The Impact of OOT Calibration Weights

In the pharmaceutical industry there has always been a great concern on the impact of out of tolerance calibration weights (OOT’s).  “As Found” OOT weight calibration results have always set off alarms and initiated investigations in the industry, not just for calibration weights but for all instruments.  In regards to calibration weights used specifically for internal balance checks, the impact of the OOT “As Found” results from the weight calibration certificate in many cases is surprisingly negligible.

Specific Issues with Out of Tolerance Calibration Weights

The first thing that needs to be determined is how far out of tolerance was the calibration weight and how does that value compare to the readability of the balance that respective weight was being used on.  In most cases the tolerance for the calibration weights are very tight, ASTM Class 1, ASTM Class 0, Ultra Class, etc.  So in many cases the OOT impact or the amount that the calibration weight is out of tolerance would not be detected or seen on a particular balance.  For example, lets say we have a 100 mg calibration weight that has a Class 1 tolerance of 0.010 mg and was found to have a value of -0.014 mg on the weight calibration certificate.  We then know that the calibration weight was -0.004 mg past the allowable Class 1 tolerance.  If we were using this calibration weight on an analytical balance, with a readability of 0.01 mg (or 0.00001 g)  at the 100 mg test point, then the -0.004 mg value can not be measured or displayed.  So in this case, there would be no impact for the OOT condition.  The calibration weight would need to be either downgraded to ASTM Class 2 or replaced with a new Class 1 calibration weight.

The second issue that would need to be addressed in assessing the impact of OOT calibration weights is what is the internal balance check tolerance being applied by the end-user.  A 0.10% of the applied calibration weight is a very popular tolerance in the pharmaceutical industry.  Lets use the same data from the previous 100 mg calibration weight example with the -0.014 mg value from the weight calibration certificate, but this time we will say the calibration weight is a ASTM Class 0 calibration weight with a tolerance of 0.005 mg and the balance the weight is being used on is a micro balance with a readability of 0.001 mg (or 0.000001 g). The 100 mg calibration weight is now -0.009 mg out of the ASTM Class 0 tolerance.  This -0.009 mg value can be measured and displayed on the micro balance, so this value now needs to be compared to the applied internal balance check tolerance (0.10% of the calibration weight), which in this case would be 0.100 mg.  The 100 mg calibration weight’s OOT value of -0.009 mg is well within the 0.100 mg of the internal daily balance check, so again, the impact of the OOT calibration weight would be negligible.

The above two examples are metrological or scientific in nature, where the data is available and can be used to help determine the impact.  But what if the amount that a weight is OOT can be both be displayed on the respective balance and is also past the allowable internal balance check?  In this case we must keep in mind that no matter how out of tolerance a weight is, as long as the weight is only being used for internal balance checks, the accuracy of the balance is not affected by this respective OOT condition.  The calibration weights are used as independent checks and have no effect on the accuracy of the balance.  So, just because calibration weights were found to be OOT from the weight calibration certificate results does not necessarily mean that the accuracy of any previous calibrations or measurements performed on the balance should be deemed inaccurate.  But a thorough investigation would be required to ensure that the balance has been calibrated and was weighing accurately.   The investigation at a minimum should consist of a review of the most recent calibration by the balance vendor and a review of any additional internal weight checks (different weights and or different test points) performed on that particular balance.

The one sure scenario in which the impact of the OOT calibration weights would be of significant concern is when the calibration weight was used to “calibrate” the balance.  “Calibrate” the balance would be defined as going into the balance’s menu program, as per the operating instructions, and adjusting the accuracy and linearity of the balance by following the calibration steps for that balance with the specified calibration weights. This should be performed by an experienced balance calibration service technician.

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