Difference Between Nominal Accuracy and Certified Accuracy

How to interpret the accuracy of a measuring instrument?

Nominal accuracy refers to the accuracy that a manufacturer claims its measuring equipment has, while certified accuracy is an objective measurement of the true accuracy of the product.

In other words, nominal accuracy is a subjective statement, while certified accuracy is a verified fact. It is important for users to understand the difference between the two when making purchasing decisions, especially for measuring equipment used in applications that require high accuracy, such as quality control (QC) and quality assurance (QA).

Accuracy: More than a specification

Accuracy is more than a number mentioned in a specification sheet. Accuracy specifications not only reflect the manufacturer's emphasis and commitment to quality, but also reveal the manufacturer's technical capabilities, knowledge and proficiency in developing and producing high-quality, reliable measuring instruments.

International Standard ISO 17025, recognized for excellence

ISO 17025 is an international standard that specifies technical and administrative requirements for calibration measurement laboratories, providing a recognized framework for ensuring the accuracy, reliability and interoperability of test and calibration results.

In short, if a measurement equipment manufacturer's calibration procedures are accredited to ISO 17025, it means that its calibration laboratory has met the high standards set by the accreditation in terms of technical competence, quality management, and the ability to produce accurate and reliable calibration results.

Manufacturers that are accredited to ISO 17025 can display the accreditation mark or logo on the calibration certificates of their products. The calibration certificate lists evidence of the calibration, including details such as the calibration date, the conditions under which the calibration was performed, the measurement uncertainty, and the traceability of the measurement results.

ISO 10360, more testing, more assurance

ISO 10360 is a series of standards that specifies performance requirements and acceptance and revalidation procedures for different types of coordinate measurement systems. If an optical 3D measurement device is accredited to ISO 10360, it means that the device has been tested and meets the requirements set out in this international standard.

These requirements include probing form and dimensional errors, unidirectional and bidirectional length measurement errors (volumetric accuracy), planar shape measurement errors (flatness), etc. Volumetric accuracy testing usually involves multiple measurements of calibration tools of different lengths in different directions within the working volume.