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ISO 17025 queries

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Guest user Created:   Oct 23, 2020 Last commented:   Oct 25, 2020

ISO 17025 queries

1. What is the difference between certification and accreditation? 

2. I do not understand the concept of measurement uncertainty. 
Can you help me with it? 

3. I do not understand clause 6.5 Metrological traceability. 
What do they mean by the measurement uncertainty of each calibration must be available?

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ISO 17025 DOCUMENTATION TOOLKIT

Step-by-step implementation for smaller companies.

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Tracey Evans Oct 25, 2020

You asked:

1. What is the difference between certification and accreditation? 

Certification is the process where an independent body provides formal written confirmation (assurance) via a certificate that a management system, service, person or product, meets specific requirements. 

When a laboratory formally demonstrates to an independent third party assessment body, the consistent operation of a management system according to ISO 17025 and technical competency for a defined scope of work; the body formally declares the laboratory competent by awarding a Certificate of Accreditation. The laboratory is known as a conformity assessment body and is referred to as accredited, not certified. The third party assessment body, known as the accreditation body themselves need to be accredited to ISO 17011:2017 (Conformity assessment - Requirements for accreditation bodies accrediting conformity assessment bodies) to accredit laboratories.

Organisations that demonstrate they meet the requirement of a Management system standard such as ISO 9001:2015 also undergo a conformity assessment by an independent certification body, but it does not include assessment of competency. 

You also Asked

2. I do not understand the concept of measurement uncertainty. 
Can you help me with it? 

Measurements by principle involve an experimental process to determine the true value of something of interest, for example amount of chromium in drinking water. The result is often obtained after a number of steps where each is influenced by possible systematic and random errors. A single result is therefore an estimate of the true value of what is being measured. Imagine repeating the test a number of times, under various conditions, over a long period. Each of these conditions, for example different personnel, reference materials, equipment performance would impact on the quality of the result by influencing the pattern and width of dispersion of results.  Measurement uncertainty  (MU) is the parameter which defines the interval around the measured value, within which the true value lies with some statistical probability. A simple example of this parameter is standard deviation. Bear in mind MU is made up of many components. The probability distributions of some components can be evaluated over repeated measurements (e.g from quality control charts). Others will typically be evaluated from information such as calibration or reference material certificates or assumed probability distributions based on experience of the technique.

The importance of MU is the risk of a false accept or false reject of a result when making conformity statements. Consider if the upper tolerance threshold for chromium in water was 50 µg/L and a laboratory reports a result of 48 µg/L. This appears as a pass if MU is not taken into account. If the uncertainty MU for the test was 10 µg/L, statistically the range the result could have actually fallen (equally probability) is the from the measured value minus 10 µg/L up to measured value plus 10 µg/L, meaning  between 38 and 58 µg/L. This means there is a chance of a false pass if the MU is not considered – result could be as high as 58 µg/L.  This is why decision rules (ISO 17025 clause 7.8.6 Reporting statements of conformity) are reuired in certain cases.


You asked

3. I do not understand clause 6.5 Metrological traceability. 

What do they mean by the measurement uncertainty of each calibration must be available?"

Metrological traceability means there is confidence in the validity of the result as each step that could influence the measurement has a calibration traceable to the International System of Units. What “measurement uncertainty of each calibration must be available” means is that any equipment, such as balances, are calibrated by a calibration laboratory that conforms to ISO 17025 and the certificates state the measurement uncertainty of the calibration. Likewise the certificates supplied with weight pieces used for verification, certified reference materials, volumetric equipment and other items must be supplied with reported measurement uncertainty. In practice, the MU of, for example, the weighing step in preparing a sample cannot be smaller than the MU of the balance in that range of operation.

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