Loading Effect of Oil Aerosol Challenge during Filter Leak Testing

Polydisperse oil aerosol is used to challenge the “integrity” and measure the leakage of filter media, the filter assembly, framework, and connections, etc. Therefore a common question posed: “How much aerosol is needed to test a typical filter?” The answer: approximately eleven (11) drops for a 24” x 24” x 12” filter operating at 1,000 cfm.

The example below uses 10-ug/l (micrograms per liter) of PAO-4 as an upstream challenge.

Assuming a typical 24″ x 24″ x 12″ HEPA filter has an average of 65 pleats and the pleat size is 22.5″ x 10.5″, this equates to approximately 213 ft2 of media.
To determine the amount of liquid introduced to this filter, the necessary scan time of the filter must be calculated. Using a photometer with industry standard rectangular probe:

Assuming a scan rate at 2in/sec, as specified by most filter leak scan testing standards, 12 seconds is necessary to span one 24” pass. A 24” x 24” cross section requires eight (8) passes, with overlapping strokes, and, thus, a total time of 96 seconds or 1.6 minutes.

The quantity of oil liquid necessary to generate a 10ug/l upstream challenge aerosol for a 1000CFM filter is calculated by:

1,000 cfm x 28.3 (CFM to LPM conversion) x 10 ug/l x 0.00001 (g/ug) = 0.283 grams/min of liquid.
Therefore:
1.6 min x 0.283g / 0.819 g/ml (density of PAO-4) = 0.55ml PAO-4

For low viscosity liquids, it is generally considered appropriate to allow 20 drops per 1 ml of liquid. This would equate to eleven (11) drops of PAO-4 necessary to challenge a 24” x 24” x 12” 1000CFM filter using a 10-ug/l challenge.
For a point of reference, this is comparable to eleven drops of liquid spread over a surface area approximately equal to that of the “inside paint” (i.e. 3 second area) of a basketball court.