Utility managers and operators rely on flow meters to provide critical information for process monitoring and control. They require and fully expect the flow data to be accurate and reproducible.
The Importance Of Flow Meter Calibration
Calibration is a comparison of a flow meter’s performance against a known standard. Most often, standards include those of the National Institute of Standards and Technology (NIST). Calibration laboratories must meet the requirements of the International Organization for Standardization (ISO) and the International Electrotechnical Commission (IEC).
Without calibration, flow measurement relies only on an algorithm, such as a flow coefficient for the meter. This algorithm contains uncertainty. Additional input adds more uncertainty — piping configuration, friction, water temperature, and other factors.
An uncalibrated meter will produce flow readings with greater than acceptable inaccuracy.
The Drawback To Calibration
When a meter is calibrated, there’s an assumption that all factors were input correctly and properly compared to a known standard. Meter accuracy is only as good as the quality of its calibration. If the assumption of proper calibration is wrong, inaccurate flow readings, process issues, or regulatory violations may result.
Calibration Best Practices
Factors that ensure flow meter calibration meets ISO/IEC standards include:
Traceability
The International Vocabulary of Metrology (VIM) defines traceability as the “property of a measurement result whereby the result can be related to a reference through a documented unbroken chain of calibrations, each contributing to the measurement uncertainty.” Basically, the flow meter’s performance must be traceable back to a known documented standard. In the United States, this would ultimately be to NIST standards, through the National Voluntary Laboratory Accreditation Program, known as NVLAP.
Repeatability
Repeatability is an indication of precision. Ideally, a flow measurement should provide the same result each time it is repeated. Repeatability data should be included in the determination of uncertainty.
Uncertainty
What calibration really determines is the amount of uncertainty in the meter reading. The term “accuracy” is more qualitative than scientific, and what users are more familiar with. However, “uncertainty” is the correct way to express the results of calibration.
Each of the known standards has some associated uncertainty. There is also uncertainty associated with every piece of equipment used and measurement taken during the calibration. All these uncertainties are combined to provide a total confidence level for the meter’s performance.
Calibration results should be provided in a report or certificate with a clear uncertainty statement.
If possible, fluid temperature, pressure, solids, or viscosity should be replicated during calibration. This may not be possible. If that is the case, field conditions including pipe size and configuration, velocities, and fluid properties should be available. This information can be used by the utility to determine meter accuracy in the field by extrapolating the lab data.
Detailed knowledge of the field conditions will help to ensure proper meter calibration. Outside factors can be included in the flow coefficient, reducing uncertainty.
How To Have Confidence In Meter Calibration
Flow meter manufacturers typically have their own calibration facilities. The manufacturer provides initial calibration data to customers when they purchase a meter. When selecting a meter, ask whether the meter is calibrated at a laboratory accredited through NVLAP. By doing so, you have the assurance that the lab was independently evaluated to meet ISO/IEC 17025 standards for calibration. The evaluation of the calibration laboratory includes both technical and quality management standards.
NVLAP Accreditation
Calibration by a lab accredited through NVLAP provides the highest level of confidence in your flow meter. NVLAP is administered by NIST. NVLAP is also a signatory to the International Laboratory Accreditation Cooperation (ILAP), a global cooperative of laboratory and test accreditation bodies. Accredited labs must meet the stringent management and technical requirements of the ISO/IEC standard 17025:2017.
Proficiency Testing
Laboratories must participate in proficiency testing to demonstrate their technical competence initially and at least every two years. These tests include interlaboratory comparisons with other NIST traceable calibration facilities. Failure to participate in testing or to failure meet proficiency requirements is considered a major nonconformity.
NVLAP notifies nonconforming laboratories of the actions they need to take to resolve the issue. If the laboratory does not respond or cannot meet the proficiency requirements, accreditation is denied or suspended.
Onsite Assessment
NVLAP assessors audit these calibration labs initially and biannually to check component uncertainties and calibration traceability and documentation. They review the process the laboratory uses to guarantee accuracy. NVLAP accreditation requires correlation flow meter testing be performed to ensure repeatability from the time of calibration certification.
During an assessment, assessors review the laboratory’s quality manual and management system documentation. They interview staff, review records, examine equipment, confirm traceability on tools, and observe calibration demonstrations. The assessor may ask for additional information to facilitate the review, including key personnel resumes and job descriptions, staff training records, and competency evaluations.
The assessor discusses any nonconformity details with the staff during the assessment. At the end of the assessment, a closeout meeting is held. Results of the audit are provided to the lab in a written report. The assessor explains the audit results, and laboratory staff can ask questions.
Laboratories must respond in writing that all nonconformities are corrected before initial accreditation, or within 30 days for accreditation renewal.
Monitoring Visits
In addition to the scheduled assessments, NVLAP assessors may visit and monitor a lab at any time. These visits are typically announced ahead of time but may be unannounced. Monitoring may include review of a few specific items, such as verifying reported changes in personnel or operations. A random monitoring visit may also consist of a full audit.
Get The Best Performance Out Of Your Flow Meter
Major treatment plant processes, public health and safety, and regulatory compliance rely on accurate flow readings. Ensuring your meter is calibrated by a NVLAP-accredited laboratory gives you a higher level of confidence in flow measurement accuracy.
