By Ladd Howell
For flowmeter users, gambling on calibration accuracy can have serious repercussions. Without a documented understanding of calibration instrument error sources and National Institute of Standards and Technology (NIST)-traceable calibration records, flowmeters are calibrated to an unknown standard with little or no relationship to the industry-accepted guidelines for accuracy. Worse yet, the inherent inaccuracy of the meters becomes very difficult to identify. Manufacturers seeking ISO certification suffer the most in this situation.
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 new ISO 17025. The process industries have not always been as careful to ensure their calibrations meet these established national standards.
False Sense of Security
Some industrial users of flowmeters are lulled into a false sense of security by performing frequent calibrations, while lacking an adequate understanding of the implications of various calibration uncertainties and errors. Others do not fully realize how calibration techniques based on credible metrology procedures are necessary in order to compensate for fluctuating variables, such as fluid temperature and pressure or atmospheric temperature and pressure. Additionally, the number of measurement transfers involved in attaining traceability to NIST erodes the traceability to the point where, in actuality, it is essentially worthless.
Meanwhile, there are other users that believe that since their end product is manufactured to specification, the quality of their calibration source or its traceability to industry standards for accuracy, do not need to be re-examined. And some users accept on good faith their vendor’s assurances of "our flowmeters don’t require calibration."
Value of Primary Standard Calibrations
As a general rule, the extent to which a flowmeter calibration uncertainty varies depends on whether a primary or secondary standard calibration system is used. Flow is considered a "derived standard," as no single standard for flow exists.
An instrument can be considered a primary standard if it is not characterized by the same method that it is being used for. A primary standard instrument is based on measurements of natural physical parameters (i.e., mass, distance, and time) for which standards exist. The instrument should also be carefully analyzed to ensure precision error is minimized and — through traceability, bias, or systematic error — is documented and understood. Primary standard flow calibrators typically have uncertainty orders of magnitude better then secondary standards. The most common examples of primary standard calibrators include positive displacement (PD), continuous flow loop, and time-weigh systems.
Conversely, secondary standard calibrations are not based on natural physical measurements. Instead, they calibrate the flowmeter against a master meter, i.e., 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.
Why Fix What Isn’t Broken?
The issue of calibration traceability and uncertainty may lead some flowmeter users to ask, "Why should I care if my flowmeter 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?
If your in-house calibration equipment or outside calibration service centers use primary standards or secondary standards that have been 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.
About the Author
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, Ariz.
For More Information: www.ftimeters.com