Calibrating flow meters

Why does a flow meter need to be calibrated?

The calibration of flow meters is essential for various reasons, especially in industrial applications where precise measurements play a central role. One of the most important aspects is measurement accuracy and process reliability. Flow meters measure the volume of a medium within a system, and incorrect measurements can lead to incorrect dosing, inconsistent production processes and potentially dangerous operating conditions. Regular calibration ensures that the measured values remain within the specified tolerances and thus safe operation can be guaranteed.

Calibration also has a direct impact on the quality and efficiency of processes. In sectors such as the food, chemical and pharmaceutical industries, accurate measurements are crucial for ensuring product quality. Incorrect measurement values can lead to inferior products or rejects, which can result in significant economic losses. Furthermore, many industries require compliance with legal regulations and standards (such as ISO, DIN, OIML) that demand regular calibration. Companies must be able to prove that their measuring instruments are functioning correctly in order to meet regulatory requirements and avoid possible sanctions.

Last but not least, calibration also helps to optimize operating costs. Inaccurate flow meters can lead to too much or too little material being used, resulting in unnecessary costs. Regular calibration can help to use resources efficiently and minimize material waste. Inaccurate readings can lead to inefficient operating conditions and cause unnecessary energy consumption. Calibration also helps to prevent equipment failure, as incorrect measurements can lead to unexpected problems that can be detected and rectified by timely calibration. Finally, in safety-critical applications, calibration is essential for safety and risk management to minimize potentially catastrophic consequences of incorrect measurements.

When does a flow meter need to be calibrated?

The calibration of a flow meter is necessary in various situations to ensure measurement accuracy, process reliability and compliance with legal requirements.

Which measured values are collected during the calibration of flow meters?

When calibrating flow meters, various measured values are collected in order to compare the performance of the measuring device to be calibrated (test item) with that of a measuring standard. In essence, the flow rate or mass flow of the measuring standard is compared with the output signal of the test specimen.

Typical output signals of the measuring device to be calibrated that are checked during calibration are:

• Frequency
• Voltage (often 0-5 VDC or 0-10 VDC)
• Current (typically 4-20 mA)
• Visual displays
• Data communication protocols such as Mod-Bus or CAN-BUS

Calibration aims to determine the relationship between the actual flow measured by the more precise standard and the corresponding output signal of the flow meter. This allows errors and deviations to be determined and, if necessary, the meter to be adjusted to optimize its accuracy.

Methods for the calibration of flow meters in industry

In the industry, there are various proven methods for calibrating flow meters, which are used depending on the area of application, the type of measuring device and the required accuracy requirements. These methods can essentially be divided into different approaches, each offering specific advantages and disadvantages. Some of the most common methods that are relevant for the calibration of flow meters in industrial environments are explained in more detail below.

These methods include the

• Comparison with a reference flow meter (Master Meter Calibration),
• the gravimetric calibration (mass-based method) and the
• Volumetric calibration (volume-based method).

Each of these techniques is based on different physical principles and offers specific possibilities for checking and, if necessary, correcting the measuring accuracy of flow meters.

Calibration under operating conditions

Calibration under operating conditions is an important aspect of ensuring that a flow meter achieves its specified accuracy in a real operating environment. This method takes into account that the environmental conditions under which a flow meter is operated can have a significant impact on its performance.

Factors such as

• Temperature,
• Print,
• viscosity of the measured medium and the
• profile

can influence the measurement results.

When calibrating under operating conditions, an attempt is made to simulate the actual operating conditions as accurately as possible. This may mean that the calibration is carried out directly at the place of use of the flow meter and the actual measuring medium is used. The use of the correct test medium is crucial, as differences in density, viscosity or electrical conductivity between the calibration medium and the operating medium can lead to inaccuracies.

Legal regulations and standards

The calibration of flow meters in industry is subject to various international and national standards and legal regulations.

One of the most important standards is ISO 17025, which sets requirements for calibration laboratories and ensures that measurements are carried out with high precision and traceability.

The ISO 5167 standard is relevant for measuring the flow rate with differential pressure flow meters such as Venturi nozzles or orifices. ISO 4185, on the other hand, regulates flow measurement using volumetric methods.

There are specific regulations for the trade and commercial billing of liquids, such as OIML R 49 for water meters and OIML R 117 for liquids other than water, for example in the fuel and chemical industries. Within the EU, the MID Directive (Measuring Instruments Directive, 2014/32/EU) plays a central role, as it regulates the conformity of measuring instruments for the European market.

In potentially explosive atmospheres, such as in the chemical or oil and gas industry, flow meters must comply with the requirements of IEC 60079, which regulates explosion protection. In Germany, the calibration law and the calibration regulations also apply, which stipulate that measuring devices for commercial purposes must be calibrated regularly. In addition, there are industry-specific guidelines such as VDI/VDE 2641 for industrial calibration and the ANSI/ASME MFC-14M standard, which is used in the USA to assess the accuracy of flow meters.

In summary, choosing the right regulations and standards depends on the industry, the medium (e.g. water, gas, oil) and the legal requirements. Companies should ensure that their measuring devices are appropriately certified and regularly calibrated to ensure accuracy and legal compliance.

Who performs the calibration?

The calibration of flow meters can be carried out by various bodies. These primarily include specialized calibration laboratories that have the necessary equipment and expertise to perform precise calibrations. These laboratories can be independent service providers or belong to manufacturers of measuring devices, such as DDM Sensors.

An important aspect when selecting a calibration laboratory is its certification or accreditation. Accreditation, for example by the DAkkS (German Accreditation Body) in Germany, confirms that the laboratory is competent to carry out calibrations according to internationally recognized standards and that the results are traceable to national or international standards. This creates confidence in the quality and reliability of the calibration results.

How are the calibration results documented?

The documentation of the calibration results is an essential part of the calibration process. It serves as proof of the calibration performed and provides important information about the performance of the tested flow meter. As a rule, a calibration certificate is created that contains detailed information on the calibration process and the results.

Such a calibration certificate typically contains the following information:

• Identification of the calibrated flow meter (e.g. manufacturer, model, serial number)
• Details of the measuring standard used (e.g. type, serial number, calibration certificate)
• Date and place of calibration
• Specification of the calibration method used
• The ambient conditions during calibration (e.g. temperature, pressure)
• The measured values of the standard (GN) and the test specimen (KG) for different measuring points
• The calculated performance indicators, such as the K-factor, the meter factor or the error of the meter
• The specification of the measurement uncertainty of the calibration
• A reference to the traceability of the measurements to national or international standards
• The date of the next recommended calibration (if applicable)
• The name and signature of the responsible person and details of the calibration laboratory

For documentation purposes, DDM Sensors carries out an initial calibration, an adjustment if necessary and finally a final calibration. The calibration certificates are provided in written and/or electronically readable form. It is important to keep the calibration certificates of the measuring devices in order to be able to trace the calibration history.

Which factors influence the calibration?

The calibration of flow meters can be influenced by various factors that directly affect the measurement accuracy and reliability of the devices. Here are some of the most important factors:

• Temperature:
  • The viscosity of liquids and gases changes with the temperature, which can influence the measuring accuracy.
  • The thermal expansion of sensors or cables can lead to deviations.
  • Electronic components in the measuring devices can provide deviating values in the event of temperature fluctuations.
• Print:
  • Changes in the operating pressure can influence the flow profile and the density of the medium.
  • With gases, a higher pressure leads to an increase in density, which can influence the calibration.
• Density and viscosity of the medium:
  • Especially with non-Newtonian liquids, changes in viscosity can affect the measuring accuracy.
  • Changes in density (e.g. due to temperature or pressure fluctuations) affect certain measurement methods such as Coriolis or ultrasonic measurements. DDM offers to determine the density and viscosity of customer-specific media at different temperatures in order to achieve optimum viscosity correction.
• Contamination and deposits:
  • Biofilms, sediments or deposits in the measuring section can change the cross-section and falsify the measurement.
  • Magnetic or electrostatic deposits can affect the measuring accuracy of electromagnetic flow meters, for example.
• Air bubbles or gas components in liquids:
  • With ultrasonic or electromagnetic flow meters, the presence of air bubbles can lead to measurement errors.
  • An uncontrolled gas content can falsify density and volume flow measurements in Coriolis measuring systems.
• Electrical and magnetic interference:
  • External electromagnetic fields or vibrations can affect electronic sensors, especially electromagnetic flow meters.
• Calibration frequency and maintenance:
  • Insufficient or infrequent calibration will lead to greater deviations over time.
  • Regular maintenance and inspection of the measuring devices are necessary in order to obtain accurate values in the long term. It is important to keep the calibration certificates in order to track the calibration history and adjust maintenance and calibration intervals accordingly.

Each of these factors can affect the calibration of a flow meter and should be considered during selection, installation and maintenance.