We first published this article in April 1999. It's a pretty timeless article that's well-suited for our anniversary look back at the content we've produced during Control Design's 15 years. Part I examines the concepts and terminology used to define accuracy of measurements. Part II explains how to combine errors within an instrument or system to provide an estimate of total error.
The importance of accurate measurement is critical to control system designers and engineers. Both of them have a direct incentive to make sure their machines are providing the most accurate outputs possible. The thickness of their paychecks is ultimately determined by how well customers benefit from the accuracy and precision in the equipment they build. That customer will judge the OEM by how well the machine produces his product and by what it took to produce it.
In other end user machine environments, accuracy may seen less important. There may be less concern if readings aren't exact, as long as there's enough usable product to keep the boss off their back. Only if they're lucky, is that likely.
These days, increased competition and government regulations have boosted the demands for improved operating efficiency, business unit accountability, cost leadership, and quality certifications. The accuracy of measurement and control systems is of greater concern. Extensive applications of computers, data collection facilities, and databases are relying on accurate measurements.
When an OEM provides a system to accurately measure all the control variables, from web thickness to milled dimensions to chamber temperature, it helps the end user eliminate waste, improve efficiencies, and reduce costs.
Businesses are putting stricter accountabilities at lower levels in business units, and that requires accurate internal accountability for the unit as well as for intercompany or intracompany transfer.
ISO9000 certification is of increasing importance in the process and discrete industries, and instrument accuracy is an important part of that. The government is putting more and more regulation on the process industry, in particular, requiring more accurate measurement and data collection.
So accuracy is important. But what is accuracy? The language of accuracy is not universal, and any discussion depends on a common understanding of terminology. (For definitions of commonly used terms, see "An Accurate Glossary")
Absolute Accuracy or 'Repeatability'?
By definition, all accuracy is relative: how accurate a measurement is compared to a standard.
When discussing the error of an instrument or system, we need to determine what form of accuracy we need for a particular function. Absolute accuracy refers to how close a measurement is in relation to a traceable standard (see the traceability pyramid in Figure 2).
"Repeatability," on the other hand, refers to how accurately a measurement can be duplicated or repeated. The term "repeatability" in this context is the common field usage and not the ISA definition of the term. The common field usage is essentially the same as the ISA term "reproducibility;" that is, the combination of linearity, repeatability, hysteresis, and drift.
If it is important that you make a measurement in reference to an absolute value, we are talking about absolute accuracy. When most people talk about accuracy, they are talking about absolute accuracy. In order to have absolute accuracy for your measurement, you must have traceability from your measuring device to the National Institute of Standards and Technology (NIST)/National Bureau of Standards (NBS) reference standards (the "golden rulers"). The accuracy of your measurements is directly dependent on the accuracy of your calibrators, which is directly related to the care and feeding of your calibrators, the calibrator's calibration cycle, and the traceability of your calibrators.
In a plant where ambient and process conditions can vary substantially from a reference condition, the OEM needs to understand how the end user will attempt to maintain accuracy, and this can be a daunting task to simulate. Calibration cycle and methods, instrument location, instrument selection, maintenance, recordkeeping, and training all become important issues in maintaining instrument accuracy. A formal calibration program is the only way to ensure accuracy of instrumentation. This is essential for achieving and maintaining an ISO 9000 certification.