Metrological Tolerance
A calculated statistical value defines the range within which the true value of a chemical concentration is asserted to lie at a specified confidence level. During compliance testing for circuit board assemblies, expanded measurement uncertainty accounts for all cumulative errors in sample preparation, extraction, weighing, and instrument drift. This figure is expressed as a margin of error that is added to or subtracted from the reported analytical result.
Statistical Calculation
Lab operators calculate this value by multiplying the combined standard uncertainty of the method by a coverage factor, which is usually set to two to achieve a ninety-five percent confidence interval. The calculation incorporates variances from the balance used to weigh the PCB material and the repeatability of the injection system. For instance, if a gas chromatography run measures a phthalate concentration of nine hundred milligrams per kilogram with an expanded measurement uncertainty of one hundred and fifty milligrams per kilogram, the true concentration could exceed the restricted limit of one thousand milligrams per kilogram.
This possibility of exceeding the limit requires the testing lab to report the result as inconclusive unless the measured value is sufficiently below the threshold.
Decision Threshold
The application of this uncertainty calculation is mandatory for all accredited laboratories certifying materials under the RoHS directive. It stops applying once a rapid, qualitative screening technique is used solely to confirm the complete absence of a substance, where precise quantification is not required for decision-making.
