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Depending on the configuration of your process and how important that specific measure is for your performance, some flow meters should be calibrated whereas others can be left uncalibrated.
Usually, flow meters part of a secondary loop system are redundant as any drift in the measure will be corrected by the action of a primary loop or an operator that makes set point adjustments based on some other process input. This is true for most flow meters in circuits that act as an intermediate loop for a given process. These loops are generally measured by some other equipment and they are unlikely to require absolute flow measurement accuracy. It is usually enough for flow meters to offer stable, consistent performance in the short term for this type of configuration.
Flow meters that are likely to require routine calibration include those impacting product and feedstock quality and quantity, energy/fuel quantity and critical process operating points.
The primary motivation for guardbanding is to control the risk of accepting an out-of-tolerance unit or rejecting an in-tolerance unit.
The ISO 17025 standard requires that measurement uncertainty be taken into account when statements of compliance are made. In other words, when a calibration laboratory calibrates an instrument and produces a calibration certificate indicating that the instrument “passed” or “failed”, it is important that, for each test point, the measurement uncertainty be first calculated and then used in the determination of the test result.
The ISO / IEC 17025 standard requires that the guardbanding technique be applied for each test point when the test uncertainty ratio (TUR) is less than 4: 1. The technique of guardbanding can lead to a situation where it is not possible to determine, within the confidence interval, the statement of compliance or non-compliance for the device under test. In this case, the declaration of conformity is “undetermined.”
Declaration of conformity for TUR* lower than 4: 1
• Ustd : Uncertainty of the standard (% of reading)
• Uinstrument : Uncertainty of the measurement system (% of reading)
• Resolution equipment under test : Resolution of the equipment under test (% of reading)
Zone of compliance calculated according to a new tolerance
(*) Le TUR se définit comme suit :
• TUR : Test Uncertainty Ratio
• Tolerance : Tolerance of the equipment under test
• Ustd : Uncertainty of the standard
• Uinstrument : Uncertainty of the measurement system
• Uresolution : Resolution of the equipment under test
ILAC G8 : Guidelines on Assessment and Reporting of Compliance with Specification
CLAS Requirements Document 3 (November 2009): Minimum Requirements for Measurement Standards for Laboratory Certification
ISO/IEC 17025 is an international standard for evaluating the competence of calibration laboratories. It includes portions of the ISO 9001 standards relating to organization and management plus it encompasses the entire calibration system to produce metrics for the:
- Calibration rig components,
- Administrative systems for process operations,
- Personnel proficiency,
- Documentation supporting the traceability
- Total measurement uncertainty for the entire calibration facility
By it’s stringent procedures, ISO/IEC 17025 ensures that the entire calibration process is traceable to the International System of Units (SI), personnel are technically competent and methods and practices in place produce precise and valid results.
Contrary to other claims of traceability (like NIST Traceable*) which are not governed by the authorities and where the liability of the claims of traceability is the responsibility of the individual laboratory, ISO/IEC 17025 involves a rigorous third party assessment to make sure that all of the requirements identified in the standard are met and periodic reviews are used to determine if the standards are maintained.
The ISO/IEC 17025 accreditation procedure also includes the evaluation of the total measurement uncertainty (accuracy and repeatability) of each component on the calibration rig and inter-laboratory proficiency testing to provide individual laboratory confidence by comparing ISO/IEC17025 accredited labs to each other.
* Unfortunately, NIST does not govern calibration facility claims of traceability. The responsibility of NIST-traceable claims resides with the individual laboratory.
Underwriters Laboratories (UL) calibration requirements stipulate that inspection, measuring and test equipment (including flow meter) must be calibrated at least annually for their intended function and use or whenever the flow meter has been subject to some form of abuse that may affect the measurement device fitness for use.
The manufacturer’s most commonly recommended calibration interval for precision test equipment is also one year. The one year interval optimizes the cost of calibration versus the cost of an out-of-tolerance for many users. This interval achieves the desired reliability target and helps to resolve early minor problems that affect process consistency and repeatability for most industries.
However, if in practice a response to an out-of-tolerance measurement leads to significant costs or if you are using a new meter or one located in an hostile environment, or if you are requiring FDA approval, it is recommend to calibrate on shorter calibration schedule in order to investigate early minor deviation from the standards until you get an accurate “footprint” of the meter and then switch to longer interval of calibration.
Key elements in determining compliance to national and international standards are the validity and accuracy of inspection, measurement and test results. As flow meter performances degrade over time, these instruments must be calibrated to provide the necessary level of confidence in the results of the tests being conducted.
There are several reasons why a flow meter falls out of calibration: physical modifications may occur from corrosion or dirt within the process; internal parts degrade slowly and will eventually have an impact on meter performance or flow meter may receive mechanical or pneumatic impacts resulting from process variations, improper installation or initialization or procedure. The most common reasons for calibrating flow meters are:
Reasons for calibrating flow meters
|Coriolis||Coating and wear of the flow tube|
|Thermal||Contamination of sensor or/and restrictor; sensor wear; modification of the orientation of the sensor|
|Turbine||Bearings affected by chemicals or dirt; bearing service affects calibration; rotors wear|
|Variable Area||Material build-up; plugging; tube abrasion|
|Magnetic||Liner damage; electrode coating|
|Positive Displacement||Dirty liquids, corrosion and abrasion change the volume; bearing wear degrades accuracy; gear service affects calibration; solids can cause plugging|
|Differential pressure||Orifice plates, nozzles and venturis are subject to wear: orifices plate get knocked out of position. Pitot tubes become clogged|
|Ultrasonic||Changes in the sonic properties of the fluids; lack of contact between transducer and pipe wall|
|Vortex||Flow meter mounted improperly in pipe; vibration; shedder wear|