In the pharma industry there has always been 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 pharma industry, not just for calibration weights but for all instruments. In regards to specifically calibration weights used for internal balance checks, the impact of the OOT “As Found” results on the weight calibration certificate in most cases is surprisingly negligible.
There first thing that needs to be assessed is how far out of tolerance was the calibration weight and how does that value compare to the readability of the balance it was being used on. In most cases the tolerance for the calibration weights are very tight, Class 1, Class 0, Ultra Class, etc. So in many cases the OOT condition or amount that the calibration weight is out of tolerance would not be detected or measured on a particular balance. For example, if we have a 100mg calibration weight that has a Class 1 tolerance of 0.010mg and was found to have an OOT value of -0.014mg from the weight calibration certificate. Then we know that the calibration weight was -0.004mg past the allowable Class 1 tolerance. If we were using this calibration weight on a Mettler Toledo XS205DU analytical balance, with a readability of 0.01mg (or 0.00001g) at the 100mg range, then this -0.004mg value can not be measured or displayed. So in this case, there would be no impact of the OOT calibration weight. The calibration weight would most likely be downgraded to Class 2 or would need to be replaced to 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. A 0.1% of the applied calibration weight is a very popular tolerance in the pharma industry. Lets use the same data from the previous 100mg calibration weight example of the -0.014mg value from the weight calibration certificate but this time we will say the calibration weight is a Class 0 calibration weight with a tolerance of 0.005mg and the balance the weight is being used with is a Mettler Toledo XP6 micro balance with a readability of 0.001mg (or 0.000001g). The 100mg calibration weight was -0.009mg out of the Class 0 tolerance. This -0.009mg 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 (0.1% of the calibration weight), which in this case would be 0.100mg. The 100mg calibration weight’s OOT value of -0.009mg is well within the 0.100mg of the internal daily balance check, so the impact of the OOT calibration weight would be negligible.
The above two examples are that of metrological or scientific in nature, where the data is used to help determine the impact. It is also important to remember that the accuracy of the balance is not affected by the OOT calibration weights used in the respective internal balance check. The calibrations 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, this does not mean that the accuracy of any previous calibrations or measurements performed on the balance should be deemed inaccurate or questionable.
The one scenario in which the impact of the OOT calibration weights would be of concern is when the calibration weights have been 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 of the balance by following the calibration steps for that balance with the specified calibration weights. This is usually performed by an experienced balance calibration service technician.