Raising the Standard: Legal-for-Trade Belt Weighing in North America By: CST (Control Systems Technology) with original article by the late Lorne Danielson, TD Micronic
· Accuracy ·Flowrates for Certification ·Long-Term Stability ·Quantity of Test Material
These reforms are shaping belt weighing into a trusted method for regulated transactions in ports, terminals, and mineral operations where measurement disputes can have large-scale financial and operational consequences. Accuracy Modern belt scale standards now support certified accuracy levels that were previously unachievable with dynamic systems. The OIML offers Classes 0.2, 0.5, 1.0 and 2.0, where each class represents double the verification error (+/-0.1% for Class 0.2). NTEP standards in the U.S. have similarly advanced, enabling certification at 0.1%, a threshold that had been considered the upper limit of belt scale precision. In Canada, Measurement Canada allows legal-for-trade approval at either ±0.5% or ±0.1%, depending on the material’s value. Since 2006, scales trading in high-value bulk commodities must meet the stricter 0.1% requirement. This shift in standards represents a significant step toward commercial belt weighing systems achieving the same level of trust as static scales. Flowrates for Certification Historically, belt scale certification tests were conducted at a single, controlled flowrate. While this simplified the process, it failed to reflect real-world variability and often resulted in discrepancies under normal conditions. Today, both NTEP and Measurement Canada require testing at multiple flowrates, typically low (20–30%), nominal (60%), and high (100%), to demonstrate consistent accuracy across operating conditions. This multi-point verification aligns with OIML standards and ensures that scales perform reliably, even when flowrates deviate from the norm. This is crucial in ensuring operational accuracy day-to-day and not just in test environments. Long-Term Stability Conveyor belts experience wear, stretch, and environmental exposure that can shift calibration over time. Unlike static scales, these changes are more complex and harder to detect without regular testing. NTEP addresses this with a mandatory permanence test conducted 6 to 12 months post- certification to confirm performance has not drifted. Additionally, annual recertification is required for all legal-for-trade belt scales under NTEP.
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