April 19, 2018 03:00:00 PM

Feller Statistical Corrections for the BioCapt Impactor

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The BioCapt® impactor uses precision-cut slits to ensure laminar flow and improve biological collection efficiencies for viable particles in accordance with ISO 14698-1/Annex B. The impactor’s design enables the capture of microbiologically viable particles on a suitable media surface for incubation and enumeration. These values represent calculated statistical results made using Feller’s formula (Feller, 1950):


Pr Probable statistical total of viable particles required to produce r positive holes
N Number of holes
r Number of viable particles counted on the agar plate


A sample of the results for flow rates of 25-50 LPM are shown below:


The values in Table 1 demonstrate that biological efficiency and accuracy is guaranteed by ensuring a negligible probability of different viable particles entering the same unoccupied space.

For flow rates of 25–50 LPM, the design of the impactor shows that no correction is necessary until after 57 viable particles in the  r  and  Pr  columns. This means making an error when counting viable particles is extremely small because the probability of two viable particles entering the same unoccupied space is negligible. Worst case scenario, counting 200 viable particles results in an error of less than 3% when considering a  Pr  value of 206.

Download the Feller Statistical Corrections paper to view the full list of values calculated from Feller’s formula for flow rates of 25-50 LPM and 100 LPM.