Scale Calibration Tolerances

Why Tolerance Matters More Than the Reading

A scale calibration doesn't just verify that the scale reads correctly — it verifies that the scale reads correctly within the tolerance the applicable standard requires. A reading that's off by 0.1% may be within tolerance for a Class III truck scale at mid-range capacity and out of tolerance for a laboratory analytical balance at low load. Understanding which tolerance applies to your equipment is the foundation of interpreting calibration results correctly.

NIST Handbook 44 Tolerance Classes for Commercial Scales

NIST Handbook 44 establishes tolerance classes for scales used in commercial weighing transactions. The most commonly encountered classes in industrial Michigan applications: Class III (general commercial weighing — floor scales, truck scales, bench scales in receiving and shipping): Acceptance tolerance is 1 scale interval at loads from zero to 50% of maximum capacity, and 1.5 scale intervals from 50% to maximum capacity.

Maintenance tolerance (the wider tolerance applied to scales already in commercial use) is 1.5 scale intervals from zero to 50% and 2 scale intervals from 50% to maximum. For a floor scale with a 10-lb graduation, the maintenance tolerance at mid-range is ±15 lbs.

Class IIIL (vehicle and axle-load weighing — truck scales, vehicle scales): Uses the same framework as Class III but with tolerances expressed relative to the number of scale divisions and adjusted for the higher capacities involved. A 100,000-lb truck scale with 20-lb graduations has maintenance tolerances of 30 lbs at mid-range. Class II (precision commercial weighing — some grain and precious metals applications): Tighter tolerances,typically 0.5 scale intervals acceptance and 1 scale interval maintenance.

USP Tolerances for Pharmaceutical Laboratory Balances

USP General Chapter <41> (Balances) establishes that a balance used in compendial weighing procedures must meet two performance requirements: repeatability — the standard deviation of at least 10 repeated weighings of a test load must not exceed 0.10% of the test load; and eccentricity — the difference between measurements when a test load is placed at different positions on the pan must not exceed 0.10% of the test load. These are minimum requirements. Many pharmaceutical QC applications require tighter performance than <41> specifies, particularly for low-mass weighing where minimum weight constraints govern.

GMP and Customer-Specified Tolerances

Where neither HB44 nor USP specifies the applicable tolerance, the tolerance is set by the quality management system, the customer, or the process requirement. A batching system addition with a formula tolerance of ±0.5% requires a scale whose calibration demonstrates performance within that tolerance across the addition weight range— which determines the resolution and accuracy class of the instrument needed for the application.

Q: What happens if my scale fails tolerance at calibration?
A scale found out of tolerance at calibration is adjusted and returned to within-tolerance performance (as-left). For commercial scales, the as-found out-of-tolerance condition may trigger an OOT investigation to assess whether transactions or records produced during the affected period need review. For Legal-for-Trade scales, asignificantly out-of-tolerance as-found condition should be reported to MDARD weights and measures for assessment of whether the scale requires re-inspection.

Q: Can I set my own tolerance tighter than what HB44 requires?
Yes. HB44 tolerances are legal minimums for commercial weighing compliance — your quality management system or process requirements can specify tighter tolerances for internal calibration purposes. Cech calibrates to whatever tolerance your quality system specifies, documenting pass/fail against that standard alongside the HB44 reference.