There are no published standards are requirements in regards to weight calibration frequencies for your “precision” calibration weights. “Precision” would be defined as any calibration weights having the following, or equivalent, Accuracy Classes; ASTM Class 0, ASTM Class 1, ASTM Class 2, ASTM Class 3, and ASTM Class 4.
Calibration weight manufacturers will recommend one year to six months, depending on usage of the calibration weights and criticality of the weighing process. In servicing the pharmaceutical industry for over 30 years, most of the industry seems to have followed the calibration weight manufacturer recommendations of one year and six months for their weight calibration frequencies. About seventy five percent of the weight calibrations we perform for the pharmaceutical industry are at a one year frequency, with the rest being at six months.
With that being said, unless your weighing process is part of a cGMP critical operation, or getting very heavy usage, or the calibration weights are being used for calibrating laboratory balances, especially analytical and micro balances, then a one year calibration frequency for the calibration weights should be sufficient. If the calibration weights are being used in a cGMP Pharma critical operation, or getting very heavy usage, or are being used for calibrating laboratory balances, then a six-month calibration frequency is highly recommened for the calibration weights.
The other scenario in which the weight calibration frequency would best be at six months is if an Out of Tolerance “as found” condition would be an issue for any previous measurements or daily checks that particular calibration weight may been used for. This is a common issue in the pharma industry that usually triggers investigations and a look at available historical data.
A weight calibration frequency of once every two years would be acceptable if the calibration weights are not getting used on a regular basis or the weighing application is not critical in the least. But no matter how non-critical the weighing process or how little the calibration weights are being used, a once every two year weight calibration frequency would not be recommended for calibration weights that are Class 0, Class 1, or Class 2.
For a variety of reasons, (as found out of tolerance conditions, weight handling, traceability and confidence in your weighing process) anything beyond a 24 month weight calibration frequency for precision calibration weights would not be recommended.
So in summary, a quick and dirty breakdown of calibration weight frequencies would be as follows:
cGMP critical weighing operations; the calibration weights are being used for calibration of laboratory balances; avoidance for Out of Tolerance conditions and investigations; the calibration weights are getting very heavy usage.
The calibration weights are used on a moderate basis; there is a normal level of critacality to the weighing process; the pharma industry has adopted a one year weight calibration frequency as their industry standard.
The calibration weights are not being used that often; the calibration weights are not part of critical weighing process.