Suitability of Used Impactors – Why the ‘Effective Diameter’ Criterion Needs the ‘Jet-to-Plate’ Criterion to Assure Measurement Capability in a Good Cascade Impactor Practice (GCIP) Environment
Daryl L. Roberts1, & Jolyon P. Mitchell2
1Applied Particle Principles, LLC, 9194 Edison Street, Blaine, Minnesota, USA
2Jolyon Mitchell Inhaler Consulting Services Inc., 1154 St. Anthony Road, London, Ontario, N6H 2R1, Canada
We review conditions for asserting that used multi-stage cascade impactors (CIs) are performing aerodynamically in the range specified by the manufacturer for new apparatuses. This “aerodynamic similarity” concept is the fundamental basis for assessing the suitability of used CIs because impactors function largely on the basis of known aerodynamic principles. This criterion includes not only an average nozzle diameter (effective diameter or Deff) but also the perpendicular distance from the nozzle exit plane to the plane of the collection surface, normally denoted as S, scaled to the individual nozzle diameters (normally denoted as W). Typically users ignore this S/W ‘jet-to-plate’ ratio and compute only the effective nozzle diameter from optical stage mensuration data. By failing to account for this necessary aspect of aerodynamic similarity, users open themselves to the possibility that the S/W ratio exceeds the manufacturer’s specification and additionally allows the possibility of concluding that clearly absurd nozzle size distributions are within specifications. Such a result would potentially invite regulator scepticism about the validity of Deff. We explain this more complete approach to assessing CI measurement capability by considering several conditions encountered by inhaler testing practitioners. These conditions range from new, to commonly encountered ‘slightly imperfect used,’ to ‘absurd’ impactor stages. We show that the S/W specification is a technically essential component of Good Cascade Impactor Practices (GCIP). The assessment of the S/W ratio is simple and recommended as part of periodic, possibly annual, impactor stage mensuration.