Calibration and Reference Weights

Difference between Calibration and Reference Weights

Over the years some of our clients have asked us to explain the difference and definition between weight standards used in the calibration process and those used in reference checks. We have noticed some of our customers refer to their weights as calibration weights when in fact they are being used as reference weights. This has occurred during day to day activities or during an audit. This can open up a lot of questions as to what the weights are actually being used for. First an explanation of calibration weights. Requirements for weights used in the calibration process (or for that matter any equipment used in the calibration process) are defined in ANSI/ISO/IEC 17025:2005 5.5 Equipment and 5.6 Measurement Traceability. The accepted guideline for weights used in the calibration process is that the weights be submitted to a metrology lab with NVLAP Accreditation or SI Units traceable to the National Institute of Standards and Technology. The NIST Traceable number for the weights used in the calibration process is recorded on your Certificate of Calibration that you receive from our company. The reason for this is the measurements will show the true mass in a vacuum, the conventional mass in air, and the uncertainty based on a minimum of 10 repeatability measurements. Weights in what are known as analytical kits generally are taken out to a minimum of 1 microgram, or 6 places below the decimal. With weights 5 grams or less going out to a 1/10th of a microgram, or 7 places below the decimal. Weights used in the calibration process should be submitted on an annual basis to a metrology lab to be recertified. The metrology reports should be made available to any customer whose instruments are being calibrated with these weights. If a client has instruments that require a combined uncertainty report in addition to their Certificate of Calibration, the uncertainty of each specific weight used in the uncertainty test should be included in the combined uncertainty report for that specific instrument. One component of the test accuracy ratio is determined by taking the conventional mass value of a specific weight standard used in the calibration process and dividing it by the resolution of the instrument being calibrated.

Weights used as reference checks on a particular instrument don't require the same standards used in the above definition. The Weight Standard Verification Report from Analytical & Precision Balance Co., Inc shows the mean average based on 4 repeatability readings of a comparator between the client's reference weight and a Class 1 with NVLAP/NIST traceability. This shows the client's reference weights have been submitted to a neutral, non biased third party for verification. Once a client's instrument has been calibrated it must be within tolerance across its full weighing perimeter. These readings are recorded in the "After Service" line on the Certificate of Calibration issued by Analytical & Precision Balance Co., Inc. The reference check is used to validate that the instrument is within tolerance in between calibration cycles. We recommend our clients record the readings of their initial reference checks as soon as the instrument has been calibrated. We also suggest using at least two different weights. The reason for the two weights is to check for linearity and if the instrument is out of tolerance at either weight value. If so, then either the instrument is out of tolerance or one of the weights has been compromised. This serves as a backup cross check of the reference weights when combined with the Weight Standard Verification Report mentioned above. The more samples that are being weighed on a particular instrument over the course of a day, the more often reference checks should be done. A log should be kept for each weighing instrument with the time and date and reference check values. An example would be reference checks are being done on an instrument every 8 hours that it is being used and the reference checks are being done about the same time each day. Then, during one of the reference checks the instrument shows to be out of tolerance. In this case the only samples or weighings to be questioned would be those done over the previous 8 hours when the last reference check showed the instrument to be within tolerance. This along with the regularly scheduled reference checks using two weights and the reference log will show an auditor that the owner of the instrument has a logical, common sense S.O.P. with regards to their weighing instruments. We advise our customers not to calibrate their instruments using their reference weights, as this will void their ISO 17025 Certificate of Calibration with Analytical & Precision Balance Co., Inc. We also advise our customers not to refer to their reference weights as calibration weights.

The purpose of this document is to assist our customers in regards to their weighing instruments and their weight standards to ensure they have the highest quality, most accurate instruments possible and to define the difference between a weight being used in the calibration process and in reference checks. We would like to thank our clients for using our services. Feel free to call or email us if there is anything we can do to help you.

Mike Williams,
Analytical & Precision Balance Company, Inc.