Reference Center – Weights FAQS

For a proper calibration, weights need adequate time to establish and maintain a thermal equilibrium with the laboratory before any measurement can be done. One-piece weights take the longest time to calibrate - adjustments are made on a polishing wheel that generates heat and requires 4–8 hours to reestablish thermal equilibrium before any calibration/tolerance checks on a balance. A quality calibration can not just be performed in a matter of days and if someone tells you otherwise then they simply are not following good measurement practices and the quality of the calibration will be drastically inferior. In fact, it is our supposition that if this occurs you are not getting the service that you paid for and you are simply paying for a calibration sticker.

Check for contaminants, scratches, and discolorations. Be sure to look on the bottom of the weight, since occasionally matter attaches itself to the bottom and it may go unnoticed.

The weight error is the measured value correction plus its associated uncertainty (please refer to a Troemner NIST/NVLAP Weight Calibration Certificate for an example). You can use the weight to determine the balance error, but you can never use the balance to determine the weight error. This means that if you want to determine the weight error, you must have a mass standard with a certificate that enumerates the value correction plus its associated uncertainty or you must have another certified weight to make a comparison. If you want to determine the balance error, it is relatively easy if you have a certified mass standard with the aforementioned data. With this information you should know how the weight performs plus or minus its associated uncertainty value. Therefore, the balance error can be deduced by performing multiple measurements to see how well the balance executes a measurement. This, of course, can be recorded and tracked over time, so that a balance uncertainty can be ascertained as well.

It is recommended that if you use your weights infrequently to moderately, you should have them recalibrated once a year. However, if your weights are used very frequently or several times a day, then you should have them calibrated once every six months. Although there are no standards or regulations that specifically address recalibration intervals, this is the industry norm in order to ensure that these standards are still within their specified tolerances. It is not unusual to see significant “as found” failure rates on those weights that are not recalibrated on an annual basis.

“Accreditation” is a formal recognition by an authoritative entity that an organization or individual is competent to carry out specific tasks. In the field of metrology, a third party such as NVLAP or A2LA, totally unaffiliated with the organization in question, ensures the calibration is being performed correctly by technically competent individuals. Accreditation translates to a cost savings in minimizing or eliminating the need for onsite visits by the customer. If your calibration service provider is not accredited, you are taking a risk with your quality control program.

The only real difference between these two organizations is that NVLAP is affiliated with the US National Institute of Standards and Technology (NIST) while A2LA resides in the private sector, however, both agencies audit to ISO/IEC 17025 (formally ISO Guide 25). Specific criteria that is covered by both organizations during an assessment include: equipment and environment, methods and procedures, validation of uncertainties, quality system and reviews, record keeping complaints, measurement, traceability, organization and staffing.