Originally Published September 2000
Technology news
Supplier of Woven Meshes Introduces Testing Method for Filtration Efficiency
Synthetic monofilament meshes used for filtration are typically identified by the value of their mesh apertures. This value, which is determined in a laboratory, may not correspond exactly with the size of the particles that the mesh can trap under different circumstances, according to Marco Mietta, R&D engineer at SaatiTech S.p.A. (Veniano, CO, Italy), a manufacturer of precision woven meshes. To provide users with more-precise data, the company has developed a "particle-counting test system and related testing procedures that have enabled SaatiTech to calibrate its family of small-pore-size filter meshes," explains Mietta. The filtration efficiencies of Saatifil meshes with openings smaller than 45 µm, as determined by this study, are published in a catalogue that is now available upon request.
A particle-count test system developed by SaatiTech S.p.A. enables calibration of the firm's small-pore-size filter meshes.
Divided into six sections, the brochure initially defines the parameters that are traditionally used to describe woven meshes used for filtration. "Among the properties that are identified," says Mietta, "the average value of the mesh opening is the most important. It can be measured based on electronic analysis of a microscopic image or by means of the bubble point test." The latter method is an indirect measure that is obtained by converting the amount of pressure required to pass the first bubbles through a fabric. Because the result depends on the value of a mathematical constant, it may be inaccurate, according to Mietta. "You might think that the mesh opening corresponds to the size of the particle that is trapped, but that is not strictly true," says Mietta. "There are external factors such as final working conditions, the type of contaminant that covers the fabric, and the minute tolerances of the mesh opening that can influence the results." SaatiTech has sought to refine the process by taking into consideration filtration efficiency, which introduces progressive data into a description of filtration performance.
SaatiTech produces a range of medical-grade fabrics for filtration applications in biopsy bags, oxygenators, flow control instruments, IVD components, and related devices.
The specially designed equipment, described in the paper, comprises a filtration test bench with two laser particle counters. The 2–120-µm latex spheres used as the contaminant are suspended in water, and the testing circuit is constructed in a single-pass configuration. "The contaminated liquid passes through the filter media a single time, and filtration efficiency is then calculated based on the particle counts obtained during the test, before and after filtration," explains Mietta.
Filtration-efficiency data on meshes such as he one pictured are available in a recently published catalogue.
The core section of the brochure lists filtration-efficiency test results, in diagram and table form, for the company's range of finer polyester and nylon meshes. Obtained using a multidispersed contaminant (i.e., differently sized particles), the results are better suited for practical applications than those obtained by means of a method that uses a monodispersed contaminant, according to Mietta. "The use of contaminants of varying dimensions has greater relevance to actual applications," he notes. "For some specific practical uses, however, a contaminant composed of a single particle size can be used, with theoretical results calculated according to previously obtained data," says Mietta. Filtration-efficiency data can be reprocessed by calculating the number of particles within a restricted micron range, he adds, thus avoiding the need to do another dedicated test using a real monodispersed contaminant.
The brochure also includes a series of comments on the test method and results that have been obtained. Notably, Mietta points out that the data are indicative of a fabric's normal performance, which may be affected by such factors as the filtrate used, flow rates, and the concentration and dimensional distribution of the contaminant. The shape of the contaminant can also affect filtration efficiency: spherical particles, for example, are more efficiently trapped by the filter than oval particles, which can pass through smaller mesh openings if they are oriented sideways.
SaatiTech's equipment has been used successfully to conduct preliminary tests in the development of woven products that must achieve specific performance specifications such as infusion sets. "The technical expertise and state-of-the-art equipment at the facility has enabled us to produce these new filtration-efficiency technical sheets," says Mietta. "They can be used as a practical reference tool to understand the effective behaviour of our range of monofilament polyester and nylon filter fabrics. It is our view that a practical evaluation of filtration performance is more reliable than theoretical measures of the mesh aperture that are currently in use," he adds. —Norbert Sparrow
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