meters may require minimal calibration. However, changes in gas composition and temperature can impact accuracy.

Thermal Mass Flow Meters
Coriolis Mass Flow Meters

Coriolis Mass Flow Meters

The accuracy of Coriolis mass flow meters is generally less affected by changes in fluid properties. While they may have stable performance, periodic validation against known standards is recommended to ensure accuracy. Because Coriolis flow meters offer exceptional accuracy, they are frequently used as the reference flow meter for calibration.

Coriolis Mass Flow Meters

It's crucial to understand that even flow meters with reduced calibration requirements benefit from occasional calibration. Manufacturers' recommendations and industry best practices should be followed to ensure continued accuracy. Compliance with relevant standards and regulations may dictate calibration intervals and validation procedures for specific applications.

How often should a flow meter be calibrated?

The frequency of calibrating a flow meter depends on several factors, including the type of flow meter, the application, the required level of accuracy, environmental conditions, and industry standards. Here are some general considerations regarding the calibration frequency of flow meters:

Manufacturer's Recommendations:

Manufacturers often provide specific information within the product manual about how frequently their flow meters should be calibrated to maintain accuracy. The manual may also indicate a proper way to maintain the device to improve its overall lifespan. If there is no information included with the device about calibration intervals, speaking with the manufacturer is a good place to start.

Industry Standards and Regulations:

Some industries have established standards and regulations that dictate calibration requirements. Examples include standards set by organizations such as the American Petroleum Institute (API), International Organization for Standardization (ISO), and the National Institute of Standards and Technology (NIST).

Criticality of Measurement:

The criticality of the measurement is a significant factor. Flow meters used in critical processes or safety-critical applications require more frequent calibration to ensure the highest level of accuracy.

Environmental Conditions:

Harsh environmental conditions, such as extreme temperatures, corrosive atmospheres, or exposure to contaminants, can impact a flow meter's performance. In such cases, more frequent calibration may be necessary to compensate for environmental effects.

Process Changes:

Changes in the process, such as alterations in fluid composition, flow rates, or other parameters, can affect accuracy. When significant process changes occur, it may be necessary to recalibrate the flow meter to account for these variations.

Historical Performance:

Consider the historical performance of the flow meter. If a flow meter has shown stability and reliable performance over an extended period, it may require less frequent calibration compared to a meter with a history of performance issues.

Is flow meter calibration expensive?

The cost of flow meter calibration can vary based on several factors. Here are some factors that can influence the cost.

Type of Flow Meter:

The type of flow meter being calibrated is a significant factor. For example, calibrating a rotameter is likely to be cheaper than a Coriolis mass flow meter.

Calibration Facility or Service Provider:

The choice of calibration facility or service provider can influence costs. Accredited calibration laboratories with state-of-the-art equipment and skilled personnel are generally more expensive.

Traceability and Standards Compliance:

If the calibration process needs to adhere to specific industry standards or regulatory requirements, additional costs may be incurred to ensure traceability and compliance.

Shipping Costs:

If the meter needs to be transported to a calibration facility, costs associated with shipping and handling to transport the meter to and from the calibration facility should be factored in.

Process Downtime:

Potential downtime during the calibration process should be considered. You must factor in how much profit loss will be incurred if the system that the meter is in must be down for the duration of the entire calibration process.

Additional Services:

Some calibration providers may offer additional services, such as detailed calibration certificates, data analysis, and recommendations for adjustments or repairs.

Emergency or Expedited Services:

If urgent or expedited calibration services are required, providers may charge additional fees for priority scheduling and quicker turnaround times.

The cost of calibration should be weighed against the potential risks and consequences of inaccurate flow readings for critical processes. You should also consider the long-term benefits and potential cost savings provided by accurate flow meter performance.

At KOBOLD, we will have you return the device to us for an evaluation. We will inspect and if necessary, check the unit on our “in-house” equipment to determine how the unit is performing. We will advise you of the cost for routine maintenance if applicable, repairs and or replacement before we do any work.

If the price of the repair services exceeds 50% of the cost of a new unit, we will suggest replacing the unit. If the unit also requires certification, we will quote certification service as a separate line item, independent of the service charge.

Who performs flow meter calibration?

Most manufacturers have a way to verify if a unit is passing or failing, even if they are not an accredited lab. They should be able to adjust, repair, or replace components to ensure it will pass at any accredited laboratory. Assuming it is economical to do so.

In some cases, it may be better to replace the flow meter instead of repairing an older model. Here at KOBOLD, we will advise you accordingly to provide you with the most economical choice.

As we previously mentioned, the OEM may have the ability to calibrate and certify the meter or have access to the latter if requested.

If KOBOLD oversees the calibration, what does that process look like?

If you have chosen to have KOBOLD oversee the calibration of your existing flow meter, please contact us at 412-788-2830. We will help you to determine what the best course of action is.

If we can help, RMA will be issued for your item. Once it is received, it will be evaluated in-house to see if a repair is needed and whether it makes sense economically to do so.

If it is, we will send your unit to a Calibration Lab with a good reputation that we have confidence in. Once the lab is finished, the unit will be sent back to us, we record the documents and return the unit back to you.

If it is ascertained that device replacement makes more sense than repair, we are available to partner with you to find the best and most economical solution for your application.

How long will it take to get a flow meter calibrated through KOBOLD?

Although the timing will depend upon the specific product and where it will need to go to be calibrated, an average time for calibration is 2 to 3 weeks. If you have something to be calibrated, please call us at 412-788-2830 for the most accurate turnaround time.

If KOBOLD isn’t performing the Calibration, how do I know my flow meter is calibrated properly?

A flow meter receives a Certificate of Calibration when the unit is tested by an Accredited laboratory and passes all the criteria as specified on the OEM (Original Equipment Manufacturer) datasheet.

The datasheet typically provides crucial information about the published accuracy and conditions for the calibration lab to reference during their process. The lab will re-create those conditions or have an equivalent method of re-creation to perform the test and determine if the unit is working as advertised.

Once the lab establishes the unit is performing within the specifications, the lab will issue a certificate indicating the unit has passed all the criteria as provided by the OEM. The certificate itself is traceable with a unique identifier that the lab maintains for future reference.

A common phrase that may be on the certificate is “As Found, As Left”. This means the lab recorded 2 sets of measurements, as the unit was received (As Found), and the state in which it will be returned (As Left). If no adjustments were necessary, there would only be 1 set of measurements if data was included with the certificate. A calibration lab is not obligated to make a passing data point better. Adjusting an already passing data point is inefficient and may create unforeseen issues. Depending on the failure data, an end user may use “As Found” data to determine if a product recall is necessary.

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KOBOLD USA is a subsidiary of KOBOLD Messring GmbH, a world-leading instrumentation engineering business founded in Germany in 1980 by Klaus J. Kobold. With patented technology and superior service, the company quickly established itself as one of the global leaders in sensor and control systems with high quality products. The KOBOLD brand name became synonymous with superior quality and technological advancement in instrumentation engineering.

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