Precise weighing processes reduce rework and raw material loss, boosting profitability. Automated weighing systems also eliminate safety risks by handling and transferring masses in closed environments.
Accurate weighing requires good laboratory techniques and practices. It also relies on having a consistent weighing process. This is important for achieving repeatable results and meeting product quality standards.
Accuracy
Whether the process is weighing raw materials on a receiving dock, filling and labelling finished goods’ boxes for inventory, or checking product consistency on an assembly line, the weighing process requires a precise measurement. Any errors or omissions in this critical step can result in costly mistakes that can be difficult to correct.
Fortunately, there are several steps you can take to ensure that your process is accurate. Using an OIML or NTEP approved balance that has been properly installed and calibrated is one important first step.
Moisture can wick into the junction box and interfere with the signal lines to the load cells, decreasing accuracy. For this reason, it is best to use a waterproof NEMA 4-rated junction box and plug unused junction box holes. Finally, it is best to keep your weighing system in its rated temperature range.
Errors
Weighing errors often occur as the result of many factors that are difficult to control. This makes it important to use a dedicated sample management or LIMS software that provides a streamlined testing regime that meets your scientific requirements without taking up your whole working week.
Human errors are common and can happen despite the best efforts of alert and careful operators. Fingerprints on the rim of the container, oily fingers or residue on a lid can all add mass and alter readings.
Temperature changes also contribute to weighing errors. This is why it is critical that the balance, reference standards and test objects are in thermal and environmental equilibrium before the mass calibration starts. This is why each mass SOP includes instructions that the weights and balance should be allowed to equilibrate for 24 hours before starting the weighing.
Safety
Using accurate weighing methods, companies can reduce their exposure to risk, ensuring product quality, operational efficiency, and regulatory compliance. This involves choosing precision instruments, establishing standard operating protocols, and regular calibration and maintenance schedules.
Pre-weighing ingredients or components prevents over- or under-estimating the quantity needed. It also helps ensure that all ingredients are added in the correct order and proportion, preserving the overall quality of the end-product. In manufacturing, this is particularly critical for the pharmaceutical and food sectors where precise ingredient weighing can save time, money, resources, or even lives.
The weighing system must be capable of supporting the combined weight of both the container and the measured sample to avoid damage and ensure accuracy. Additionally, the weighing system should be inspected and calibrated at regular intervals to account for environmental changes that can affect measurement uncertainty and minimum weight. The weighing equipment should be placed in an area with minimal vibration to prevent interference with the load cells’ output signal.
Repeatability
Generally speaking, repeatability is the ability of a balance to return the same result for the same object under the same conditions. This differs from accuracy, which is the weighing instrument’s ability to measure an object within its permissible error limits (see Figure 1).
In many applications it is important for the repeatability of a weighing process to be close to zero. However, this depends on the application and the user’s weighing needs.
A balance should be exercised before taking readings to increase the repeatability of its measurements. This is done by placing a weight equal to the load that is to be measured and then arresting or removing it from the pan. This allows the balance to “warm up” and eliminates some of the larger variations present in initial weighings. Good Weighing Practice recommends that all balances be exercised ten times before performing routine testing. The four significant contributors to measurement uncertainty are identified by the GWP standards as: (U_tot is total measurement uncertainty; U_RP is repeatability; U_EC is eccentricity; and U_NL is nonlinearity). Each of these contributes to a different part of the overall tolerance limit.