Flowmeter calibration : The need of hour

Flowmeter calibration and proving systems may include simple verification, traceable calibration, or in-field proving. These systems use several different methods to assess flowmeter performance, including volumetric and gravimetric testing and master-meter comparison.

In today’s industrial marketplace, where many companies must adhere to strict specifications to ensure quality standards are met, proper flowmeter calibration has never been more critical. Flowmeter calibration and proving systems may include simple verification, traceable calibration, or in-field proving. These systems use several different methods to assess flowmeter performance, including volumetric and gravimetric testing and master-meter comparison. Calibration involves using traceable standards to test an individual meter and compare it to a laboratory standard.

In an effort to shed some light on the calibration options available to end-users, the following article discusses the operational characteristics of calibration systems for flow measurement instruments. In particular, it focuses on liquid applications for the calibration of flowmeter technologies. There are many acceptable flowmeter types available to the end-user today, all of which use a different method to arrive at a measurement. Flowmeters are selected for applications based on many different variables, such as mass/volumetric flow, response time, conductivity of fluid, and solids content. Flowmeters generate an output, such as frequency, analog, current, or visual display. The calibration methods discussed in this article are used to calibrate a range of flowmeter technologies, but are most appropriate for fast-responding, frequency-output measurement systems.

Calibration interval schedules are determined by the critical nature of the application or internal procedures of the organization. “As Found” or “Before” calibrations can be completed on an accelerated schedule, and, with acceptable deviation in the data, the calibration intervals can be expanded. Once an unacceptable deviation occurs, the calibration integral has reached the maximum timeframe.  Repeatability is the ability of the flowmeter to reproduce its output indefinitely under constant operating conditions at any point over its specified operating range.

Calibration Need
Flowmeter technologies are responsible for accurately measuring applications for many different reasons, including, but not limited to, batching, distribution, management, profitability, and disposal. Managers of these meter technologies are ultimately responsible for the accuracy of measurements and can be held accountable for the performance of their instruments. In today’s competitive environment, companies are increasingly micro-managing processes in an effort to increase profits and decrease production. Various flowmeter designs require calibration due to procedures, moving parts, or buildup and/or corrosion on the inside of the pipe. For any or all of these reasons, meter calibration is critical to business success. For example, flowmeters with moving parts may wear, thus affecting the calibration/output of the device. With ultrasonic or magnetic flowmeters, electronic performance depends on adjustment and cleanliness of the pipe/electrodes. Corrosion within the pipe or on the magnetic electrode can make it difficult for these technologies to deliver a correct output for the actual flowrates.

With buildup and/or corrosion in a pipe section, Coriolis mass flowmeters, for example, may shift in their calibration  indicating increased mass flow due to the additional weight of the flow tube. Only routine calibrations of flow measurement instruments can help companies manage their application within specifications. Without accurate calibrations, money may be going down the drain. Calibration of turbine flowmeters, for example, has become a critical requirement for all applications, whether it be power generation, process, accountability, or disposal. Turbine meters are normally calibrated before and after installation; after repair; when changing applications or products; when changing product viscosity; or to chart the flow patterns of the meter during a period of time.

Most companies follow their quality control guidelines or have written ISO 9000 procedures for the intervals of their calibrations. The most common interval is an annual calibration. However, many users have their flowmeters calibrated semi-annually or even quarterly depending on the nature of the application. Flowmeter calibrations are usually completed on a primary standard calibrator to meet a 4-to-1 accuracy ratio requirement better than the unit under test (UUT). In many applications, “As Found/Final” or “Before/After” calibrations are required for comparison of the previous calibration. An “As Found” calibration will determine if the flowmeter still meets its original manufacturer’s specifications. If the unit meets these specifications, the calibration is stamped “As Found/Final Calibration.” Should the calibration not meet the original specifications, adjustments or parts replacement may be required prior to final calibration.

Primary vs. Secondary Standards
In the metrology world, there are two different types of calibration standards primary standards and secondary standards. A primary standard measurement is made using fundamental components (mass, length, time, etc.). An instrument is considered a primary standard if it is not characterized by the same method it is being used for. Conversely, secondary standard calibrations are completed with a master meter having been calibrated on a primary standard.

The flowrate is derived from the master meter and other application inputs (e.g., temperature and pressure). Secondary standard calibration uncertainty increases with the introduction of additional inputs to derive the flowrate and repeatability of the master meter. In many applications, this uncertainty is sufficient to meet the user’s acceptance specifications. In most cases, a 4-to-1 accuracy increase on the primary or secondary standard is acceptable to complete a calibration on the unit under test.

Common Calibrator System Designs
There are three basic designs for calibrator systems used in the flow measurement industry for calibration of flowmeter technologies. Positive-displacement (PD) and time-weight calibrators are normally found in a metrology laboratory, where flowmeters removed from the application are sent in for calibration. Depending upon the uncertainty of the meter requiring calibration, an in-line prover or flow transfer standard (FTS) may be brought into the field to perform a meter calibration/validation. The following details some common calibrator designs:

Time-Weight Calibrator:
The time-weight calibrator was one of the earliest flow calibration techniques. The system essentially consists of a bucket on a scale with a stopwatch to time the filling of the bucket. In practice, the bucket becomes a large tank with a highly accurate weighing system, and the stopwatch becomes a sensor-based timing system with a crystal-controlled clock.

The time-weight calibrator design maintains a steady flowrate during the measurement period, ensuring constant temperature and pressure conditions. A data acquisition system carries out the control of the system and the measurement of test parameters.Time-weight calibrators incorporate large reservoir tanks, pumps, diverter valves, and flow straighteners of various diameters.

Once the desired flowrate for the calibration run is set, the diverter valve changes the normal flow path back to the reservoir to a flow path into a holding tank. At the end of the run, the diverter valve bypasses the fluid through the flow loop again, and the system weighs the actual volume of fluid to determine a mass flowrate. Using the time-weight method, it is necessary to tare the scale before each run so only the addition of the liquid being metered by the flowmeter is weighed. The weighed amount (mass) of liquid is then divided by the time required to take the sample (mass flowrate or lb/sec). During the sample the electronics are also collecting the output of the unit under test for the correlation with the flowrate during the sample flowrate. Although the time-weight calibrator is a primary calibration standard, it is very large in comparison to other primary standards available on the market.

etup and calibration time is extensive for completing even one calibration point. Time-weight calibrators are also expensive to maintain and operate due to the age of the technology. The calibrated flowrate is obtained by dividing the discrete volume of the prover with the time it takes to displace its volume. This calibrated flowrate is then compared to the output of the flowmeter being calibrated. The same methodology is applied to the calculation as the time-weight system, only using volumetric flow.