What is a Herzberg?

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.)

Herzberg filtration speed: The time taken to filter $$\pu{100 mL}$$ water at $$\pu{20 ^\circ C}$$ through a filter area of $$\pu{10 cm2}$$ at a constant pressure of $$\pu{5 cm}$$ water column (Thomas Scientific).

The values are given in seconds. So, the example OP has given has Herzberg filtration speed of $$\pu{375 s}$$. That means the relevant filter paper takes $$\pu{375 s}$$ to filter $$\pu{100 mL}$$ of water under above Herzberg filtration conditions. Thus your understanding is partially correct (except for $$\pu{10 cm}$$ water column):

If that is a reliable description then it looks like "375 herzberg" means that it took $$\pu{375 s}$$ 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.

Keep in mind that the temperature is important (Ref.1). This measurements are done at only $$\pu{20 ^\circ C}$$. This reference described Herzberg's research as:

Herzberg used a constant-head apparatus, based on the principle of the Mariotte flask, which forced water through a horizontal disk of filter paper $$\pu{10 cm2}$$ in area and discharged it into a measuring flask. With a head of $$\pu{5 cm}$$ of water at $$\pu{20 ^\circ C}$$, the instrument was used to measure either the time of filtration of $$\pu{100 ml}$$ of water or the volume of water filtered in $$\pu{1 minute}$$. He reported a range of filtering rates of $$23$$ to $$\pu{760 ml/min}$$ per $$\pu{100 cm2}$$ of area for 30 filter papers (Ref.2).

Thus, it is safe to assume that the definition given in Thomas Scientific is based on these conditions.

References:

1. Herman Bogaty, Frederick T. Carson, "Measurements of rate of flow of water through filter paper," Journal of Research of the National Bureau of Standards 1944, 33, 353-362 (PDF).
2. Wilhelm Herzberg, In Papierprüfung (Paper testing); Fifth Edition, Springer Verlag: Berlin Heidelberg, Germany, 1921 (ISBN: 978-3-662-23207-1).
• can you cite a source, since your definition contradicts the one provided by OP (5 cm pressure vs 10 cm)? Aug 10 '20 at 15:29
• Thanks; your source is actually one of the two links I cited in my question ("this old paper on filtration flow rate measurement"). The thing is, it describes Herzberg's work, but otoh it doesn't appear to define a named unit based on that work (although I could not find an English version of Papierprüfung, but I haven't looked hard yet). Plus the 5cm vs 10cm head thing. :) Hmm... Aug 10 '20 at 17:03
• @ Andrew: I have edited to give additional infomation. Although Thomas Scientific is a vendor, they have followed exactly what Herzberg has done to find various flow rates of various filter papers. I think that would clear OP's doubts about definition. Aug 10 '20 at 18:04
• Seems legit, or at least as convincing as it'll ever be. Aug 10 '20 at 18:14