Absolutely not a nominal issue

Dear Editor:
After reading the article in your recent publication (“Absolute & Nominal: Cartridge Filters & Ratings,” Vikas Thusoo, WC&P, November 2002), I felt it necessary to relay some possible inaccuracies in the article:

1) Section “Filtration,” Paragraph 2, p. 52—“In today’s market, manufacturers use three types of rating to evaluate filters: nominal rating, absolute rating and beta ratio.”

Response: In today’s market, manufacturers use two types of ratings to describe filter cartridges—nominal and absolute. Beta ratio and percent efficiency are empirically based calculations that describe the efficiency of the filter. The value of this calculation, based on particle challenge test data, is used to classify the cartridge in terms of absolute or nominal.

2) Section “Filtration – Nominal rating,” Paragraph 1, p. 52—“This means that a filter with a nominal rating of 5 microns (mm)…”

Response: In most technical prose (mm) refers to millimeter (1×10-3 meters). Micron is used to represent micrometer (µm) equal to (1×10-6 meters). Example—a 5 mm (5,000 micron) particle is easily visible to the unaided eye. A 5 µm (0.005 mm) particle is not visible to the unaided eye.

3) Section “Filtration – Beta ratio,” Paragraph 2, p. 53—“Keep in mind that a ratio of 1,000 (b1000)…”

Response: A Beta ratio is typically expressed as  β1000 for a beta 1000 rating using the Greek alphabet.

4) Section “Types of cartridge filters,” Paragraph 3, p. 53—“Sometimes the filter is designed to reduce porosity from the periphery to the core.”

Response: Gradient density filters are designed to reduce pore size from the periphery to the core. Porosity reduction is a typical negative consequence of reducing the pore size. Higher porosity is a desirable attribute, lending higher dirt holding capacity to the filter structure.

5) Section “Pleated filter medium,” Paragraph 2, p. 54—“Filtration ratings can be anywhere between 10 mm down to 0.04 mm absolute, which is generally used…”

Response: Typical liquid absolute ranges for depth and pleated cartridges are slightly expanded to include 100 µm to 0.02 µm.

It is reasonable to consider that a missing font can explain points two and three. The rest of the points I submit for your consideration. Please feel free to contact me with any questions. Also, if you need additional cartridge filtration experts to assist in reviewing articles, please let me know.
Mr. Thusoo did a very nice job introducing this topic to your readers. Due to the lack of standards in the filtration industry, this topic presents some challenges to satisfy all readers. Thank you for your time and consideration.

Patrick Buzzell
USFilter Corp.
Londonderry, N.H.

Author response: Thanks for forwarding Mr. Patrick Buzzel’s comments on the article. He has made some interesting observations and I am pleased to reply point-wise as follows:

Point 1—All filter ratings are meant to define, with varying levels of accuracy, the removal efficiency of a filter at a certain micron rating. When a manufacturer calls a filter to be 5 micron nominal, the efficiency is generally understood to be <90%. At 5 micron absolute, the same filter now becomes (say) 99% efficient. In submicronic or critical applications (like pharmaceuticals), 99% efficiency is not enough and end-users demand a quantifiable idea on the filter efficiency, something for which “nominal” and “absolute” terms fall short. It is, therefore, such that the term Beta Ratio has become a widely accepted and used method of evaluating filter cartridges.

Points 2 & 3—Right. The units should read as microns and not mm. I checked the last edited copy of the article that I sent to the editors and it definitely uses the micron symbol, i.e., mm is by no means a symbol for micro-meter (microns).

Point 4—Porosity refers to mean pore size. As such, it is incorrect to say that a “higher porosity is desirable.” Pore size or micron rating will be defined by the customer’s application. Perhaps what Mr. Buzzell is referring to is “flow rate.” Of course, higher flow rate is preferable provided that efficiency is not compromised.

Point 5—Right. Pleated filters are of higher micron ratings also; however, the reference was generally to applications where defined pore size necessitates the use of pleated filters. Such filters are between the range specified.

Please thank Mr. Buzzell on my behalf for his positive comments.

Vikas Thusoo
Toronto, Canada

Editor’s note: Our apologies for the missing “µm” symbols. Sometimes when files are translated between programs, they default to “mm.” It is customary in some circles to use the letter “u”—or “um”—as an alternative to avoid that problem. Likewise in the use of “b” for “β.” We’ll do a better job of proofing. A corrected version of this article will be posted to our website.


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