Weighing scale calibration - How to calibrate weighing instruments
Weighing scales and instruments are widely used in the industry for various measurements, including small laboratory instruments measuring a few grams and large industrial instruments measuring the mass of trucks. These instruments should be calibrated regularly to ensure they are measuring correctly and accurately. A proper metrologically traceable calibration is the only way to know how accurately weighing instruments are measuring.
Many weighing instruments are used for legal measurements or measurements used as the basis for monetary transfer, and these are part of a legal or statutory verification program based on legislation. Often, calibration of weighing instruments is based on a quality system (such as ISO9000), health care, traffic (air, marine) safety, or forensic investigation. There are dedicated regulations for weighing instruments and their calibration (EURAMET Calibration Guide, NIST Handbook 44, OIML).
The main focus of this article is to look at the practical considerations and the different tests you should perform when calibrating your weighing instruments. Preparations before calibration include understanding the technical characteristics of the weighing instrument, the accuracy requirement, and what to do if the calibration fails (adjustment). The calibration process typically involves the whole measurement range being calibrated and the calibration performed in the location where the instrument is being used.
The calibration procedure should specify where to place the load during the test and calibration results (in certificate format) should also document the locations. The test load used in an eccentricity test should be at least one-third (1/3) of the max load of the weighing instrument, preferably using just one test load, if possible. For a weighing instrument with multiple ranges, the eccentricity test should be done with the highest range.
The eccentricity test is important to determine the difference caused by the location of the load, but it is not necessary to have an accurate calibrated load. If the eccentricity test is used to determine the errors of the indication, a calibrated load should be used.
In conclusion, weighing instruments are essential tools for various applications, and regular calibration is crucial to ensure their accuracy and reliability. By following the guidelines provided by the European Institute of Standards and Technology (EISM), users can ensure the proper functioning and accuracy of their weighing instruments.
The eccentricity test involves zeroing the indication before the test, moving the test load to locations 1-5, and recording the indication in each location. The zero may be checked between locations to ensure no changes have occurred. If necessary, the instrument can be zeroed between each test. The instrument can be tared when the load is in location 1, making it easier to see any differences between locations.
A repeatability test is performed to determine the instrument's repeatability. This test involves replacing the same load on the same place on the load receptor multiple times, in identical and constant conditions and with identical handling. The load used should be close to the maximum load of the instrument, and the test should be repeated at least 5 times in a row. For instruments with a high range (over 100 kg/ 220 lbs), it should be repeated at least 3 times.
In the repeatability test, the instrument is zeroed, the load is placed on the load receptor, and indication is recorded after stabilization. The load is removed, and zero indication is checked.