Uncertainty of measurement & Technical Specifications SAAMF Roadshow Durban CSIR NML Eddie Tarnow...

Uncertainty of measurement & Technical Specifications

SAAMF Roadshow Durban

CSIR NML

Eddie Tarnow

Metrologist: Torque & Automotive

14 June 2006

© CSIR 2006 www.csir.co.za

What is “Uncertainty of Measurement”?

• The true value of the measureand is never known• A value is assigned based on an estimate, within a certain level of

confidence• This results in a range of possible values

0 10 20


What is “Uncertainty of Measurement”? (2)

• The range of values, within which the true value is believed to lie, with a certain level of confidence, is termed “The Uncertainty of Measurement“

• If an instrument, with an accuracy specification of +/- 1 % of reading, is used to perform a measurement, the true value will lie in the range of -1% to +1%

• This is however very simplistic because there are often a number of factors which influence the uncertainty of measurement



• It should be noted that the uncertainty of measurement resulting from a calibration can usually be one of two scenarios:-• If the instrument has been calibrated according to the manufacturer’s

recommended procedure and is found to perform within its specified accuracy, the uncertainty associated with the instrument (imported traceability) is equal to the instrument accuracy specifications

• If no conformance to specification is checked, and the calibration service provider simply assigns a value to the instrument at a particular measurement point, the uncertainty quoted on the calibration certificate can be used as the applicable uncertainty of measurement associated with the instrument provided it is used to perform the measurement at EXACTLY the same point and under the same conditions

• NOTE: The uncertainty of measurement quoted on a calibration certificate applies ONLY to the calibration results quoted in the certificate and NOT to subsequent measurement results during use



• Subsequently the uncertainty applicable for the measurement setup must include ALL possible influence factors such as imported traceability, display resolution, environmental conditions, operator, measurement setup, etc.

• This implies that in order to evaluate the uncertainty of measurement applicable at the time of performing a measurement or test, a detailed analysis of these factors is required

• Internationally, uncertainty of measurement is estimated according to an ISO document called, “Guide to the Expression of Uncertainty in Measurement” also known as the “GUM”.


Estimating the uncertainty of measurement

• The GUM describes the following basic methodology to estimate uncertainty of measurement:-• Model the measurement• Identify the sources of error• Categorise as type A (statistical), or type B (other)• Manipulate appropriately

• Standard uncertainties• Sensitivity co-coefficients

• Combine to obtain combined standard uncertainty• Expand to obtain an expanded uncertainty at the require level of confidence• Report the result

• This process should be followed for all test and calibration measurements performed in the industry where conformance is declared

• Degree of detail dependant on risk


The Impact of Uncertainty of Measurement on conformance declaration

• ISO/TS 16949:2002 states that calibration and testing activities should be accredited to ISO 17025.

• ISO 17025:2005, “General requirements for the competence of testing and calibration laboratories”, states in para 5.10.4.2, “when statements of compliance are made, the uncertainty of measurement SHALL be taken into account”.

• This implies that in the automotive industry, uncertainty of measurement MUST be taken into account when declaring compliance or non-compliance of components, to the technical specifications.


The Impact of Uncertainty of Measurement on conformance declaration (2)

• ISO 14253-1:1998, “Geometrical Product Specifications (GPS) – Inspection by measurement of work-pieces and measuring equipment – Part 1: Decision rules for proving conformance or non-conformance with specifications”, indicates the following scenarios:-


How can I learn about the estimation of uncertainties?

• The training division within the National Laboratory Association, CMeTSA, offer a course entitled, “Introduction to Estimation of Uncertainty of Measurement (GUM)”.

• There are various metrology consultants who can provide guidance in this regard.

• The SAAMF can be approached for advice on getting assistance.

Questions??

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