A Primer on Precision Scale Calibration and Test Weights

Precision digital scales and analytical balances must be recalibrated for accuracy to be in compliance with international standards and to maintain good laboratory practices.  Analytical balances such as the Kern ABJ series can be recalibrated using internal motor-driven weights.  The internal calibration sequence is initiated manually.  Semi-micro analytical balances such as the Kern ABT series automatically recalibrate using internal weights under four scenarios:  1) Four hours after the previous calibration; 2) When there is a fluctuation in temperature of 0.5^oC (0.9^oF); 3) When the balance is switched from standby to weighing mode and condition (1) or (2) has been met or 4) If the balance was disconnected from its power source.

Other scales require external recalibration, which is performed by using test weights under procedures that should be spelled out in your company’s Quality Management manual.

Test weights themselves are manufactured to assure extreme accuracy because a balance will never be more accurate than the test weight used to adjust it.  Weight sizes used for the recalibration exercises depend on the scale or balance being used. In the case of laboratory balances and scales guidelines are generally displayed in the scales’ calibration adjustment modes or in an operations manual.

Test weights should also be recalibrated as part of ongoing GLP.  The frequency of recalibrating test weights depends on the frequency of use, the conditions of use and the security needs of your organization in terms of standards compliance.  Tovatech suggests that test weights used intensively should be recalibrated at least every six months, otherwise at least once a year.   Recalibration is done under strict laboratory conditions. Weights that pass are issued a recalibration certificate detailing the exact conditions under which the recalibration was made.

Sometimes test weights that fail recalibration can be brought back into tolerance limits by repair or remanufacture, otherwise they must be replaced.  Because these weights can represent a substantial investment, extreme care should be taken in handling and storage.  For example, acids on your hand could cause etching that could invalidate the accuracy of a 1 mg test weight – especially one with a tolerance of ± 0.003 mg.  For this reason gloves should always be used when handling test weights.  Gloves, along with tweezers and dust brushes are supplied by companies such as Kern as a further means to promote safe handling.  The weights themselves are stored in fully lined wooden boxes and milligram weights in removable plastic boxes.

Examples at the other end of the scale (so to speak) are stackable stainless steel test weights and heavy duty cast iron test weights up to 2000 kg (~4409 pounds) that likewise represent a substantial investment.  These weights must also be calibrated and certified.

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What practices does your company follow in recalibrating your scales and balances?  How and how often do you recalibrate the test weights?