Flow meter accuracy and repeatability are fundamental to the success of industry where liquid flow rate measuring devices are used; therefore they are closely scrutinized according to set criteria. Many techniques are used to ensure the liquid flow rate measuring device is performing correctly. Depending on the requirements of the flow meter and the environment where it is being used, testing of liquid flow rate measuring devices can occur via three routes: calibration, in-field proving and verification.
The reason for calibration is to find out whether deviations are there due to process usage or manufacturing. As a result, calibration processes may occur in two scenarios: in re-calibration after use or in the last step during manufacturing. There are many design standards which govern calibration systems such as master meter comparison, gravimetric methods, and volumetric methods. All these methods are capable of producing results with uncertainties of 4 to 1 or better compared to the meter that is tested. Also, ISO 17025 and NIST standards govern the whole calibration system. For instance, The National Institute of Standards and Technologies defines principles for traceability. ISO 17025 provides rigorous, third-party accreditation. Together, both sets of standards define metrics for administrative systems, calibration of rig components, documentation supporting traceability, personnel proficiency and measurement uncertainty for the calibration facility.
In the first scenario, an operational flow meter may be returned to the laboratory for creating documentation to determine meter conformance to set standards or to fix electronic or mechanical deviations within the meter. This may occur due to component drift or sensor aging.
In the second scenario, the manufacturer establishes the zero point stability. Once established, the linearity of the flow meter is tested.
This process may be conducted in situ with a customer or out-of-process with a master meter, scale, transfer standard or flow proving cart. Within in-field proving, the accuracy of the proving meter must be at par with the customer’s expectations. Testing runs can occur continuously or totalize for comparison. Unlike in a calibration lab where the conditions are controlled, in in-field proving competency rests with the holder of the master meter, the ability to recreate test conditions during the lifecycle of the process meter and employed practices.
Why is field proving used?
- To alter a flow meter’s calibration so that it matches the master meter’s standard.
- To find justification for returning or removing a process meter to rectify deviations or re-establish factors per manufacturer’s tolerances.
Liquid flow rate verification is used to confirm long-term stability. Verifications may be processed comparisons conducted under actual flowing conditions or electronic simulations without real flow.
Electronic verifications do not involve field tests of the process meter. When paired with integral meter diagnostics, it can pick up changes within the sensor. Electronic verification may be used to inspect converter or transmitter errors, component drift and hardware failure Test results are compared with the manufacturer’s specifications and tolerances during the process meter’s lifecycle. Sometimes electronic verification tools need to be returned to the manufacturer to ensure proper calibration and functionality of the electronic components. Verification tests are relied upon to satisfy agency, government or quality systems requirements.
The testing of the liquid flow rate measuring instruments is integral to industry and must be conducted via calibration, in-field proving or verification.