Flow Calibration
Fluke Calibration GFS-2102, Dynamic Gravimetric Gas Flow Standard (Photo courtesy of Fluke Calibration)

Flowmeter calibration is often described as “vital” to accurate flow measurement. And accurate flow measurement is vital to a number of important processes that make the world go ‘round, from producing clean water to generating electricity to the custody transfer of fuel.

Calibration Defined
The term calibration is reserved for manufacturers that use traceable standards to establish or correct factors specific to an individual meter. As part of the calibration process, the meter to be tested is compared with the laboratory master standard.1

“Until the flowmeter is calibrated, the output is meaningless. The meter has nothing to reference the output to, nor is it scaled to produce meaningful units of measure,” explains Ron Madison, Manager of Calibrator Sales & Marketing at Badger Meter (www.badgermeter.com, www.flow-dynamics.com). “Calibration is a metrology process that brings flow measurement into a credible uncertainty result. The calibration must be traceable to an authoritative agency, such as NIST [National Institute of Standards and Technology], so that every calibrator aligns itself to a single government primary standard source.”

Over the years, the demand for higher-accuracy meters has increased, due, in part, to stricter government mandates and more stringent in-house requirements for critical applications. To meet increased demands, manufacturers of flow measurement devices continue to develop instruments that provide higher performance and new technology features.

According to Larry Renda, a regional product manager for the Pressure and Gas Flow Calibration Products at Fluke Calibration (us.flukecal.com), such features include on-board microprocessors, integrated circuits and memory that provides storage of current calibration data, data logging, higher resolution analog and digital outputs, advanced characterization, greater operational flexibility, and so forth.

The trend toward higher accuracy flow devices, of course, drives the requirement for more accurate flow calibration systems, Renda says. But he makes the case that high-performance flow calibration standards aren’t always necessary for field-class flow measurement devices. Renda says fundamental calibration standards for field-class devices are often far too complicated, expensive, don’t provide a continuous flowrate indication, or opportunities for automation.

“There is a class of maintenance shop and metrology lab instruments that possesses the necessary accuracy along with the simplicity of use and the economy to properly manage this vital function,” Renda says.

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The Calibration Landscape
While the highest flowmeter calibration standards might not be necessary for all meter types, given the application or particular fluid involved, they are playing an important role in shaping today’s common calibration practices.

“Flow calibration is vital to the health of flow measurement, but with varying degrees of precision,” Madison says. “As an example, custody transfer [of oil or gas] will require frequent meter proving, which often requires leaving the meter inline and moving a prover to the field location.” In contrast, he says a tank filling process may not need calibration until a level issue becomes obvious. The answer, Madison points out, lies in the criticality of the precision required to produce, sell or measure flow.

Flow lab accreditations have been changing the landscape of the way flowmeter users approach calibration. A benefit of accredited calibration labs to the end-user is that customer audits are not required if the calibration lab is -accredited by the National Voluntary Laboratory Accredited Program (NVLAP), which incorporates IEC/ISO 17025. For example, he says the nuclear industry searches out NVLAP calibration facilities knowing that they can depend on third-party audits to ensure calibration competence and uncertainty claims.

There are several design standards for calibration systems, as noted by Jerry Stevens and Jason Pennington’s article “Calibration, Proving, & Verification” (Flow Control, flwctrl.com/CalProvVerify). Standards may include, but are not limited to, volumetric methods, gravimetric methods, and master meter comparison. These methods can produce results with an uncertainty of better than 4-to-1 as compared with the meter to be tested. In addition, there are standards that govern and encompass the entire calibration system. NIST and ISO 17025 define standards and requirements with varying degrees of complexity for calibration facilities and procedures. NIST (www.nist.gov) defines standards for traceability that reside with the individual laboratory for maintenance or self-compliance. ISO 17025 is a more rigorous, third-party accreditation.

Best Practices & Common Pitfalls
The importance of, or need for, flowmeter calibration varies based on application and fluid conditions; however, there are some general best practices fluid handling professionals can follow to ensure proper flow calibration.

At every accuracy level of flow calibration, Renda says it is imperative to ensure that the calibration standard that is being used has sufficient accuracy and the necessary traceability to accomplish the calibration per the requirements of the end-user. In addition, he says two very important assumptions also must be met:

  1. The rate of flow between the unit under test and the calibration standard is in a steady state
  2. All of the media that is measured by the calibration standard is also measured by the unit under test at the same; that is, that there are no leaks, no significant temperature changes in intermediary volumes, etc.

Renda says the most common mistake he sees end-users make is not properly analyzing their requirements for the accuracy and characteristics of the calibration standard they choose. “Since the vast majority of the devices that they are supporting are of a continuous flow-indicating nature, the calibration device should be of this type as well,” he says. “That way, so long as our two previously mentioned assumptions are met, the flowrate determined by the calibration standard and the unit under test should exactly agree.”

Madison offers the following best practices for successful flow calibration:

  • Develop a maintenance calibration schedule from historical calibration data. By tracking every calibration performed on the flowmeter, the data will reveal degradations and shifts that will give the user insight as to how often a calibration is required.
  • Find a quality certified calibration lab, preferably an NVLAP facility, and do not change suppliers. Once you do change suppliers, you enter into an unknown bias between labs that is difficult to account for. This bias shift in the calibration results is not traceable, nor easily identifiable.
  • Manage the way you pull meters for calibration, to avoid unnecessary downtime. Purchase spare meters, which makes it possible to rotate meters for calibration.
  • If you are going to use a master meter or flow-transfer standard in a line for a secondary standard calibration, design the piping to allow for easy installation and removal of the master meter.

In the area of liquid flowrate calibration, Renda says the most critical mistakes are made with regard to the viscosity, density or heat content of the liquid being measured. “Each of these can affect the flowmeter based upon its operating technology,” he says. “Matching the calibration of the flow-measuring device with its application is very important.” Renda says users should also be mindful that viscosity and density vary with operating conditions and that the heat content of a medium changes with the calibration media.

Likewise, Madison says end-users should investigate whether or not their meter technology is affected by viscosity shifts due to temperature changes. This shift can be compensated for, but the calibration needs to be defined correctly with the minimum and maximum temperature ranges, along with the fluid specification, he says. “This will improve measurement accuracy and is likely to save money by avoiding costly product waste and additional calibration test time.”

As an example, he says positive-displacement meters are not viscosity-sensitive because they measure a mechanical volumetric quantity. However, if the viscosity is approximately less than 10 centistokes, there is likely to be seepage around the measuring parts. This can be accounted for in the calibration, but if the viscosity varies due to temperature, the measured volume might also vary as a result of seepage or the lack of.

A common misconception end-users have is that their meters must be calibrated by the original manufacturer, Madison says. But some manufacturers do not have NVLAP accreditations, or even primary standards. “They may only provide water calibrations and use conversion charts for other liquids,” he says. “Obviously, this is dependent on the type of flowmeter used. As an example, a Coriolis flowmeter measures direct mass and can be calibrated in water for other liquids and gases; while a precision turbine meter requires a hydrocarbon calibration mixed to a precise viscosity for petrochemical fluids.” He encourages users to look for quality NVLAP-accredited calibration facilities, which have primary standard calibrators with a range of flow and media capability, for the highest quality calibrations.

Lastly, Madison says the end-user must be familiar with the difference between secondary standard and primary standard calibrations. “If price is the motivator, the end-user might well be getting secondary standard calibrations,” he says.

Good Protocol
When it comes to good flow calibration protocol, end-users should obviously consider the accuracy requirements of the device they are calibrating and keep good historical calibration records for ongoing reference to avoid downtime and associated costs.

Renda says they should also consider whether automation can and should be applied. “If the flowmeter has a visual or manually read output, automation might not make sense,” he says. However, even in these cases there are times when, due to the design of the flowmeter, the automation of secondary parameters (temperature and pressure) makes sense. Renda says this is the case of variable-area gas flowmeters, as these devices require a correction for the variation of the design conditions to that of the actual calibration conditions.

Madison says good meter calibration protocol rests on good historical calibration records and a calibration plan that will not shut down operation.

“Calibration maintenance software is a low-cost investment, which provides historical information and displays the calibration results over time, which is required to evaluate the repeatability and life of the meter,” he says. “Remember that the fluid being measured, coupled with the flowmeter technology, dictates the life of the calibration in that set of circumstances. A flowmeter may retain its calibration longer in one application than in another, due to environmental and fluid conditions.”

Future Trends
Calibration experts see automation, online monitoring and self-contained field calibration features as trends for flow calibration going forward.

“The manufacturers of process flowmeters are trending toward self-contained field calibration features,” Madison says. “A few of these already exist in magmeters, thermal mass, ultrasonic, dual-rotor turbine meters, etc. This is done by leveraging electronics, in an effort to check the health of the meter and calibrate the meter with some limitations.”

Renda says he sees continued improvement in the accuracy of field-class flow measurement equipment driven by industry’s desire for automation, improved process efficiency, and lower cost of ownership (through reduced calibration costs). “I think that wireless interface technologies of remote flow measurement equipment will be more important in the future to reduce the cost of installation, providing that they are secure,” he says.

Amy W. Richardson is the former managing editor of Flow Control magazine. If you have comments about this article, email Matt Migliore at matt@grandviewmedia.com.

1. “Calibration, Proving, & Verification”, Flow Control, flwctrl.com/CalProvVerify.