Weighing scale instruments are used in a variety of industries for numerous measurements. There are tons of weighing instruments, from small laboratory ones that measure a few grams to industrial weighing—as with any device, weighing instruments need to be calibrated regularly to ensure that they measure correctly and accurately. A proper metrologically traceable calibration is the only way to know how accurately weighing instruments are measuring.
Before calibrating a weighing instrument, some considerations need to be considered. You will want to know the technical characteristics of the instrument (max weight, d value), the accuracy requirement (max error allowed and uncertainty), and what to do if the calibration fails (adjustment).
When the instrument is being tested on-site, you will need to turn it on before calibration to allow the temperature of the weights to stabilize to the same temperature where the calibration is to be done.
Do a few pretests to ensure that it is usually working. Suppose the weighing instrument fails in calibration and is adjusted. In that case, you should make an “as found” calibration before adjustment and an “as left” calibration after adjustment.
Different Calibration Tests
During calibration several tests should be done during calibrations.
- Eccentricity test
- Repeatability test
- Weighing test
- Minimum weight test
When using a weighing instrument, the load is not always placed perfectly in the center. The results of a weighing instrument can vary slightly depending on if the load is placed in different locations on the receptor. The eccentricity test is performed to test how much effect the site of the load has.
The test has the reference load placed on the load receptor in a few specified locations. The calibration procedure should establish where to place the load during the test, and calibration results should document the location. The objective of the eccentricity test is to find out the difference caused by the site of the load.
All instruments, including weighing, may suffer from repeatability issues. When the same load is measured several times, the result is not always precisely the same. A repeatability test is done to determine the instrument’s repeatability.
The repeatability test is performed by replacing the same load on the load receptor multiple times in the same place. The test should be done in identical and constant conditions and with similar handling.
The load should be close to the maximum load of the instrument and a single load. The repeatability test usually is done by repeating the measurement at least five times in a row. It should be done at least three times for instruments with a high range.
The weighing test aims to test the accuracy (calibration) of the weighing instrument throughout its whole range in several steps, with increasing and decreasing weight. Start with zeroing the device without any load. Set the limitations of the first test point, wait for stabilization, and record the indication. Continue increasing the loads through all the increasing test points. With multi-range instruments, each range is to be calibrated separately.
Minimum Weight Test
A minimum weight test is a test that is required within some industries, like the pharmaceutical industry. The minimum weight test aims to find the minor load that can be measured while still achieving reliable measurement results and fulfilling the accuracy requirements. When the measured value gets smaller, typically, the relative error of the reading becomes higher. The weighing instrument should not be used to measure any loads more minor than the minimum load.
Other Tests and Calibration
Some other tests are not done during standard calibration but can be done as a type of approval or initial verification. Some of these tests are a tare test, discrimination test, variation of indication over time, and test of magnetic interaction. If you are still trying to decide what requirements you need for your lab or are overwhelmed with doing it yourself, we can help. SRP control systems can help you care for your equipment as we are an ISO 17025 accredited laboratory that can calibrate your equipment to meet your needs. There will be no surprises, and your instruments and equipment will be accurate and compliant.