ASTM F902-84(1990)

1.1 This practice provides a way of depicting a unit volume of a filter medium by combining the two separate measurements of permeability and porosity (fractional void volume) into one value: the average pore diameter defined by a fluid during viscous flow through the medium. Diameter is that term in the Hagen-Poiseuille law relating to viscous flow through a circular pipe.

1.2 This practice has two limitations. Where the average pore diameter approaches the length of the mean-free path of a gas molecule, laminar flow cannot occur and the practice cannot be used. Where a fluid may attack the filter medium, that fluid cannot be used; nor, would the medium ever be used to filter that fluid.

1.3 This practice is useful not only for those filter media that are examined by bubble-point measurements, but also for thicker and coarser media that do not lend themselves to bubble-point analysis.

1.4 Average diameter in this practice differs from an average diameter deduced from fluid-intrusion measurements. This practice involves viscous flow, the other pore volume, or cross-sectional area. In viscous flow the flow rate under a given driving force is a function of the square of the cross-sectional area.

1.5 Where Test Method F316, an extended bubble-point test, determines the diameter of a mean flow pore, that diameter differs from the average flow pore in this practice by definition of what is measured. Footnote reference 2 offers a more detailed analysis of extended bubble-point data than does Test Method F316 to explain the concept of average and against which one may compare the concept of mean offered by Test Method F316.

1.6 Methods F778 addresses the details of measuring the gas permeability of filter media. Where Test Method E128 uses air to measure permeability, the method employs an air pressure of 10 cm Hg, or 0.13 atmos gage upstream to zero gage pressure downstream. This practice suggests, as does Methods F778, that the differential should be no more than 0.05 atmos so as to avoid the extra effort of correcting for differences in gas density between the two faces of the medium.

1.7 This practice is applicable to "homogenous" filter media where the average pore size on one face is not too different from that on the other face. The procedure is not applicable to filter media composed of two or more layers of material where the average pore size in one layer is very different from that in another layer. The procedure does not define "too different" or "very different," but instead requires that the investigator, in reporting test results, make some statement about the apparent composition of the filter medium tested.

1.8 This standard may involve hazardous materials, operations, and equipment. This standard does not purport to address all of the safety problems associated with its use. It is the responsibility of whoever uses this standard to consult and establish appropriate safety and health practices and determine the applicability of regulatory limitations prior to use.