Both European and US Pharmacopoeias lay down certain criteria which the cascade impaction system and test criteria selected for the inhaler must fulfil prior to and during use.
The performance and reproducibility of a cascade impactor are dependent on a number of factors, the most critical being the nozzle dimensions (and their spatial arrangement) on each stage together with the air flow rate passing through it.
Providing these critical dimensions are within the quoted specification, then the impactors concerned can be expected to give comparable results.
The process of measuring the nozzle diameters and other critical dimensions of cascade impactors is called stage mensuration.
Both Ph.Eur. and USP recommend stage mensuration of impactors prior to use and periodically thereafter.
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