Many manufacturing standards are not explicit about scale calibration and testing requirements. This article dispels some common myths about scale performance and explains how calibration and testing based on risk assessment support compliance and quality.
Weighing is often the first step in a chain of quality-control activities at a chemical process industries (CPI) plant. Accurate weighing is critical to overall product quality, particularly when batch uniformity is required. Furthermore, accurate weighing helps to address the most demanding set of often-contradictory challenges facing the CPI today: improving consumer safety while increasing productivity and lowering costs to remain competitive.
The complexity of weighing, however, is often underestimated. The accuracy of the weighing instrument is influenced by many factors, such as mechanical abuse, damage, wind turbulence, frequent temperature changes, vibration, cleaning agents, water, accumulated debris, electrical disturbances, and corrosion. Such factors and the resulting inaccuracies can put productivity and profitability at risk. Raw materials, personnel, and assets are wasted when poor-quality products need to be reworked or disposed of. In extreme cases, errors can trigger tedious, costly, and image-damaging recall actions that negatively impact the company brand.
These factors can be difficult to mitigate. However, regular maintenance combined with weighing-instrument calibration and testing can help to ensure that inaccuracies caused by poorly performing scales do not damage product quality. Inspection, routine in-house testing, and calibration that take business risks into account will improve quality and reduce costs associated with out-of-specification (OOS) measurement errors.
Measurement uncertainty and calibration
All measurements (including weight) have some degree of measurement uncertainty. A measurement is not complete unless it includes the associated uncertainty. For example, if an instrument with a 0.01-g calculated measurement uncertainty shows a reading of 100 g, the weight should be reported as 100.00 ± 0.01 g — which indicates that we can be “reasonably certain” the true weight lies somewhere between 99.99 g and 100.01 g.
Measurement uncertainty is determined through calibration, which is performed by accredited service professionals. It is distinct from routine in-house testing, which assesses the device’s performance between calibrations (Table 1).
| Table 1. Routine testing and calibration help ensure weighing equipment is reliable and productive. | ||
| Service Event | Goals | Frequency |
| Routine Testing (in-house) | Evaluate a device’s performance between calibrations based on tolerances | Based on process tolerance and risk |
| Calibration | Determine a device’s measurement uncertainty | Based on process tolerance and risk |
| Verification | Determine a device’s fitness for purpose based on measurement uncertainty | Based on process tolerance and risk |
| Calibration with Adjustment | Adjust a device to meet its manufacturer’s specifications when necessary | Based on device’s pre-calibration status |
Would you like to access the complete CEP Article?
No problem. You just have to complete the following steps.
You have completed 0 of 2 steps.
-
Log in
You must be logged in to view this content. Log in now.
-
AIChE Membership
You must be an AIChE member to view this article. Join now.
Copyright Permissions
Would you like to reuse content from CEP Magazine? It’s easy to request permission to reuse content. Simply click here to connect instantly to licensing services, where you can choose from a list of options regarding how you would like to reuse the desired content and complete the transaction.