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Whatman lists the flow rates for their filters in a unit called a 'Herzberg':

Filter Paper Description

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.)

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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).
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    $\begingroup$ can you cite a source, since your definition contradicts the one provided by OP (5 cm pressure vs 10 cm)? $\endgroup$ – Andrew Aug 10 '20 at 15:29
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    $\begingroup$ 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... $\endgroup$ – Jason C Aug 10 '20 at 17:03
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    $\begingroup$ @ 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. $\endgroup$ – Mathew Mahindaratne Aug 10 '20 at 18:04
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    $\begingroup$ Seems legit, or at least as convincing as it'll ever be. $\endgroup$ – Jason C Aug 10 '20 at 18:14

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