Accuracy and Calibration Failures in Science and Engineering

3Technical Root Causes Behind Calibration Failures

Abstract

The shift in particle count calibration standards resulted from inaccuracies and instability in SRM 2806, driven by variable particle shape and refractive index in Medium Test Dust (MTD). Outdated calibration techniques lacked SI traceability, causing imprecise size thresholds. Advances in measurement technology, like SEM, revealed OPC misclassifications. NIST’s recharacterization led to SRM 2806a, with improved test dust stability and validated particle sizes. Poor communication hindered SRM 2806a’s adoption, with labs using legacy curves, delaying detection of the shift. The change corrected technological and material inconsistencies, exposing flaws in historical data.

Keywords: ISO medium test dust (MTD), optical particle counters (OPCs), calibration techniques, measurement uncertainty, particle geometry, batch-to-batch consistency, SI-traceable standards

The underlying cause of the shift in particle count calibration standards was the inaccuracy and instability of the original SRM 2806 due to several interrelated factors:

Limitations of the Original ISO Medium Test Dust (MTD)

• MTD used in SRM 2806 had variable particle shape and refractive index, which affected light scattering in optical particle counters (OPCs).
• Particle sizing relies on optical response, which can vary based on particle geometry, not just size.
• Over time, batches of MTD differed, introducing lot-to-lot inconsistency.

The root cause behind ...