2012

MEASURE FOR MEASURE

Managing the Measurement System

by Graeme C. Payne

From time to time, a debate arises about what inspection, measurement and test equipment (IM&TE) should be calibrated. Typically, the discussion involves two camps: one maintaining that all measuring devices must be calibrated and the other holding that only measuring instruments used for final inspection and testing need to be calibrated.

Both sides have some merit, but, as is often the case, the best answer lies somewhere between these two positions.

Open to Interpretation

Paragraph 7.6 of ISO 9001 says the organization shall determine what measurements or monitoring is necessary to “provide evidence of conformity to product requirements,” and what devices are needed to perform the measuring or monitoring tasks.1 Among several other requirements, the standard also states, “Where necessary to ensure valid results, the measuring equipment shall be calibrated.”

Problems arise from interpreting these or other parts of paragraph 7.6. Those who argue on one side sometimes say every measuring instrument a company uses—even a thermostat in an administrative office—might have some effect, however tenuous, on the quality of the company’s product. Those on the other side often hold that an instrument need not be calibrated if it is used only for setup or alignment (or a similar process) of a part because the part will be inspected later using calibrated tools.

Calibration is the process of comparing a measuring device to a measurement standard to determine how true the reading is relative to the definition of the measurement unit. All IM&TE is subject to change over time, from causes such as mechanical wear, electronic drift, environmental changes or many other factors.

In this context, a measuring instrument includes anything that is used to establish a physical parameter. There are obvious instruments, such as voltmeters, micrometers, measuring tapes, pressure gages, scales and conductivity cells. But the term “measuring instrument” also includes surface plates, angle blocks, torque wrenches, plug gages, special setup jigs, feeler gages, pH buffer solutions and much more.

Calibration of IM&TE has long been identified as a quality cost, usually an appraisal cost,2 and businesses are almost always trying to reduce costs of producing products or services. A quick evaluation might indicate some cost savings are possible by reducing the amount of calibrated IM&TE. The problem is how to determine what needs to be calibrated and what doesn’t.

Phil Stein’s test was to ask whether, in terms of the product being made, it matters if the measurement result is ever wrong. If it does matter, then the measuring device must be calibrated.3

If the answer is no, then the next question should be, “Why are you performing the measurement?” If the measurement is not needed, it can probably be skipped to streamline the process. If the measurement is needed but is wrong (because the tool is uncalibrated), the effect on downstream processes (rework, missed operations or cost) can be evaluated.

The engineering and management problems in such a discussion should be determining what measurements are necessary to ensure the quality of the product, when and where those measurements need to be made, and what measurement characteristics are required of the IM&TE.

All of these are a subset of the overall business or quality management system and together could be called a measurement management system.

ISO to the Rescue

As fortune would have it, there is an ISO standard for that. Paragraph 7.6 of ISO 9001, refers readers to editions of ISO 10012-1 and 10012-2, which have since been superseded by the combined ISO 10012:2003 Measurement Management Systems—Requirements for Measurement Processes and Measurement Equipment.4 This standard provides a model for planning an organization’s total measurement system.

ISO 10012 is a useful guide to determining and documenting all measurement management requirements. An organization’s measurement management system must ensure its products meet all known requirements.

Measurement requirements ultimately derive from the product specifications and in turn drive the measurement planning process. The measurement requirements place a minimum limit on the required accuracy and precision of measurement tools. The product realization process determines where the measurements can be and should be made, which are not the same.

The standard describes this through a concept called metrological confirmation, defined as the “operations required to ensure that the measuring equipment conforms to the requirements for its intended use.”5 Metrological confirmation is a planning and analysis process—part of the overall measurement management process—that establishes what measurements are required and whether each measuring instrument is suitable for its intended use.

The process answers questions such as:

  • At this step in the product realization process, what measurements are needed and what are the tolerances?
  • Does the measuring instrument have the required range, resolution, accuracy and precision?
  • Does the instrument have the stability required in the work environment?
  • Are the operators properly trained in its use?
  • Have all measurement uncertainty influences been identified and accounted for?
  • Is the measurement process in a state of statistical control?
  • Is the measuring instrument calibrated and is the calibration traceable to the appropriate measurement unit of the International System of Units?
  • Is the measurement instrument protected from unauthorized adjustment or alteration?
  • Is the measurement process error proofed and are effective corrective and preventive action processes in place?
  • Is each specific metrological confirmation reviewed at defined intervals?
  • Is the measurement management system monitored and audited?

Another popular misconception is that ISO 10012 deals only with the calibration system. Actually, only seven of its 120 “shall” items directly or indirectly deal with calibration.

ISO 10012, although often overlooked, can be used to find that middle ground between the “calibrate everything” and “calibrate the absolute minimum” camps. The standard gives guidance for managing all measurements that happen during the product realization process, wherever and whenever they happen.

Taken as a whole, ISO 10012 provides a rigorous method for determining whether every measurement being made is necessary, is being made with appropriate measuring instruments, is being made correctly and is reviewed at regular intervals. The documentation from the planning and analysis becomes objective evidence for making fact based decisions about which tools truly need to be calibrated.

REFERENCES

  1. ANSI/ISO/ASQ Q9001-2000, ASQ Quality Press, 2000, paragraph 7.6.
  2. Joseph M. Juran and Frank M. Gryna, Juran’s Quality Control Handbook, fourth edition, McGraw-Hill, 1988.
  3. Philip Stein, “Don’t Whine, Calibrate,” Quality Progress, November 2000.
  4. ISO 10012:2003 Measurement Management Systems—Requirements for Measurement Processes and Measurement Equipment, International Organization for Standardization, 2003.
  5. Ibid, paragraph 3.5. 

GRAEME C. PAYNE is president of GK Systems, a consulting firm specializing in measurement science. He is a contributor to The Metrology Handbook (ASQ Quality Press, 2004) and is the 2006-2007 chair of the ASQ Measurement Quality Division. Payne is a senior member of ASQ, a certified quality technician, calibration technician and quality engineer.

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