Understanding the Weight Traceable Certificate

Our Weight Traceable Certificate is a NIST Traceable report documenting weight calibration results.

The Weight Traceable Certificate documents the testing environmental data (Temperature, Pressure, Relative Humidity and the Air Density) taken at the time of the weight calibration. The air density is used to apply and adjust for the required air buoyancy corrections for the weight calibration results. In addition to the environmental conditions the weight certificate also contains the descriptive and identification info (S/N, ID No., Range, Accuracy Class) for the respective calibration weights under test.

Decoding the Information on Your Weight Calibration Certificates

Both the as found and as left values, for all the calibration weights that were calibrated, are documented on the weight certificate. The weight calibration values documented on the certificate are conventional mass values. The conventional mass values could also be called “mass in air”. These weight calibration values are relevant when using the values in measurements with balances. The conventional mass values should not be referenced when using the calibration weights in a vacuum chamber or when “true mass” values are required.

Also documented on the weight certificates are uncertainty values. These values are the uncertainty of measurement values for both the as found and as left weight calibration values. For example, if a 100g Class 1 weight has an as found value of 0.050mg and an uncertainty value of 0.034mg, then the weight certificate is stating that the value of that weight is 0.050mg +/- 0.034mg. The uncertainty values are calculated at a 95% confidence interval using a coverage factor of k=2.

Lastly, the stated tolerance and the as found disposition columns are contained in the weight traceable certificate. The stated allowable tolerance for the calibration weights is based on the fixed accuracy class values, which are referenced from national or international standards. The most recognized domestic calibration weight standard would be ASTM E 617-97 which uses a numerical accuracy system. The OIML R 111-1 calibration weight classification standard would be most recognized internationally. The OIML standard has an alpha-numeric accuracy breakdown.

The as found disposition column is strictly a “pass/fail” disposition for that calibration weight for the as found value only. If the resulting as found value of the calibration weight is outside of the allowable tolerance for that respective weight class then the as found disposition would state “Out of Tolerance”. If the resulting calibration weight value is within or at the allowable tolerance for the respective weight class then the as found disposition would state “In Tolerance”. Again, this would be for as found calibration weight values only.

It is also of note that a calibration weight will not be certified to a respective accuracy class if the resulting weight calibration value when added to the uncertainty of measurement value brings the calibration weight out of the allowable tolerance range. For example, if the resulting weight calibration value for a 500mg ASTM Class 1 calibration weight is -0.0089mg and the uncertainty of measurement of that weight calibration is 0.0016mg, then the weight calibration value added to the uncertainty of measurement would bring the potential calibration weight value to -0.0105mg which is outside of the allowable tolerance range of 0.010mg. The calibration weight would then have to be removed from service or downgraded to ASTM Class 2.

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