Process Accuracy vs Process Uncertainty (Risk Mitigation: Calibration and the Customer’s Process)

Abstract

The requirements for the process accuracy may be dictated by the requirements of the expected output of the process or product. The ratio between the process accuracy (i.e., the acceptance limits of the manufacturing process, or it could be the product itself) and the instrument accuracy (i.e., the equipment used to measure the process) is the Process Accuracy Ratio (PAR). Just as a calibration standard’s accuracy is an incomplete representation of a calibration process, the sole use of accuracies of the manufacturing measurement process may omit large errors that could change the outcome of the measurement or test. The measuring process can be impacted by many factors. We will define the inclusion of possible sources of error as uncertainty components of the process and thus more exactly define the ratio as the Process Uncertainty Ratio (PUR).Calibration laboratories have the same need to determine the uncertainties of their measurement processes. An important part of metrological traceability is uncertainty and a good measurement assurance program. We have to build an uncertainty budget for the measurement process whether it relates to calibration of instruments or the use of instruments to measure a manufacturing process or the end product. Metrology laboratories that are accredited to ISO17025 are required to calculate the uncertainties of their calibration processes. In a similar manner, you can demonstrate that all the possible components of error are accounted for when determining the process uncertainty. Questions that should be asked: What possible components of error will affect my output or product (time, atmospheric conditions, uncertainty of the calibration performed on the instrument I am using, proper use of the instrument upon which pass/fail decisions are being made, et.al.)? What is the potential risk involved if the process isn’t evaluated for potential components of error? We may find that the accuracy of the instrument used to measure the process may not be the biggest contributor of error in the process. Once we determine the potential errors, we can begin to eliminate as much of that error as possible through statistical process control (SPC) or other means. As the customer of calibration services, it is your responsibility to ensure the calibration received supports your process requirements and that you take into account all sources of error when using instruments to make decisions about your manufacturing processes or concerning your product’s quality.