Evaluation of the Uncertainty in Measurement Versus the Limit of Specification: Qualitative and Quantitative Aspects of Compliance

In many cases, the declaration of compliance of a result of measurement is not clear. This is observed when there is a partial superposition of the expanded uncertainty of a quantity with its limit of specification. In these cases, a judgment is required based in pre-established criteria between the client and the supplier. The consequences of an inadequate decision may be disastrous. Either the supplier will have financial losses or it will lose credibility with its client. This work demonstrates how to calculate the probability, for an appropriate confidence level, that the uncertainty associated with the measurement is within the limits that were established in a contract, declaring the compliance or non-compliance relative to the specification, and study two methodologies for helping the claimer to take the decision of compliance or non-compliance, using as example the heat capacity of natural gas, calculated from its gas chromatography. This approach encourages the client/supplier relationship. This occurs because the results are reported to both parts with a probability of compliance instead of a single value. The first methodology is based on the increase in the size of the sample. The second proposal is based in the calibration of the chromatograph against certificate reference materials with more exact and precise uncertainty values.