Evaluating Filters with Scanning Electron Microscopy

Filtration systems provide clean air to breath, water to drink and oil to run engines. Filters separate dust, pollen, bacteria, sand, and other particulates from a filtration feed. Some filters are a woven mesh from metal materials. Others, like high-efficiency particulate air (HEPA) and ultra-low particulate air (ULPA) filters, are made from glass or polymer fibers. Filter shapes vary, from pleated cylinders or panels to bag designs, depending upon the application. Efficient filters are needed in personal, business, and manufacturing environments.

HEPA Filter

Scanning electron microscope image of HEPA filter 

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 SEM for fibers
 Automated fiber analysis
 Elemental ID with EDS

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Advantages of SEM for Evaluating Filtration Fibers & Membranes

Advanced microscopy techniques are imperative to evaluating the performance of a filter. Optical inspection, like this optical image of nanofibers on the left, has been the industry standard for decades allowing scientists to measure various fiber properties and monitor large contaminants. Optical inspection has become insufficient for many new applications because fiber dimensions, especially nanofibers, are often below the resolution limit of an optical microscope.Optical vs SEM pulsed-air-filter.png

Optical vs SEM images of air filtration media

With a higher depth of field and greater image contrast, use of a scanning electron microscope (SEM) is becoming the new standard for characterizing filtration materials, as shown in the SEM image of nanofibers (right). An SEM image affords a quick and high resolution visualization of filter media. Elemental analysis, via energy dispersive X-ray spectroscopy (EDS) with SEM, allows for the identification of elements in the fibers or particulates.

Benefits of the Phenom SEM for Filtration Fiber Characterization

The Phenom SEM was designed to bring high quality SEM imaging into environments where a traditional SEM was not manageable or practical.  The Phenom SEM:

  • Is accessible and intuitive to use
  • Produces superior images
  • Provides high productivity and workflow 

The Phenom scanning electron microscope does not require a dedicated operator nor additional infrastructure. The innovative column design allows imaging of fibers without sputter coating. 

 FiberMetric HEPA filter

 Measure dimensions of filtration fibers by SEM

Fiber Analysis for Filtration Media

The Phenom desktop SEM uses FiberMetric, proprietary fiber measurement software, which measures fiber diameter, orientation and pore sizes.  Using the integrated motorized XY stage, data acquisition and analysis can easily be automated. This software/hardware combination allows for hundreds of images to be acquired and analyzed, accumulating in tens of thousands of measurements, providing statistically significant data.

Elemental Analysis for Foreign Particulates in Filtration

Elements in the filtration fibers or contaminants can be identified using energy dispersive X-ray spectroscopy (EDS).  An EDS detector is integrated in the Phenom SEM and can show elements at a point, across a line, or mapped in an area.

 SEM image of filtration fibers with EDS spectrum and data of particulate

Scanning electron microscope image (left) of HVAC filter with particulates and corresponding EDS spectrum (right)