Film aids quality control of membrane filters

Tactile pressure measurement has evolved from an art to a science over the last few decades.

Tactile pressure measurement has evolved from an art to a science over the last few decades.

What once was a trial and error system with little or no accuracy employing carbon paper, clay-bluing dyes and tinfoil has vastly improved.

Engineers and technicians can now determine force distribution and magnitude to an accuracy of +/-4%.

Tactile pressure sensor film (TPSF) technology is nondestructive, relatively accurate, extremely efficient and easily calibrated.

Generally speaking, any situation where two components either contact or impact in a normal (perpendicular) fashion is a viable candidate for using the TPSF technique.

TPSF can be used in both laboratory environments simulating manufacturing conditions and also during the live process itself.

TPSF is a single-use system, capturing maximal pressure loading at a static moment in time.

The film is also quantifiable, yielding fairly precise values of force magnitude of any point across the contact surface area.

TPSF could be considered an extension of finite element analysis (FEA) with one substantive distinction: the film is a post-process interpretive tool that actually collects stress and force data.

Microcapsules embedded in the film rupture at precise pressure levels, producing a visible colour change and a high-resolution image of pressure distribution and magnitude.

This quantifiable colour change is therefore directly proportional to the amount of pressure applied.

The user is able to visually inspect the film for significant variations aberration and general uniformity.

Spatial resolution of the TPSF system is 0.005 to 0.015mm, yielding ultra-high-definition imagery of force profile.

Pall RAI needed to find a quick method of quantifying flatness on some of its tangential flow filtration cassettes without having to manually measure many points through the use of callipers.

Pressurex TPSF was selected, as it was easy to use and much faster than callipers.

Tangential flow filtration cassettes are rectangular shaped membrane filters that filter fluid as it moves across the membrane as opposed to directly into the membrane as in most types of depth filters.

This movement of fluid across the membrane area helps also to prevent the membrane from fouling.

The production process is an adhesive-based procedure bonding membranes into filter mesh screens using compression fixtures.

Layers of membrane subassemblies are then bonded to feed screen meshes through the use of adhesive.

The final assembly is then encapsulated and cut to size.

Filter flatness is a very important specification during the flow cassettes production because these filters seal against their flat end faces on both top and bottom of each cassette.

Pressurex TPSF helped quantify the sealing flatness profile of these cassettes quite easily.

During the test process, the TPSF was cut into about eight different shapes, with the most common shape being approximately 177.8 x 203.2mm.

The TPSF was easily affixed to the test surface without any slippage being observed.

Distribution of the pressure across the filter face was deemed more important as Pall RAI was more concerned with flatness uniformity than peak stresses.

The visual inspection ability of the TPSF allowed the company to determine easily the relative flatness.

Before the use of the film as a quick quality-control tool, measurement was done manually through the use of callipers.

Due to the overall expense of the film, a real-time capacitance tactile pressure sensor system (CTPSS) was decided for use in R and D and manufacturing engineering departments as a means of qualifying filter flatness.

TPSF is a single-use system, like photo film, capturing a snapshot of maximal pressure loading at a specific moment in time.

What the TPSF reveals is a very high-resolution image of both pressure distribution and pressure magnitude.

The way the film works is quite simple.

Microcapsules embedded in the film rupture at precise pressure levels causing them to release chemical contents, which then interacts with a developer component, and thus produces a visible colour change.

This quantifiable colour change is directly proportional to the amount of pressure applied.

The user is able to visually inspect the film for significant variations aberration and general uniformity.

Spatial resolution of the TPSF system is 5-15um, yielding ultra-high-definition imagery of force profile.

The colour change of the film is both instantaneous and permanent, allowing for the film to be both immediately analysed and then saved for archival purposes.

Once a compressive load has been applied, the TPSF can now be analysed in several different ways.

The quickest and most economical technique is by visual comparison to a colour calibration chart (conceptually similar to using Litmus paper); this was the method chosen by Pall RAI to review their pressure rest results.

The engineer can match the films resultant colour to this calibration table can, to obtain an approximate indication of pressure magnitude.

Voids, pits, micro-cracks, warping, and surface aberrations are immediately apparent on visual inspection.

Typically the visual interpretation method yields accuracy of +/-15% full-scale.

A more sophisticated and accurate technique of analysis and interpretation of TPSF is by using an optical imaging tool.

The pressure analysis system consists of Windows-based software and a specially calibrated scanner that reads and interprets the film.

Images rendered by the image analysis are accurate to +/-2 to +/-4% full-scale, and are accompanied by a wealth of statistical data about your interface.

Key features of the image analysis system include the ability to produce histogram data of the image being analysed.

The system provides not only for analysing the entire interfacial surface, but also for the study of small and problematic areas (which can be enlarged and enhanced and carefully scrutinised).

Where implemented, TPSF as a measurement tool has generally resulted in a quite positive return on investment.

With relative ease, the two fundamental attributes of seal efficacy, namely total force and average pressure, can be visually assessed.

A technician can be trained in the TPSF technique in a matter of hours.

As a result of using the TPSF technique, manufacturers are able to assess optimal surface stress distribution required for a given interface.

The TPSF technique takes manufacturers and users a step closer to full-process standardisation.

Taken together, the TPSF and the optical analysis system (where used) have become powerful tools in the arsenals of manufacturers' production and R and D facilities.

Used by itself, TPSF can be an extremely efficient and reliable method of visually inspecting filter flatness in a quality control environment.

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