The first thing that needs to be determined is what calibration weights are needed. This entails choosing the calibration weight class, weight range, and weight test points. Ideally, a weight should be more accurate than the weighing (test) instrument, though this is not always possible. Whenever possible, the calibration weight range should bracket the user range of the balance.
If an analytical balance that has a readability of 0.0001g (gram), and has a needed measurement range of 100g (grams) down to 100mg (milligrams), with a critical measurement point at 1g (gram), then the NIST Traceable calibration weights needed for this internal check would be at a minimum 100g, 1g, and 100mg. The accuracy for the calibration weights should be ASTM Class 1. ASTM Class 1 is a very precise calibration weight accuracy and the most common in domestic laboratory applications.
So, if we have those three weights incorporated into our internal check program, we have bracketed our measurement range for that particular analytical balance. In addition, with the very accurate ASTM Class 1 weights, we can feel very confident that when the weight is placed on the analytical balance and we see a very large error or discrepancy (Example: The 1g weight has a balance reading of 1.0030g) we know that there is an inaccuracy with the balance and it is time to contact the balance calibration vendor and more importantly that particular analytical balance needs to be put out of service until the inaccuracy is corrected.
The only time that ASTM Class 1 weights would not be sufficient would be in micro balance applications. Micro balances are extremely fine balances with readabilities of 0.000001g (gram) or even 0.0000001g (gram). For micro balance applications ASTM Class 0 or Ultra Class (Manufacturer Accuracy Class) weights would be needed. These are the most accurate weight classes and are better suited for the extremely fine readability of the micro balances.
The internal calibration weight balance check is not intended to duplicate the balance calibration performed by the balance calibration technicians during regularly scheduled preventative maintenances. It is a part of your quality assurance program to ensure that your weighing instruments are measuring accurately. The checks should be performed as per a standardized operating procedure (SOP) along with results being documented in the respective log book.
Also keep in mind, that the internal calibration weight checks alone are not sufficient. A regularly scheduled preventive maintenance and calibrations with a qualified vendor is still needed. And lastly, make sure your your internal calibration weights are calibrated on an appropriate interval (six months, annually, etc.) to ensure accuracy and NIST traceability.
For more info please see our “Internal Calibration Weight Verifications (Checks)” under Technical Articles in this site.