Abstract: The dynamic serial dilution apparatus of the invention, consists of an injection/mixing chamber and a precision injector from which foreign material may be impelled into a flowing fluid leading to the device to be calibrated. This arrangement allows the controlled injection of a concentrate of foreign material into pressurized piping systems with a negligible level of incidental undesirable artifacts. The injection/mixing chamber assures even dilution of the concentrated stock solution and thorough mixing with the carrier fluid without adding undesirable artifacts. As a result, the particle counter being tested sees only a dilute, homogeneous sample of specific, known concentration. Altering the delivery rate of the stock contaminant solution or the flow volume of the carrier fluid permits the particle concentration in the sample to be easily varied. The carrier fluid, typically deionized water, can revert instantly to a condition free of foreign material upon cessation of the foreign material injection.