Flow is a crucial measurement in any system, large or small. It helps indicate the status and efficiency of the upstream equipment and what to expect for downstream equipment. It also helps operators determine the process output, balance mass throughout the system and determine characteristics about the transported fluid.
To ensure that a system is operating optimally and as designed, flow meters need to be calibrated regularly. Calibration is a measurement of the flow meters’ accuracy. Operators do not simply want to read and evaluate any data that their instruments provide, they want accurate data that reflects the actual operating conditions as closely as possible. Professional calibration services can help with that.
Flow meters are calibrated before they leave the manufacturer, and they are typically calibrated again upon installation and startup of a system. Internal meter components wear, corrode and fall out of alignment over time, which cause the instruments to deviate from their calibration standards. Wear and corrosion can be caused by particulate matter in the pumped fluid, the inherent characteristics of the fluid or changes in the system.
Since accurate flow meter readings depend on multiple factors, the calibration of flow meters considers such factors as well. Temperature, pressure, density and viscosity are known influencers of flows and must be clearly defined and established to correctly calibrate the meters. In addition, the correct standard needs to be selected to ensure that the measurement is compared to an appropriate value.
Though facility operators and maintenance staff are encouraged to monitor the status and evaluate the accuracy of their flow meters, it is wise to hire a professional flow meter calibration service to perform detailed assessments of the instruments. Here are some of the ways that professional calibration services help ensure the accuracy of the measure and therefore the accuracy of the system flow readings.
Uncertainty and Traceability
Uncertainty is caused by normal use of the instrument in changing conditions. It is also a result of the type of flow meter, the history of the flow meter’s use, the characteristics of the pumped fluid and other properties of the system in which the flow meter is used.
Absolute certainty is impossible, which is why calibration procedures always provide a tolerance level. Many calibrations have a tolerance level of plus or minus 0.1 percent. Professional calibration services know the exact permissible level of uncertainty for each flow meter type and can calibrate to the correct standard. They also have access to advanced calibration technology capable of potentially decreasing the tolerance level and provide more accurate results.
Traceability is another major factor that professional calibrators consider. Traceability is the ability to link a specific calibration measurement back to the reference standard through an unbroken chain of calibrations, each of which add to the level of uncertainty of the specific measurement.
Accuracy, Precision and Repeatability
Flow meters need to prove their ability to provide consistent results under unchanging conditions. This is known as flow meter repeatability. Flow meters are considered good quality and in good operating condition when they demonstrate repeatability.
There is often confusion between the concept of repeatability and the concepts of accuracy and precision. Accuracy is determined by calculating a flow meter’s deviation from a known standard or the actual flow value. Precision is determined by measuring the deviations between all flow meter readings. To clarify further, accuracy measures flow meter data with respect to a standard and precision measures flow meter data with respect to other flow meter data.
Repeatability is different than accuracy but somewhat similar to precision. Repeatable flow meters provide precise measurements under the exact same conditions using the exact same measuring instruments. The resolution of the device and the standard the device is measured against influences the repeatability.
Professional calibration services know the level of accuracy and repeatability to expect from different types of flow meters and can evaluate the results with respect to the operating conditions. For example, a professional calibrator may determine the result of a measurement is reasonable for an operation with unique flow characteristics, whereas the same measurement would be erroneous for another more standard application.
Linearity Graphs and Interpreting Permissible Errors
Many flow meters output linear data, but some do not. Therefore, nonlinear flow data needs to be linearized to determine whether the instruments are operating within their design limitations. Once all calibration data is determined, performance error is determined. Error is defined as the difference between the actual calibration measurement and the standard value. Relative error, or error with respect to the actual value of the measurement, is usually expressed as a percentage.
Professional calibrators know the range of permissible errors for flow meter types and can interpret the result to determine whether the calibration itself is acceptable. Just as operators want reliable data to measure the status and effectiveness of their system, professional calibrators want to ensure that the result of their calibration fall within a defined range of accuracy.
There is no golden rule for calibration frequency, so the schedule needs to be determined with respect to the system and fluid characteristics as well as the accepted tolerance level for the operation. Some applications that require extremely precise and accurate measurements might perform basic calibration on a weekly basis and more detailed calibration on a quarterly schedule. Other applications may opt for monthly calibration and annual service inspections.
Professional calibrators can help determine the frequency with respect to the system needs, flow meter types and fluid characteristics. Unexpected changes to flow meter readings may also indicate a need for calibration, especially when system conditions remain unchanged. Professionals can quickly evaluate the history of calibrations and the system data to determine whether the instrument needs to be measured or whether a problem in the system may have occurred.
Professional Calibration Services Offer Convenience and Confidence
Overall, professional calibrators provide operators with confidence in the calibration results and can help evaluate the results to differentiate problems with the flow meter from problems within the system. Of course, in-situ calibrations can be performed as part of regular preventive maintenance programs, but professional services often provide more accurate measurements and evaluation of each device. The more reliable a system’s instruments are, the more confident operators are in the system performance.