Whatman lists the flow rates for their filters in a unit called a 'Herzberg':
What is this?
I've been searching around and I did find information about Herzberg flow rate testers, for example this old paper on filtration flow rate measurement and this summary of filtration terminology, but I can't seem to find two sources that agree with eachother.
The clearest looking description I found was that second link above, which states:
Whatman quantifies liquid flow rate for its range of filters by using a Herzberg flow rate tester. Prefiltered deaerated water is applied to the test filter (effective area $\pu{10 cm2}$) at a constant hydrostatic head ($\pu{10 cm}$). The rate of the flow is measured in seconds per $\pu{100 mL}$. Flow rate can also be measured by the modified ASTM method which uses a quadrant folded filter held in a wire loop. It is not considered to be as reliable or consistent as the Herzberg test.
If that is a reliable description then it looks like "375 herzberg" means that it took $\pu{375 seconds}$ for $\pu{100 mL}$ of water with a $\pu{10 cm}$ constant head to flow through a $\pu{10 cm2}$ filter, which means that more Herzberg = less flow.
Although that's a little confusing too because the $\pu{2.0 \mu m}$ filter flow rate above is shown as 375 Herzberg, but these larger $\pu{2.5 \mu m}$ filters are listed as 1870. Now the $\pu{2.5 \mu m}$ filters are $\pu{200 \mu m}$ thick compared to the $\pu{2.0 \mu m}$ filters at $\pu{160 \mu m}$, and also I have no real understanding of how any of this works, but my initial reaction is "how could the larger pore size have a $5 \times$ slower flow rate?".
Is that correct? If not, what is this unit?
(I realize that second source is clear, but since I can't find consistent descriptions I mostly just want to make sure that's what it means.)