Fit matters: What fluid mechanics shows us about N95 mask fit testing

Recently, our fluid mechanics team applied their skills and equipment to look at the problem of leaking N95 masks.

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Recently, our fluid mechanics team applied their skills and equipment to look at the problem of leaking N95 masks. For over 33 years, TSI customers have used the PortaCount® Respirator Fit Tester to evaluate the fit of a facemask to its wearer. PortaCount® equipment objectively measures how completely a mask seals to a person’s face. No other respirator fit tester can quantitatively fit test so many types of respirators—gas masks, SCBAs, respirators, and disposables like N95s and FFP3s.

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See details, videos, and images from this fluid mechanics experiment on this page, PIV Leak Assessment of N95 Respirators

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A PortaCount Respirator Fit Test system can identify a leaking facemask but doesn’t pinpoint where the leak is located. Enter Drs. Dan Troolin, Ruben Hortensius, and Wing Lai of the TSI Fluid Mechanics team, who decided to try to capture a facemask leak on high-speed camera.

Setting up the test to find an N95 mask leak

The fluid mechanics team took on the mask leakage challenge by acquiring a few new lab pieces, including an “inhaling” mannequin head. The rest of their test setup included a flow chamber, a particle generator, and a particle image velocimetry (PIV) system, consisting of a laser, high-speed camera, and software.  

TSI fluid mechanics systems help researchers solve all kinds of air or liquid flow problems—such as how to measure flow velocity for a new heart valve or help aircraft better withstand sudden downdrafts. PIV systems use lasers to measure flow velocity. In this setup, the PIV laser light sheet would illuminate particles “inhaled” by the mannequin and show the path of the particles through or around an N95 mask.

How does a properly fitted N95 mask work?

The purpose of an N95 respirator is to protect a wearer from airborne particles. According to the FDA, “The 'N95' designation means that when subjected to careful testing, the respirator blocks at least 95 percent of very small (0.3 micron) test particles.” In order to filter out these breathable particles, an N95 must achieve a very close facial fit.

When conducted properly, a PortaCount respirator fit test can show if there is a good facial fit. Where do particles go when there isn’t a close fit? The PIV experiment was ready to show us.

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For videos, images, and more details on this fluid mechanics experiment, visit PIV Leak Assessment of N95 Respirators

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