When using laboratory balance calibration weights, also referred to as mass standards, for laboratory or calibration applications, the proper care, handling, and usage are extremely critical in maintaining the traceability and accuracy of the weights. It is essential that the weight maintain its accuracy, especially if they are ASTM Class 0, ASTM Class 1, Ultra Class, or OIML Class E2 weights. In addition for critical weighing applications and balance calibrations, you need to maintain the weight calibration certificate value from the last calibration for that weight. If a calibration weight is accidently dropped, the calibration weight essentially is useless to the user until the weight can be calibrated again. So maintaining the calibration weight’s value and compliance to the respective tolerance for the duration of the calibration frequency is essential.
CARE AND HANDLING
It all starts with the handling of the weights. Precision calibration weights are stainless steel or sometimes aluminum for very small milligram weights (20mg and below). Very good tweezers, preferably stainless steel with fine carbon tips, are best for weights 100g down to 1mg. This will help prevent the calibration weight being dropped, damaged during handling, or even lost. For the larger calibration weights, 200g and up, a clean cotton glove, that is changed as needed, works best.
Never handle the weights with bare hands, abrasive equipment, or a glove that could be dirty or contaminated. The oils from the bare hand or other contaminents will cause a weight to change its value and if it is an Ultra Class, ASTM Class 0, ASTM Class 1, or OIML Class E2 weight might even cause the weight to go out of tolerance.
Abrasive handling equipment will scratch and damage the weight causing the weight to lose mass and have its weight calibration certificate value altered and eventually go out of tolerance.
The calibration weight should be handled and placed on the balance with great care. Most of out of tolerance conditions on calibration weights are caused by wear and tear occurred during regular use. The weight class tolerances on ASTM Class 1, ASTM Class 0, Ultra Class, and OIML E2 weights are extremely fine (and can go down to 0.010 mg, 0.006mg and 0.005mg respectively) and a continuously used weight can easily go out of tolerance. A calibration weight should be handled and placed on the balance platform as carefully as possible. This technique will help reduce wear and tare and out of tolerance conditions. Again, having stainless steel fine carbon tip tweezers will also help in reducing wear and tear.
Cleaning weights is usually not something that is done other than by the weight calibration vendor. But I do recommend, for the very precise and smaller calibration weights (10g down thru 1mg), if not a periodic cleaning then at the least a camel hair brush to dust the weight off each time prior to use on the balance. There is almost always a build up of dust and debris (either from the case itself or the balance and or environment). The build up can become considerable, changing the weights value and causing an out of tolerance condition for the precision calibration weights. Also, if a weight becomes contaminated (testing material, fingerprints, etc.) it could probably be cleaned without having to go back to the weight calibration company. When a cleaning is needed and desired, again just for the 100g calibration weights and smaller, the calibration may be easily cleaned with a very fine cheesecloth and ethyl alcohol. Great care must obviously be taken so as no to damage or alter the weight during cleaning, but a very light cleaning with the ethyl alcohol should do the trick and bring the weight back to its weight calibration certificate value.