"Interpretations are issued following a study by an assigned Working Group and a subsequent confirming vote  from the cognizant Committee."

Ref (a): M. Golden email of 28 March 2011

Subject: Request for interpretation of ANSI/NCSL Z540.3-2006

Dear Mr. Golden,
Thank you for your inquiry regarding the National Standard, ANSI/NCSL Z540.3-2006, Requirements for the Calibration of Measuring and Test Equipment, herein referred to as the Standard. In reference (a) you requested an interpretation of the Standard to “determine if the method utilized by NASA, as presented by Scott Mimbs at the MSC and NCSLI Conventions meets the requirements of the Standard. Specifically, if it can be shown that a statistically significant population of “Legacy” instruments has an End of Period Reliability rate of >89%, are the requirements in Sections 5.3b and being met.”

As background, the Standard “prescribes requirements for a calibration system to control the accuracy of the measuring and test equipment used to ensure that products and services comply with prescribed requirements.” As a requirements standard, it does not specify methodology for achieving the requirements as it is recognized that there may be a variety of ways to achieve the requirements that are applicable to individual organizations implementing the Standard.

The NCSLI 174 Accredited Standards Committee recognizes that the methodology developed under the metrology leadership of NASA, commonly referred to as the “89% Rule,” has merit for limiting the probability of false accepts and relative calibration process uncertainty when applied to prescribed and controlled conditions. Although the suggested methodology does not specifically require a calculation of measurement uncertainty, it should, under appropriate conditions, provide for estimating the limit or maximum relative calibration process uncertainty. As such, compliance with sub-clauses 5.3b and of the Standard could be realized.

The NCSL International 171 Committee recently approved a project to document the 89% Rule methodology and include the results in the NCSL International Handbook for the Application of ANSI/NCSL Z540.3 – Requirements for the Calibration of Measuring and Test Equipment. This Handbook currently provides a number of methods to meet various technical requirements of the Z540.3 National Standard, notably the probability of false accept and calibration process uncertainty which, as noted, are factors being addressed through the 89% Rule. It is expected that the Handbook methodology will address the necessary conditions under which the 89% Rule would be considered appropriate for compliance with the Standard.

Further information for participation in the NCSLI 171 Committee’s activities may be obtained from Mr. Steve Doty, Chairman of the NCSLI 171 Committee, at 951-273-4701.

Thank you for your continued interest in the Z540.3 National Standard and its application within the National Measurement System.


Robert E. Fritzsche
Chairman, NCSLI 174 Accredited Standards Committee
NCSL International