New Measurement Uncertainty Requirements
Effective January 1st, 2012, all ISO/IEC 17025:2005 calibration laboratories are required to begin reporting measurement uncertainties for each measurement result. Fox Valley Metrology prides itself on being one of, if not the only lab to offer this via automated software. This automation allows us to provide this service at no additional cost.
For calibration laboratories, ISO/IEC 17025:2005 (clause 5.10.4.2) requires that “if a statement of compliance with a specification is made, this shall identify which clauses of the specification are met or not met”.
When compliance with a specification is made it should be clear to the customer which coverage probability for the expanded uncertainty has been used. Unless otherwise noted, the coverage probability will be 95%.
An explanation of In Tolerance, Out of Tolerance, and Within Uncertainty:
(Case 1) In Tolerance: If the specification limit is not breached by the measurement result plus the expanded uncertainty with a 95% coverage probability, then compliance with the specification can be stated. This can be reported as “In Tolerance”.
(Case 4) Out of Tolerance: If the specification limit is exceeded by the measurement result minus the expanded uncertainty with a 95% coverage probability, then noncompliance with the specification can be stated. This can be reported as “Out of Tolerance”.
(Case 2 & 3) Within Uncertainty: If the measurement result plus/minus the expanded uncertainty with a 95 % coverage probability overlaps the limit, it is not possible to state compliance or non-compliance. The measurement result and the expanded uncertainty with a 95 % coverage probability should then be reported together with a statement indicating that neither compliance nor non-compliance was demonstrated. A suitable statement to cover these situations would be “Within Uncertainty”.
White Paper on Resolving Measurement Disputes
Disputes over measurements can be costly for both parties to resolve and may hinder ongoing relations between suppliers and users of gages and instruments. Often it is simpler for both parties to agree to accept an average value of their readings as the final ‘size’ or the point at which their readings plus measurement uncertainties overlap.
The obvious way to avoid such problems is to agree beforehand on a method that will be used to resolve them if they arise. Often, the degree of separation between the readings dictates the best approach to take. Where the uncertainty of each party is significantly different, the party with the lowest uncertainty in the calibration would be considered more reliable.
The AMTMA offers the following methods as options you can choose from. If the Referee Method fails to bring a resolution, then the Universal Standard Method should be used due to the fact it is technically based and internationally accepted by metrologists in all disciplines.
The Referee Method
The two parties agree on a third party to provide a referee measurement that it is agreed will be considered as the actual value. An alternative on this is where the reading by either party that is closest to that provided by the referee is considered the accepted dispute.
Unless otherwise agreed to, the costs of using laboratories in this method are paid by the losing party.
The Unviersal Standard Method
National and international standards agencies have produced methods of resolving measurement disputes that focus on the uncertainty budgets of those that have produced measurements. The advantage of this method is that its technical base tends to remove personalities from the equation and may indicate that neither party to a dispute has the capability required to resolve it.
Using this method, the onus of proving a measurement falls on the party who has questioned the results of calibration. If requested, this party must provide a copy of their uncertainty budget for the measurement to the other party review. Budgets from both parties should be compared. Such a review should focus on seeking agreement between both parties respecting each element included in the budget since it will rarely, if ever, be all right or all wrong. The mathematics should take care of the rest. There may be cases where one or more elements have not been included in the budget included in the budget and when they are, the outcome changes significantly.
In the event one or more assumptions in the budget cannot be resolved, a third party can be asked to provide an opinion on them.
CHANGE IN CALIBRATION CERTIFICATE FORMAT
Many customers will soon see a change in how the gaging and metrology industry presents the N.I.S.T. number on calibration certificates. NIST is the acronym for National Institute of Standards and Technology.
NIST, A2LA, and many auditors are REQUIRING that we no longer list the NIST test report number on calibration certificates. The NIST number would typically change depending on the type of master used for calibration as well as each time the calibration masters were calibrated at NIST.
Thread Check Inc. is substituting the NIST number with the statement “ ….traceable to SI units through NIST”. This change has already taken place with many of our certificates of compliance and accuracy forms. SI is defined as System International
Many customers will be asking about this missing NIST number on their certificates. We are referring them to A2LA document P102-A2LA policy on measurement Traceability available at www.a2la.org and the NIST policy on traceability located at www.NIST.gov
Selecting Gage Blocks
Select gage blocks in accordance with the combination range required. If a large length is required, add a long block set.
Select gage blocks in accordance with the minimum length step required. Add wear block sets if necessary.
If a set containg a large number of gage blocks is selected, the number of combination gage blocks required for a length is reduced and the number of combinations is increased. The accuracy will be retained and damage will be reduced.
The specific gage block set for micrometer inspection and caliper inspection is available.
If using only one length repeatedly, it is a good idea to purchase discrete gage blocks.
The 2mm-based gage blocks, which take the base of the minimum length set as 2mm, are easy to handle and will not warp, as compared to the 1mm-based gage blocks.
