Ambiguities in pore-size rating
Within any given pore-size designation available in the market, there exists some range to its quantified properties. In its
manufacture, a membrane lot may, and indeed will, incur some variation in the in-process bubble-point measurements that are
translated into pore size. Each pore-size rating is prepared using a different casting formula. Nonetheless, the preparations
do not yield distinct quanta, although the variations within a class are less than those between classes. Each batch or even
each filter within a batch of membranes is classified to a single pore-size value, although each of the individual filters
comprising the group may have its own bubble point within the range of values that define the given rating.
As Schroeder points out, the pore size–retention correlation is not a step function (26). In its manufacture, a membrane lot
will incur some variation. In using nonuniform standards, filter manufacturers might assign their pore-size labels somewhat
ambiguously. One fabricator may, on the basis of flux, consider a membrane to be an "open" 0.1 μm. Another filter producer,
using a somewhat different rating system might classify it as being a "tight" 0.2 μm. This could give rise to a labeled 0.1
μm, not so handicapped by reduced flows, being compared with a labeled 0.2 μm, not so advantaged by enhanced flows. The more
"open" 0.1 μm may not flow faster than an average 0.2 μm but may do so against a "tight" 0.2 μm. Consequently, it becomes
an unrewarding exercise to try to compare competitive membranes each rated by their own individual catalogue descriptions
as perhaps being of the same pore size, yet exhibiting different flux rates or retentions under test.
As is usually the case in the physical sciences, reliable evaluations and meaningful comparisons must be derived by users
from performance data obtained through experimental investigation founded on suitable experimental designs. Another possibility
for initial comparisons is the standard ASTM organism challenge test designed for 0.2/0.22 μm-rated filters but extendible
to 0.1- or 0.45 μm-rated membranes as well (27).
Figure 6: Different flow rates of 0.2- and 0.1-μm rated filters.
Consequences of rating ambiguities
As shown in Table II, Tolliver and Schroeder found that the retention of 0.198 µm latex particles by five commercially available
0.2/0.22 μm-rated membranes differed significantly (21). The particle-retention mechanism was not complicated by adsorptive
sequestrations because the solution contained surfactant. Given the size uniformity of latex particles, the variation in retentions
based on sieving indicates different reference standards for the like-rated pore sizes. Nonstandard ratings also explain the
Lindenblatt et al. (28) report that the flow rates exhibited by four 0.2/0.22 μm-rated membranes, as also of four 0.1 μm-rated filters differed
markedly (see Figure 6) (28). Moreover, as indicated in Figure 7, the throughput volumes measured for the nonstandardized
similarly rated pores also differed. Comparable data are offered by Jornitz et al. (29).
Figure 7: Different throughputs of 0.2- and 0.1-μm rated filters.