Reporting the Expanded Uncertainty of the measurement is a requirement of ISO/IEC 17025 and likely always will be. However, if you look at several different certificates of calibration, their Expanded Uncertainty reported, contributors and reporting methods will vary greatly. Some include a blanket statement of uncertainty, while others simply ignore significant contributions required by several published standards. This guide describes the formulas, statements, and data reported on a Morehouse ASTM E74 calibration certificate and how it benefits the end-users.
At Morehouse, we follow the proper requirements per our accreditation and rely heavily on meeting the requirements of ISO/IEC 17025:2017 and these published documents:
• ILAC-P14 "Policy for Measurement Uncertainty in Calibration" (1)
• JCGM 100:2008 "Evaluation of measurement data — Guide to the expression of uncertainty in measurement" (2)
• ANSI/NCSL Z540.3 & Handbook "Requirements for the Calibration of Measuring and Test Equipment" (3)
Morehouse also follows general acceptable metrological practices, such as the NIST technical Note 1297, "Guidelines for Evaluating and Expressing the Uncertainty of NIST Measurement Results." (5) The Appendix on a Morehouse ASTM E74 calibration certificate uses a similar method to what is found on a calibration report from NIST. The difference is that Morehouse includes the resolution of the Unit Under Test (UUT) because it is required by ILAC-P14. Therefore, Morehouse follows the ILAC-P14 requirements by reporting Expanded Uncertainty per point and replicates NIST's method in the appendix with one modification to include the resolution of the UUT (see Figure 1).