Trial Facts to Be Considered
The question of what effective filtration area to use during 47-mm disc trials often comes up (Figure 1). Most users have simply used a number between 12.5 and 13.5 cm2.
The use of an incorrect effective filter area in comparison or scale-up calculations may lead to the wrong conclusions. The effective filtration area of common 47-mm discs within a stainless steel holder and disposable device were measured by measuring the diameter of the discolored portion of the filter disk. This provides a direct measurement of the effective filtration area, as only this area will be discolored.
Recently filter manufacturers have begun to supply encapsulated disk devices to facilitate the performance of scale-down studies. These are marketed as scale-down devices specifically for such use. The devices themselves are sealed, preventing easy inspection of the filter membrane after use. This requires that the user refer to the published filtration area for the device for comparison or scale-up calculations.
Severe nonlinearity experienced during scale-up studies led one investigator to perform destructive inspection of one such device. The diameter of the coloration within this device was measured to be 48 mm (Figure 2), which represents an effective filtration area of approximately 18 cm2.
However, the literature for this device states an EFA of 13.8 cm2, introducing an error in excess of 30%. The effective filtration area of the disposable device is 36% larger than the 47-mm disc utilized in a stainless steel holder. The main reason for this discrepancy is the fact that the stainless-steel device requires an O-ring sealing. The O-ring diameter and seal will reduce the effective filtration area. The disposable device utilizes a heat welding, which is a thin ring around the membrane.
In another instance, indicator trials performed at 47-mm scale were followed by verification trials utilizing 1,000-cm2 pleated capsule filters. The scale-up calculations also exhibited severe nonlinearity. This time it was found that a filter capsule marketed as a 1,000-cm2 device actually contained approximately 1,600-cm2 of effective filtration area (+60%). The reader is cautioned to verify the effective filtration area of any device used for indicator or verification trials.