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Flowmeter accuracy: The importance of traceable calibration procedures

November 21, 2005
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Manufacturing operations live and die by the repeatability of process conditions. It is not enough for an individual measurement device to perform in a consistent manner, run-to-run and day-to-day. Multiple instruments running the same process must also run in the same manner, and under the same conditions.

For industrial end users, inaccurate flowmeter calibrations will have a serious affect on process performance. Flowmeters with moving parts, such as turbine and positive-displacement meters, need to be calibrated periodically to determine the effects of wear on the meters. According to experts, ultrasonic flowmeters used to measure natural gas should be calibrated before they are put into service.

Without documenting the sources of calibration instrument errors, and maintaining National Institute of Standards and Technology (NIST) traceable calibration records, users calibrate their flowmeters to an unknown standard without compliance to recognized industry accuracy guidelines. Moreover, they have no way of identifying the inherent inaccuracy of their flowmeters currently in service.

Background

In the aerospace and defense industries, flowmeter users have long required traceable instrument calibrations in accordance with government standards such as MIL-STD-45662A, ANSI Z540, and the recently approved ISO/IEC 17025 standard. The process industries have not always been as careful to ensure their calibrations meet these established national standards.

The new ISO/IEC 17025 guidelines contain all of the requirements needed by testing and calibration laboratories in order to demonstrate to customers and regulators that they operate a sound management system which puts them in full control of their processes, are technically competent, and are able to generate technically valid results. Accreditation bodies recognizing the competence of testing and calibration laboratories will use the standard as the basis for their accreditation.

Despite the importance of traceable, fully documented calibrations, some flowmeter users still ask, “Why should I care if my meter calibrations are traceable to recognized standards? My manufacturing process works fine. Why fix what isn''t broken?"

The answer is simple: The only way you can be certain your flowmeter is measuring against a known and consistent accuracy standard is documented uncertainty analysis and calibration traceability. If your previous calibrations were not traceable, or the instrument uncertainty was not known, how do you know if they are accurate?

The most commonly accepted definition of traceability is found in the International Vocabulary of Basic and General Terms in Metrology, 1993, as:

“...the property of the result of a measurement or the value of a standard whereby it can be related to stated references, usually national or international standards, through an unbroken chain of comparisons all having stated uncertainties.”

Many flowmeter users accept, on good faith, their vendor’s assurance that “our meters don’t require calibration.” Companies may also believe their quality assurance program is well in hand, and as such, do not see a need to examine the traceability of their calibration source.

Some process plants develop a false sense of security by performing flowmeter calibrations on a frequent basis. Too often, however, they ignore the implications of calibration uncertainties and errors, and underestimate the importance of credible metrology procedures in compensating for fluctuating variables such as fluid temperature and pressure, or atmospheric temperature and pressure. The number of "measurement transfers" involved in attaining traceability to NIST can also erode traceability to the point where it has no real value.

Many smaller, local calibration service centers simply can’t afford precision equipment, and lack the experience to perform the sophisticated metrology procedures needed for a NIST-traceable calibration. These service centers often use simplified calibration systems containing unacceptable levels of uncertainty, and introduce additional error into the flowmeter under test by not compensating for such variables as temperature, pressure, density and viscosity changes during the actual calibration.

Calibration basics

For the sake of this discussion, it is important to present a clear definition of "NIST traceability." The term refers to the ability to quantify all of the imprecision of the components of a given measurement, and assess the errors of the final calibration result. Traceability to NIST serves the valuable purpose of documenting (i.e., quantifying, under actual dynamic flow measuring conditions) the measurement performance levels of the calibration source relative to the NIST Flow Standard.

Systematic sources of uncertainty and errors in flowmeter calibrations include: temperature variations along the flow path, liquid variations that cause viscosity to differ from calibration conditions, pressure effects on liquid compressibility, and temperature effects on flow calibration/measuring devices.

To understand the significance of the two major types of calibration error, Systematic Error (or bias) and Precision Error (or random), it is helpful to picture a dartboard. Imagine the dart thrower pitching darts at the target with the goal of consistently hitting a bull''s-eye representing perfect accuracy.

Precision error means the dart thrower’s grouping is poor—his hits are random and scattered all over the target. When the thrower’s precision improves, his hits are no longer random, but are very repeatable, dependable and tightly grouped (even though not in the bull''s-eye).

After considerable practice, the dart thrower attains a high precision and also hits the bull''s-eye dependably, and thus does not have any significant bias or systematic error. He has fine-tuned his accuracy by determining the bias of his aim, and has corrected it in order to hit the bull''s-eye. This example clearly shows the importance of calibration traceability.

It should be noted that both precision and systematic error are difficult to eliminate. Precision error can only be removed with the use of the correct instrumentation and through a complete understanding of the measurement being made. Systematic error can only be eliminated through direct traceability to NIST standards.

Any analysis of calibration traceability should consider compensation techniquesthat take into account the varying conditions under which flowmeters operate. The end user might assume a calibration has great precision when, in reality, a host of critical parameters have been overlooked. Changes in fluid make-up, temperature, pressure, viscosity and density can significantly distort a flowmeter’s output signal and result in an inaccurate measurement. Installation effects can also alter the measurement.

In addition, the environmental effect upon the calibrator itself impacts accuracy. For example, the temperature in the calibration lab can change the shapes of the vessels used to produce the flow calibration, hence influencing the accuracy of the measurements.

Primary versus 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. The most common examples of primary standard calibrators include positive displacement, continuous flow loop, and time-weigh systems.

Primary standard calibration devices are typically used and maintained by the metrology or calibration laboratory within an organization. The laboratory can then either directly calibrate the production instruments or calibrate instruments used as transfer standards for calibration of the production instruments.

Conversely, secondary standard calibrations are not based on natural physical measurements. Instead, they calibrate the flowmeter against a “master meter”—a flowmeter calibrated on a primary standard. Flow transfer standards, sonic nozzle stands and other secondary calibration systems are often many generations away from NIST and error sources are not fully understood and/or documented. Primary standard flow calibrators typically have uncertainty orders of magnitude better then secondary standards.

Value of documentation

Ideally, companies should periodically audit their outside calibration supplier to ensure adequate equipment, procedures and documentation are in place to enable NIST traceability. But if the cost and time required to perform these on-site audits cannot be justified, the use of an ISO 9000 certified or MIL-STD approved calibration supplier is a good alternative.

At the time of an audit, traceability documents should be made available, with a trained metrologist on hand to substantiate the accuracy and traceability of the calibrations. These audits have traditionally been required by military contractors, and are becoming more common among industrial flowmeter users.

However, if the calibration supplier is ISO 9000 certified, the user has assurance that quality procedures and traceability records are maintained, and as such, these costly and time-consuming audits may not be needed.

A NIST-traceable calibrator''s system software should retrieve all relevant data and organize it into a final report accurately representing your flowmeter’s performance based on established parameters. Certified data sheets containing traceability information should be provided upon request with every calibration run. These data sheets offer a paper trail back to the NIST standards for the instruments used to "calibrate the calibrator.”

If the calibration supplier cannot produce proper certifications, the end user has no verified traceability to NIST, and therefore no assurance of true accuracy. Frequent recalibrations to an unknown standard will only duplicate the mistake.

Summary

No matter what kind of flowmeters you have, they need to be checked and possibly recalibrated periodically. How often depends on the type of flowmeter and the operating conditions. Even though calibration may seem like a complex process, it has the potential of saving you many dollars and also the headaches that arise from having a meter that does not read accurately.

If your in-house calibration equipment or outside calibration service centers use primary standards documented to define sources of errors, limits, and calibration traceability, then you know your flow measurements are accurate, consistent and reliable to the extent of the limits. Otherwise, the calibration data you receive is suspect, and an inherent inaccuracy will likely be introduced into your manufacturing process and products, perhaps causing you to waste raw materials in ways that can never precisely be determined.

The importance of traceable, documented primary standard calibrations to your quality assurance program cannot be overstated. Remember, your flowmeter is only as accurate as its calibration, and your calibration is only as accurate as your calibration system.

Ladd Howell is systems business unit manager for Flow Technology, Inc. (FTI), an ISO 9001-2000 certified supplier of turbine and positive displacement flowmeters, flow electronics, and calibration systems. FTI maintains one of the industry’s largest primary standard flow calibration and repair labs at its facility in Phoenix, Arizona.

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