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.)


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.


  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).
  • 2
    $\begingroup$ 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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