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4 - Accuracy and Weighing Performance
Lightning
Passive and Active Systems Lightning is one of the largest risks for a truck scale owner. If a scale is damaged by a lightning strike, the cost of repairs and downtime can be extensive. Most truck scale providers offer weighing systems with passive grounding features, which provide the lowest level of protection. METTLER TOLEDO provides unique active grounding systems that can re-route the current of a lightning strike away from major components, protecting your investment. Lightning Protection Validation The best way to validate a truck scale’s lightning protection system is at a lightning laboratory. Lightning labs often are used to test critical aircraft components by shocking them with the same voltage and amperage levels seen in lightning strikes. These labs essentially fabricate lightning power surges and release the current directly to the truck scale electrical system. For example, METTLER TOLEDO has been able to validate the effectiveness of the StrikeShield TM protection on POWERCELL ® load cells and scale terminals at such a facility. There, components were hit with 80,000 amperes – twice the current of a typical lightning strike – and protective systems behaved as designed.
There are many reasons a business owner should care about accuracy. Among them: • Accuracy affects your business. • A conventional truck scale may not be as accurate as you think. • Legal-for-trade scale error tolerances don’t protect you from profit losses. • Not all scales offer the same level of accuracy.
All truck scales are accurate the day they are calibrated and certified by local Weights and Measures officials, but it is critical to the customer’s bulk material operation that the scale maintains accuracy over time. For a better understanding of the expected accuracy performance of a truck scale, let us first review the Weights and Measures legal tolerances. In commercial or legal-for-trade applications, scales must be regularly inspected, tested, and recertified by Weights and Measures authorities. Many scale owners (and even scale providers) assume that these recerti- fications will provide them with sufficient accuracy. In reality, the error tolerances for minimum requirements can be significant. That means even a recently certified scale could still actually be losing money for the owner with every weighment. The size of the legal accuracy tolerance for your scale depends on your location and whether your region recognizes NIST/NTEP or OIML standards. The tolerance is a function of the percentage of the scale’s capacity being utilized and the size of the scale’s weighing increment. Here, an important distinction to note is the capacity of the scale is the maximum weight value for which the terminal and load cells are configured, not the physical maximum capacity of the structure.
NTEP (United States standard) 1 scale division = 20 lb Scale capacity (lb) Truck weight (lb)
Allowable error (lb)
200 000
40,000 – 50,000 50,000 – 60,000 60,000 – 70,000 70,000 – 80,000
100 120 140 160
NIST/NTEP Accuracy Tolerance Handbook 44 outlines a step-based tolerance that increases with every additional 5% of the scale’s capacity being utilized.
OIML (Global standard) Using C3 load cells Scale capacity (kg)
Truck weight (kg) 10,000 – 40,000 40,000 – 60,000 10,000 – 40,000 40,000 – 60,000 60,000 – 80,000
Allowable error (kg)
60 000
40 60 40 60
80 000 (multi-range)
100
OIML Accuracy Tolerance There are various accuracy specification levels defined by OIML: C3, C4, and C6 are seen in truck scales. Most truck scale applications are C3 (used in the chart above). A very small percentage of truck scales are C6, which provides a stricter accuracy tolerance.
Video: Lightning Protection Tests See a video on how METTLER TOLEDO tests
lightning protection systems at www.mt.com/LightningTest
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