Average Diameters and Particle-Size Distribution

• The particular application for the particles can help to determine the criteria for sizing. Either the average diameter of a particle or its weight can be a basis for measurement. Particle density can be based upon either the mass of particles with a given range of sizes or the actual number of particles.
  
  

Cascade Impactors

• A commonly used device for the separation of particles with air is a cascade impactor. The impactor uses nozzles that direct streams of air laden with particles at impact plates. Particles with sufficient mass will strike the plates and pass through, while smaller particles will proceed to the next stage. In this stage, the air speed is increased by the use of smaller air jets. The process continues to new stages until all of the particles have passed through an impact plate.
  
  

Calibrating a Cascade Impactor

• In order to obtain accurate measurements, any precision instrument must be calibrated. The calibration may follow the manufacturer's recommendations, or it may be necessary to write a calibration protocol and standard operating procedures (SOP). Collection efficiency data is collected for each stage. Finally, a calibration spray is developed so that the accuracy of the impactor can be compared using a particle collection efficiency curve.