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.
Why Calibrate?
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. The U.S. Environmental Protection Agency (EPA, www.epa.gov) has also increased monitoring of manufacturing processes using flowmeter technologies in order to determine environmental impacts. Meanwhile, 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
The traceability of a calibration simply refers to an unbroken chain of references leading back to the U.S. National Institute of Standards and Technology (NIST, www.nist.gov). Calibration standards are calibrated by comparison to a standard of higher accuracy, which has also been calibrated by comparison to a standard of higher accuracy. This "traceability chain" continues until the governing body or standards organization specification is met and the calibration is performed against the standard of lowest uncertainty. Since flow is considered a derived standard, there is no single standard for this process measurement.
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 (Figure 1).
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. Setup 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.
• In-line Prover: Most commonly used in oilfields for loading/unloading stations where larger flowmeters must remain in service, the in-line flow prover consists of a smooth-walled, pre-calibrated displacement chamber housing a bypass piston. A shaft is attached to the backside of the piston, which, in turn, is connected to an optic sensor for the discrete volume.
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.
With the in-line prover, the bypass piston is constructed so it will not disrupt the flow in the process line. Therefore, the prover can be permanently installed in an operating pipeline, upstream or downstream of the flowmeter being calibrated. A poppet valve within the bypass piston assembly allows for the piston to be withdrawn to the start position after a given calibration run while the process flow continues undisturbed.
• Flow-Transfer Standard: Calibration system manufacturers have utilized recent advancements in computer technology to develop portable flow transfer standards. Typically mounted on carts, or offered in suitcase kits, these secondary calibration systems are designed to calibrate flowmeters that cannot be easily removed from their application. They are intended for in-line calibration and validation of meters using the actual process gas or liquid (Figure 2).
When the flow transfer standard utilizes a master flowmeter calibrated on a primary flow standard, high accuracies are obtainable. Some flow transfer standards also have the capability of measuring and correcting the influences of line pressure and temperature effects on flow.
The flow transfer standard employs an interface box that takes inputs from the master meter, temperature sensor, and pressure sensor and supplies these outputs to the system software. Based on these inputs, the software can calculate the flowrate. The software merges the output information from the master meter, temperature sensor, pressure sensor, and unit under test and generates a calibration data sheet in volumetric or mass units, which can be stored for future reference.
Flow-transfer standards are an economical source of calibration/validation when compared to larger primary standard calibrators. This solution allows users to either set up a flow loop with a pump and master meter in a manifold configuration and perform their own meter calibrations, or install a master meter inline in their existing piping and calibrate/validate the units under test based on actual process conditions.
• Positive-Displacement Calibrator: Some of the most dramatic improvements in flow calibrator technology in the last decade have occurred in the evolution of positive-displacement (PD) calibrators. PD systems are primary calibrators, which take into account the varying conditions under which turbine flowmeters operate. These calibrators are able to compensate for temperature, density, viscosity, and other variables that can distort a meter’s output. As such, they can typically achieve uncertainties in liquid of +/-0.05 percent of volumetric flowrate measurement (95 percent CL).
A PD calibrator utilizes a precision-machined measurement chamber, or flow tube, that houses a piston-seal assembly, which acts as a moving barrier between the calibration fluid and the pressurizing media used to move the piston. Attached to the piston is a shaft that keeps the assembly moving in a true linear path in the flow tube. This shaft also provides a link between the piston and the translator, which converts the piston movement into electrical pulses directly related to displaced volume. The system software divides volume by time to determine gallons per minute (GPM). These calculations are used to determine the volumetric flowrate (Figure 3).
Positive-displacement calibrations can be directly traceable to NIST via gravimetric water draw validation. Liquid calibrations with PD systems are typically performed using water or Stoddard solvent in viscosities less than 1,000 centistokes. Flow ranges vary from 0.001 to 5,000 GPM. The actual PD calibrator flow range is dependent on the flow tube.
PD calibration data are recorded and presented in tabular format consisting of the output of the meter at each data point, the corresponding fluid flowrate for each output. The ratio of meter frequency divided by the kinematic viscosity of the calibration fluid is also presented for use when temperature compensation is necessary. Typically, 10 data points are taken over the standard 10-to-1 flow range of the meter; however, as many points as required can be taken over the extended repeatable range. The more data points taken, the better defined the meter calibration curve will be.
Typical Calibration Techniques
Most flowmeter calibration service suppliers provide a choice of calibration techniques to accommodate different applications and flow measurement requirements. One of the most common techniques is the single-viscosity calibration, which consists of running 10 evenly spaced calibration points at a specified liquid viscosity. Single-viscosity calibrations are recommended when the viscosity of the liquid being measured is constant. If a higher degree of accuracy is needed, again, the more data points taken the better defined the meter calibration curve will be (Figure 4).
Manufacturers of advanced flowmeter calibrators have achieved significant improvements in uncertainties, while reducing calibration time. New technology has also allowed users to better measure and control variables, such as temperature and pressure when characterizing fluids used in flow calibration. And depending on the application, calibration data can now be presented in many different forms for monitoring, printing, or plotting.
With the advent of increased worldwide manufacturing opportunities, flowmeter calibration equipment figures to continue to evolve over the coming years in order to meet the competitive challenges of a variety of applications.
Mark Evans is an applications engineer for Flow Technology, Inc. in Tempe, Ariz. He has over 27 years of experience in the flow measurement industry, including various positions involving calibration, such as calibration supervisor, production manager, calibrator assembly supervisor, repair services supervisor, and ISO 9000 quality system development. He can be reached at 480 240-3303 or [email protected].
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