Improved Protocol for Relating Impactor Stage Pressure Drop to the Suitability for Routine Use
Daryl L. Roberts1, Nathan Maidment2, Mark A. Copley3
1Applied Particle Principles, LLC, 9194 Edison Street, Blaine, Minnesota, USA
2Vectura Group plc, 1 Prospect West, Chippenham, Wiltshire, UK, SN14 6FH
3Copley Scientific, Colwick Quays Business Park, Nottingham, UK, NG4 2JY
Measurement of the pressure drop across each impactor stage, at a fixed flow rate, has the potential to provide users with a method of ensuring suitability of cascade impactors between periodic optical inspections of the impactor nozzles. The challenge is that the technique requires less than 0.5% random variability of the measurements because of the narrow range of nozzle diameters allowed for suitability of in-use impactors.
We measured the pressure drop across each stage of the NGI, weekly, in several multi-month campaigns in the past four years. We used the same equipment as previous investigators (Flow Resistance Monitor, MSP Corporation) and initially observed the same unacceptably high random variability in the pressure drop for stages 2 to 5. We changed the method by holding the inlet pressure at 90 kPa, rather than drawing the air directly from ambient laboratory conditions. We chose this method after noticing substantial variability in the ambient pressure. We also varied the inlet pressure intentionally and found that the nozzle “discharge coefficient,” a measure of the deviation from Bernoulli’s law, changes more than expected when the inlet pressure changes.
With the constant inlet pressure, the random variability is typically less than 10% of the range of acceptable pressure drop values. We are now able to identify instances of users failing to clean nozzles sufficiently, sometimes associated with a change in the product under test. This result demonstrates one practical way that the pressure drop method can help users ensure suitability of their impactors between optical inspections.